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Control of Molecular Weight Distribution in Polyolefins Synthesized with Ziegler-Natta Catalyst Systems
Abstract
In spite of the enormous number of papers published and patents issued on Ziegler-Natta catalysis, it does not exist a review on the methods of polyolefins molecular weight distribution (MWD) control. In the present article we shall review scientific and patent literature on this argument.
After a short comment on the speculative and industrial importance of polyolefins MWD, an outlook is given on the theories of the origins of the wide MWD usually shown by polyolefins. Subsequently, a comprehensive critical survey of the possibilities of MWD control, based mainly on the type of catalytic system and on polymerization parameters, is discussed. Finally, some considerations on MWD control in industrial processes are given and a rationalized collection of the most significant patents for polyethylene MWD control since 1968 is presented.
In conclusion, the theories based on the plurality of the catalytic active species appear more convincing than those based only on physical phenomena in explaining MWD. Together with some general principles, only a better knowledge of number and types of polymerization centres and of the relevant kinetic constants could lead to a more effective MWD control. This should represent one of the future trends of research and development in Ziegler-Natta catalysis.
... The majority of produced PE grades, for example, tube and film grades of high density polyethylene (HDPE) and also linear low density polyethylene (LLDPE) are copolymers of ethylene with α-olefins. It is very important to control MWD [6] and short chain branching (SCB) distribution [7][8][9] for high density polyethylene intended for piping applications. A general method to produce polyethylene with broad/bimodal MWD and SCB over titanium-magnesium catalyst is the use of tandem reactors [8] with different polymerization conditions [10][11][12]. ...
... The composition of active component may exert a considerable effect on the molecular weight characteristics of PE and ethylene-α-olefin copolymers [6,11,13]. Supported vanadium-magnesium catalysts (VMCs) containing vanadium chloride as the active component supported on MgCl 2 differ considerably from TMCs regarding the regulations of molecular structure of PE [14][15][16] and copolymers of ethylene with α-olefin [17,18]. There are some features of VMCs distinguishing them from TMCs in homopolymerization of ethylene and copolymerization of ethylene with α-olefins as follows: ...
The kinetics of copolymerization of ethylene with 1-hexene over highly active V-Mg Ziegler–Natta catalysts [VMC: VCl4/MgCl2 + Al (i-Bu)3] at different contents of vanadium (4 and 0.1 wt% of V) was studied. Data on copolymerization ability of these catalysts, molecular weight characteristics, and compositional homogeneity of the produced copolymers were attained. It was found that the introduction of 1-hexene led to broadening of the molecular weight distribution of copolymer (an increase in the Mw/Mn value) relative to homopolyethylene due to a decrease in the molecular weight (Mn) in the region of 1–100 kg mol−1 without changes in the molecular weight in the region of Mn > 500 kg mol−1 for two used VMCs catalysts. This result testified to non-uniformity of the active centers of these catalysts in the chain transfer reaction with participation of 1-hexene. Therefore, the active centers of VMCs producing high-molecular polyethylene were virtually not involved in the chain transfer reaction with 1-hexene. At the same time, these centers were more reactive in the incorporation of 1-hexene. This led to unusual distribution of butyl branches in ethylene–1-hexene copolymers produced over VMCs, namely, to an increased content of butyl branches in the high-molecular fraction of copolymers (Mw > 1000 kg mol−1).
... The supported titanium-magnesium catalyst (TMC) of the Ziegler-Natta type with a titanium-active center is well known in the global production of polyolefins [1]. The polyethylene (PE) obtained using this catalyst has a relatively narrow molecular weight distribution (MWD = 4-6) [2,3]. It is known that polyethylene with a narrow MWD is used to produce traditional injection molding grades of high-density polyethylene (HDPE). ...
... The composition of the active component may exert a considerable effect on the molecular weight characteristics of HDPE. In particular, supported vanadium-magnesium catalysts (VMCs), which contain vanadium chloride as the active component deposited on MgCl 2 , differ considerably from TMCs regarding the regulation of molecular structure of PE and copolymers of ethylene with α-olefin [2,3,[6][7][8][9][10][11][12][13][14][15][16][17][18]. Unlike TMCs, VMCs make it possible to produce PE with a broad or bimodal MWD in one polymerization reactor with polydispersity (M w /M n values) within the range of 15-33 [7][8][9][10][11][12][13][14][15][16][17][18]. ...
Data are presented on the activity of supported vanadium-magnesium catalysts (VMCs) with different vanadium content in ethylene polymerization and the molecular weight characteristics of the produced polyethylene. The VC1 catalyst, with a very low vanadium content (0.12 wt.%), showed a sixfold higher activity per unit weight of vanadium than the VC2 catalyst with a high-vanadium content (4.0 wt.%). Additionally, the total activity of VC2 (kg PE/g cat·h) was fivefold higher when compared to VC1. The introduction of hydrogen in polymerization leads to a considerable decrease in the activity of both catalysts. The polyethylene obtained in the presence of hydrogen over both catalysts has a broad bimodal molecular weight distribution (MWD) with a distinct shoulder in the high-molecular region (Mw ≥ 106 g/mol). Decomposition of the MWD curves of bimodal polyethylene into two fractions (high- and low-molecular fractions) made it possible to determine for the first time the ratio of the reaction rate constants of chain transfer with hydrogen (KtrH) and polymer chain propagation (Kp) for two groups of the VMC active sites producing low- and high-molecular fractions of bimodal polyethylene.
... Authors of recent literature reviews and modeling studies (Zucchini and Cecchin, 1983; deCarvalho et al., 1989; Boehm et al., 1986; Galvan and Tirrell, 1986) have concluded that broad molecular weight distributions produced by heterogeneous ZN catalysts are most likely the result of multiple active sites, although in certain instances (very broad particle size distributions or high molecular weight comonomers) diffusional resistances could play some role. The most convincing evidence cited for the existence of more than one type of active site has been provided by Usami et al. (1986) who have studied linear low-density polyethylene (LLDPE) copolymers of ethylene and ...
... Alternatively, if diffusional barriers to mass transfer govern the reaction rate, catalyst sites at different radial positions in a growing polymer particle are subject to different reactant concentrations, which can account for a broad distribution of molecular weights and composition (Galvan, 1986). Authors of recent literature reviews and modeling studies (Zucchini and Cecchin, 1983; deCarvalho et al., 1989; Boehm et al., 1986; Galvan and Tirrell, 1986) have concluded that broad molecular weight distributions produced by heterogeneous ZN catalysts are most likely the result of multiple active sites, although in certain instances (very broad particle size distributions or high molecular weight comonomers) diffusional resistances could play some role. The most convincing evidence cited for the existence of more than one type of active site has been provided by Usami et al. (1986) who have studied linear low-density polyethylene (LLDPE) copolymers of ethylene and ...
A dynamic kinetic model describing gas-phase olefin copolymerization using a multiple active site Ziegler-Natta catalyst is presented. This model is capable of predicting production rate, molecular weight, and copolymer composition changes observed in an industrial reactor. The model also explains how broad molecular weight distributions and bimodal copolymer compositions can occur as has been observed for commercial linear polyethylenes.
... The examination of the blends' MWD, illustrated in Fig. 2, indicates the presence of a significant amount of low molecular weight fractions below 10 kg/mol. These fractions arise from standard industrial polymerization practices known for yielding diverse chain length distributions due to several active sites on the Ziegler-Natta catalyst [82][83][84][85] and/or from multimodal polymerization processes, where different polymer types are produced in different reactors [86,87]. Beyond the chemical structure, or more precisely, the longest chain sequences included in the crystals, it is now well documented that the quantity and molecular weight of the low molecular weight fraction below 10 kg/mol for PE [80] or 50 kg/mol for PP has an impact on the elution behavior in TREF [88]. ...
... As a result of heterogeneity of active sites TMC produce polyethylene (PE) with a broad molecular weight distribution (MWD) (M w /M n = [5][6][7][8]. Some data on the MWD of polyethylene produced over Ti-based catalysts are presented in the review [1] and in refs. [2][3][4][5][6][7][8][9][10][11][12][13][14]. ...
In this review, we summarize and discuss our experimental data published in a number of papers on the transfer reactions of polymer chains in the polymerization of ethylene, propylene, and hexene-1, and the copolymerization of ethylene with α-olefins over multisite supported titanium–magnesium catalysts (TMC). Three groups of transfer reactions are discussed in the review: (1) transfer reactions with AlEt3 cocatalyst, (2) transfer reactions with hydrogen, and (3) transfer reactions with participation of α-olefins in the case of ethylene copolymerization with α-olefins. We have found polymerization conditions where it is possible to observe heterogeneity of active sites of TMC for all three groups of the indicated reactions. It is shown that (1) the transfer reaction with AlEt3 proceeds with higher reactivity on the active sites that produce polymers with low molecular weight; (2) the transfer reaction with hydrogen, in the case of α-olefin polymerization and copolymerization of ethylene with α-olefins, proceeds with higher reactivity on the active sites which produce polymers with high molecular weight; (3) the transfer reaction with α-olefins proceed with higher reactivity on the active sites that produce high molecular weight polymers.
... It is known that conventional multisite Ziegler-Natta catalysts, particularly the TMC, produce copolymers with broad molecular weight distribution (MWD) and non-uniform molecular weight distribution of the comonomer [8][9][10][11][12][13]. Supposedly [14,15], the formation of copolymers with the indicated properties may be caused by the non-uniformity of the TMC active sites with respect to the oxidation state of titanium. ...
Data were obtained on the copolymerization of ethylene with α-olefins over supported titanium–magnesium catalysts (TMC) prepared on the same magnesium dichloride support but differing in the composition and oxidation state of titanium. The copolymerization kinetics of ethylene with 1-hexene over TMC of different compositions were studied. Data on the composition of the produced ethylene–1-hexene copolymers, their molecular weight distribution, thermophysical characteristics, and branching distribution were presented. The constants of ethylene–1-hexene copolymerization over catalysts with different compositions were calculated. The TMC containing only Ti(II) compounds as the active component exhibited increased copolymerizing ability compared to the conventional TiCl4/MgCl2 catalyst containing Ti(III) compounds as the active component. In addition, TMC with Ti(II) as an active component produces copolymers with a more uniform branching distribution. It was shown that the TMC containing isolated Ti(II) ions could be used to produce X-ray amorphous ethylene-propylene elastomers with a high yield.
... Molecular weight distribution (MWD) of polyethylene is another factor which influences the properties of polyethylene. For example, broad MWD polyethylene offers good impact resistance and good processability, while narrow MWD polyethylene will have greater toughness at low temperatures and higher resistance to environmental stress-cracking [13,14]. Table 1. ...
This paper gives a short overview of homogeneous post-metallocene catalysts based on group 4 metal and vanadium complexes bearing multidentate ligands. It summarizes the catalytic behavior of those catalysts in copolymerization of ethylene with 1-olefins, with styrenic monomers and with α,ω-alkenols. The review is focused on finding correlations between the structure of a complex, its catalyst activity and comonomer incorporation ability, as well as the microstructure of the copolymer chains.
... It was well known that molecular weight and molecular weight distribution were very important factors in determining the physical, mechanical, and rheological properties of polymers [12]. The molecular weight controls the mechanical properties of polymers and molecular weight distribution mainly controls the rheological properties [13]. It was highly desirable to have for many applications, such as an extrusion and molding processes, polymer which have a broad molecular weight distribution. ...
In this research, a novel MgCl2-supported TiCl4 catalyst in conjunction with bifunctional internal donor was synthesized. The effects of internal donor on propylene polymerization behaviors and polymer properties (morphology, M
w and MWD) were investigated. It was found that the activity of novel catalyst was higher than that of the traditional DIBP-based Ziegler–Natta catalyst, while the catalyst activity was less influenced by the ether group length of the bifunctional internal donor. It was also observed that the MWD of PP obtained by bifunctional internal donor-based catalyst was broader than that of PP made by DIBP-based Ziegler–Natta catalyst.
... It is widely accepted that different active sites of the Ziegler-Natta catalyst produce polymers characterized by different Flory-type MWD, which is the main reason for the broad MWD of polyolefins produced using ZN catalysts [51][52][53]. Deconvolution of a given MWD gives a quantitative analysis of the contribution of these different sites; see for example [53,54]. ...
Polyethylene (PE) with the annual consumption of 70 million tones in 2007 is mostly produced in slurry, gas-phase or combination of both processes.
This work focuses on a comparison between the slurry and gas phase processes. Why does PE produced in theses two processes can show extremely different properties and extremely different reaction behaviour even if the same Ziegler-Natta (ZN) catalyst is used? Generally, it is known that the reason can be found in the differences of local conditions near active sites of ZN catalysts – the question is: which conditions are relevant? How do they interact? To answer these questions, a large number of single- and multi-stage experiments using TIBA as co-catalyst has been carried out in a 1.6-L reactor varying the following parameters:
- amount of hexane (going from gas phase to slurry)
- pre-contacting time catalyst – cocatalyst
- hydrogen pressure (0 to 10 bar)
- temperature (40 to 90°C)
- ethylene pressure (1 to 12 bar)
Isothermal-isobaric polymerization rate-profiles were analyzed in terms of activation and deactivation behaviour and the PE products were characterized by molecular weight distribution (MWD), particle size distribution (PSD), crystallinity and in some special cases by TEM and SEM images.
This combination of methods allowed us to identify and explain a number of significant differences between slurry and gas phase processes. Finally, all these findings were concentrated in a new theoretical contribution, which we called GRAF (i.e. “Growth Rate Acceleration by Fragmentation”):
1- Gas-phase rate-profiles show rapid initiation followed by rapid decay (decay type), whereas slurry profiles show the “build up type curve” with long-lasting constant activity after initiation.
2- It was shown that hexane is not at all “inert” – it affects all the relevant transport and equilibrium conditions. Varying the amount of solvent could dramatically change the reaction rate profiles.
3- The higher the temperature, the lower the molecular weight and consequently the lower the molecular weight – common for both processes.
4- By increasing the temperature in slurry with the presence of hydrogen; the higher mobility of freshly produced polymers leads to faster crystallization. More and larger lamellae increase the brittleness of the particle. This promotes the fragmentation that can lead – in an extreme case – to fines generation.
5- Internal and external particle fragmentation, as a physical effect, generates new active sites, which in turn leads to a faster chemical reaction.
6- Faster fragmentation accompanied by faster generation of new active sites (GRAF) at a higher ethylene pressure leads to a higher initial slope of the rate curves.
7- Varying ethylene pressure either in slurry or gas phase experimentally confirmed the first order ethylene pressure dependency.
8- It was shown that increasing ethylene pressure might increase the solubility of hydrogen in the polymer structure leading to termination of more chains by hydrogen transfer. By introducing a “solubility function”, it was explained why the hydrogen concentration increases with increasing ethylene pressure. The change of the molecular weight as function of the ethylene pressure can be described by the following equation in which X is the hydrogen: ethylene pressure ratio: ........][12222mCHCptSCptAptMnPXkPkkPkAkkkM+++≈
9- One of the most spectacular results was the “counter effect of hydrogen”. In the gas-phase, the reaction rate decreases with increasing hydrogen pressure; but the opposite effect was found in the slurry phase.
10- Hydrogen shows a similarly strong effect on the molecular weight of the polymer produced in either gas or slurry. In the absence of hydrogen, we found slightly lower molecular weights in slurry compared to the gas-phase.
11- DSC results confirm that hydrogen addition increases the level of crystallinity coupled with a simultaneous decrease in the melting temperature. This correlates with the higher chain mobility of shorter chains. Increasing the level of crystallinity can dramatically increase the production of fines in both phases and can change the particle size distribution accordingly if the brittleness of the crystalline particles and the growth stress reach critical levels (i.e. a crystallinity degree of 75%)
12- The polymer mobility is influenced by many variables such as:
- temperature
- chain length of the polymer produced
- chain length of the dead polymer that surrounds the active sites (“matrix”)
- hexane content in the amorphous part of the polymer matrix that changes the micro-viscosity. This different chain mobility leads to differences regarding the in-situ crystallinity, which has a direct impact on the particle brittleness. As a result, the particle can break at a critical growth stress that increases with the polymerization rate. This was the core result for the GRAF development. It is now very clear that this effect can affect the polymerization rate profiles in slurry and gas-phase polymerization differently due to different sorption, swelling and micro conditions around the active centres.
13- Two-stage experiments in different phases were carried out by varying the ethylene and hydrogen pressures to prove the GRAF hypothesis. A quick change of polymerization conditions (in the 2nd step) does not always lead to the same results of the one-stage experiments performed in the same conditions, since the history of the particle (defined by the 1st polymerization step) must determine the response – an effect that is explainable with GRAF.
14- Depending on which kind of PE – ductile or brittle - is produced in which step of the two-stage polymerization, one can produce particles with identical crystallinity and MWD, but with absolutely different fragmentation behaviour.
15- The hydrogen enhancement effect – in combination with the disintegration of particles leading to new active site generation – happens if hydrogen is introduced at the beginning of the polymerization. Producing ductile polymer in the 1st step decreases the fragmentation-controlled enhancement effect of hydrogen.
16- In general, the presence of ductile PE does not suppress particle fragmentation and the resulting rate enhancement completely, but the particle disintegration can still be reduced dramatically. This is a useful tool for optimizing a catalyst.
17- Removing hydrogen increases the reaction rate by the well-known “chemical effect”, for which different explanations exist.
18- The activity during the 2nd step depends strongly on what degree of fragmentation was reached in the 1st stage. However, for activation of new sites after fragmentation, the presence of the co-catalyst is required – “back-diffusion limitation”, and the “dilution effect” can partially compensate the rate accelerating fragmentation effect.
19- The lowest fines generation was found in a two-stage gas phase polymerization for bimodal PE production: the 1st step without hydrogen (making ductile PE) and the 2nd step with high hydrogen pressure (crystalline PE distributed within the ductile phase).
20- Changing the polymer matrix properties during switching from 1st to 2nd step conditions (by means of cooling, pressurizing, depressurizing, hexane evaporation, re-pressurizing) can influence both rate profiles and PSD. This is especially the case when performing the 1st step in slurry under high hydrogen pressures and the 2nd step in the gas-phase.
21- It is useful to analyze the MWD by deconvolution in terms of the GRAF hypothesis. The chain mobility plays an important role. In multi-stage polymerizations, the MWD is a fingerprint of the polymerization rate of each step: the amount of polymer produced in each step can be predicted from the MWD.
... Authors of recent literature reviews (Zucchini and Cecchin, 1983;deCarvalho et al., 1989;Boehm et al., 1986) have concluded that broad molecular weight distributions produced by heterogeneous ZN catalysts are most likely the result of multiple active sites, although in certain instances (eg. very broad particle size distributions or high molecular weight comonomers) diffusional resistances could play some role. ...
The focus of this thesis is the modelling and control of product properties in gas phase polyethylene reactors. The main product properties of concern are melt index (MI) and density which are related to the molecular weight and composition of the ethylene/-olefin copolymer. A kinetic model is developed which accounts for the effects of gas composition, reactor temperature, and active site distribution of the Ziegler-Natta catalyst on the MI and of the polymer product. The model predicts the behaviour of MI and in an industrial reactor, as well as broadened molecular weight distribution and bimodal composition distributions, which are typical for commercial linear polyethylenes.^ Because measurements of MI and are not available on-line, a methodology is developed to infer product properties from available measurements. Simple, theoretically-based models are derived which relate MI and to reactor operating conditions. Parameters in the models are adjusted using off-line measurements, providing an effective means for inferring both MI and ^ In a series of three product grade changeovers, dynamic optimization is used to determine optimal profiles for: hydrogen and butene feed rates, the reactor temperature setpoint, the gas bleed flow, the catalyst feed rate, and the bed level setpoint. It is shown that large transitions in MI are hampered by slow hydrogen dynamics, and that the time required for such a transition can be reduced by manipulation of the temperature setpoint and the bleed stream flow. Reduction of the bed level and catalyst feed rates during transitions can significantly decrease the quantity of off-specification polymer produced. In the absence of feedback control, disturbances and model mismatch can result in product property trajectories which differ significantly from the nominal optimal trajectory.^ A novel nonlinear model-based strategy is developed for on-line product property control. This feedforward/feedback control scheme is capable of both regulating product quality about a given target and of implementing optimal transition policies with feedback. The simplified mass balance model used in the controller design contains four adjustable parameters which are updated using an extended Kalman filter (EKF). The controller and EKF provide excellent regulatory and grade transition control for the range of polyethylene products simulated. The nonlinear controller is superior to an analogous linear time-invariant internal model control (IMC) design. The control system developed in the thesis is both simple and effective, and it has great potential for improving product quality in the polymer industry. ^
... Molecular weight and molecular weight distribution were important factors in determining the physical, mechanical, and rheological properties of polymers [10]. The molecular weight controls the mechanical properties of polymers and molecular weight distribution mainly controls the rheological properties [11]. The molecular weight was controlled by H 2 [7], temperature [6], and polymerization time [5], etc, while the molecular weight distribution was controlled by the methods of physical blending of the polymers with different average molecular weight [12], cascade reactor process [13], and mixing or hybrid of different catalysts [14]. ...
Polymerization of propylene was carried out by using MgCl2-supported TiCl4 catalyst in conjunction with triethylaluminium (TEA) as cocatalyst. The effect of polymerization temperature on polymerization
of propylene was investigated. The catalyst activity was influenced by the polymerization temperature significantly and the
maximum activity of the catalyst was obtained at 40 °C. With increasing the polymerization temperature, the molecular weight
of polypropylene (PP) drastically decreased, while the polydispersity index (PDI) increased. The effect of the two-stepwise
polymerization procedure on the molecular weight and molecular weight distribution of PP was studied and the broad PDI of
PP was obtained. It was also found that the PDI of PP could be controlled for propylene polymerization through regulation
of polymerization temperature. Among the whole experimental cases, the M
w of PP was controlled from 14.5 × 104 to 75.2 × 104 g/mol and the PDI could be controlled from 4.7 to 10.2.
Using a new modification of a highly active vanadium-magnesium catalyst (VMC), data were obtained on the effect of hydrogen and hexene-1 content on the catalyst activity and the molecular structure of the resulting polymers. It was found that the formation of polyethylene (PE) with a wide bimodal molecular weight distribution (MWD) on VMC is associated with the presence of two groups of active centers in these catalysts, differing in their reactivity in the polymer chain transfer reaction with hydrogen. It was also found that the presence of hexene-1 during copolymerization leads to an additional broadening of the MWD of the copolymer due to a predominant decrease in the molecular weight of the copolymer in the low-molecular region. At the same time, the active centers of the VMC, producing a high-molecular polymer, practically do not participate in the chain transfer reaction with hexene-1. At the same time, these centers are more reactive in the reaction of insertion of hexene-1, which leads to an increased content of butyl branches in the high-molecular fraction of the copolymers. The kinetic features of highly active VMCs found indicate their promise for the production of pipe and film grades of PE using a one-reactor scheme instead of a two-reactor scheme used to produce bimodal PE on traditional titanium-magnesium catalysts.
Comparative data on the molecular weight distribution of polymers obtained by polymerization of ethylene, propylene and 1‐hexene, and copolymerization of ethylene with α‐olefins over the titanium‐magnesium catalysts (TMC) in the absence and presence of hydrogen are presented. In contrast to the ethylene polymerization, in the cases of propylene and 1‐hexene polymerization and copolymerization of ethylene with α‐olefins, the hydrogen addition is characterized by noticeable narrowing of the molecular weight distribution (MWD) due to lower contribution of the MWD component with high molecular weight. This result is an evidence of the increased reactivity of TMC active sites producing high molecular weight poly‐α‐olefins and copolymers of ethylene with α‐olefins in the chain transfer reaction with hydrogen. It is suggested that the increased reactivity of these sites in the transfer reaction with hydrogen appears after the 2,1‐addition of α‐olefin to the growing polymer chain.
Catalysts play a pivotal role in the olefin polymerization process. After decades of development, a growing number of transition metal catalysts have been widely used in the field of olefin polymerization. Among them, the performance of catalysts such as nickel, titanium, vanadium and chromium are the most attractive. During catalysis process, the chemical valence of the metal in the catalysts usually changed and formed active center with unpaired electron at their outmost electron orbital. Electron Paramagnetic Resonance (EPR) is a good means to detect unpaired electron and free radicals and avoid distraction of other species such as co-catalyst, solvent, monomer etc. This paper reviews the recent progress in the preparation of branched polyethylene with nickel, titanium, vanadium and chromium catalytic systems and EPR analysis of related catalytic systems.
The width and shape of molecular weight distributions can significantly affect the properties of polymeric materials and thus are key paramaters to control. This mini-review aims to critically summarise recent approaches developed to tailor molecular weight distributions and highlights the strengths and limitations of each technique. Special emphasis will also be given to applications where tuning the molecular weight distribution has been used as a strategy to not only enhance polymer properties but also to increase the fundamental understanding behind complex mechanisms and phenomena.
High melt flow rate (MFR) of polypropylene can improve its processability and its high isotacticity is favor of keeping good mechanical properties of polypropylene. In this study, as a convenient and effective method, the mixture of phosphate and alkoxysilane as mixed external donors for Ziegler-Natta catalyst is used to improve MFR of polypropylene. The MFR of polypropylenes prepared by mixed external donors is three to five time higher than that prepared by dicyclopentyl dimethoxysilane (Donor-D) alone, which is attributed to broad molecular weight distribution and low molecular weight of polypropylene prepared by mixed external donors. In addition, isotacticity and lamella thickness of polypropylene prepared by mixed external donors containing triisopropyl phosphate or phenyl-phosphonic acid dipentyl ester are very close to that prepared by Donor-D, which lead to keep high thermal properties and good mechanical properties for the polypropylenes. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44704.
Five new alkoxysilanes with different sizes of hydrocarbon substituents were first synthesized and employed as external donors for propylene polymerization with a Ziegler–Natta catalyst. The effects of these and industrial alkoxysilanes with different sizes of hydrocarbon substituents on the microstructure of polypropylene were studied by SSA and 13C NMR. The results showed that isotactic sequence length of polypropylene increased with the size of R2 substituent on alkoxysilanes in the five donor systems, similar to the regularity of molecular weight, isotacticity, and thermal properties of polypropylene. Although the polypropylene produced by double cycloalkane substituents on alkoxysilanes had lower lamellae thickness L1, its contents were the highest. Moreover, excess large volume of hydrocarbon substituents on alkoxysilanes might be detrimental to improving isotactic sequence length of polypropylene. Polypropylene prepared by MIPDMS had a more uniform distribution of the stereodefects, indicating that MIPDMS might be used for the industrial production of BOPP.
A dynamic mathematical model for the copolymerization of olefins with Ziegler-Natta and metallocene catalysts in a series of continuous stirred-tank slurry-reactors was developed. The model accounts for the presence of multiple active site types and can be used to predict reactor productivity, molecular weight and copolymer composition averages.
This study focused on elucidation of effect of fumed silica added in the commercial Ziegler-Natta catalysts by physical solid–solid stirring. The added fumed silica seemed to cover around the catalyst revealed by SEM-EDX and apparently resulted in decreased activity of ethylene polymerization, but increased molecular weight of polyethylene as measured by GPC. Based on the in situ ESR measurement, it can be concluded that fumed silica can inhibit the reduction of Ti3+ by triethylaluminum (TEA). It is suggested that the delay reduction of Ti3+ may be beneficial for propylene polymerization.
A fragmentable support material for Ziegler–Natta catalysts is presented based on micrometer-sized aggregates of polystyrene nanoparticles. Hydroxyl anchoring groups are introduced by copolymerization of hydroxymethylstyrene in emulsion process to immobilize the catalysts. The catalytic activity in ethylene slurry polymerizations is found to be directly correlated to the hydroxyl group content of the supports. Furthermore, the fragmentation behavior of dye-labeled support aggregates into the initial nanoparticles is demonstrated using laser scanning confocal fluorescence microscopy as a nondestructive method. These supported catalysts fulfill two important design criteria, high fragmentability and high catalyst loading, and produce high-density polyethylene with medium molecular weight distributions (MWDs = 3–4). These values lie between those obtained using single-site metallocene-based (narrow MWD < 3) or inorganic supported multi-site Ziegler–Natta-based (broad MWD = 4–12) polymerizations without the need of blending. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014
The data on the effects of polymerization duration, cocatalyst, and monomer concentrations upon ethylene polymerization in the absence of hydrogen, and the effect of an additional chain transfer agent (hydrogen) on the molecular weight (MW), molecular weight distribution (MWD), and content of vinyl terminal groups for polyethylene (PE) produced over the supported titanium-magnesium catalyst (TMC) are obtained. The effects of these parameters on nonuniformity of active sites for different chain transfer reactions are analyzed by deconvolution of the experimental MWD curves into Flory components. It has been shown that the polymer MW grows, the MWD becomes narrower and the content of vinyl terminal groups in PE increases with increasing polymerization duration. It is assumed to occur due to the reduction of the rate of chain transfer with AlEt3 with increasing polymerization duration. The polydispersity of PE is found to rise with increasing AlEt3 concentration and decreasing monomer concentration due to the emergence of additional low molecular weight Flory components. The ratios of the individual rate constants of chain transfer with AlEt3, monomer and hydrogen to the propagation rate constant have been calculated. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
The effect of different polymerization media like n-hexane, cyclohexane, isooctane, n-decane, toluene, varsol, and light normal paraffin (LNP) on the kinetics of the slurry polymerization of ethylene using a highly active Ziegler Natta (ZN) catalyst for synthesizing UHMWPE was studied. Attempts have been made to determine the solubility of ethylene in the above polymerization media in a very basic manner and to correlate same with the process activation energy based on the Arrhenius plots. The ethylene solubility seemed to depend on the number of carbon atoms in the media, besides other parameters like geometry, dipole moment, etc. It is obvious and well understood that the monomer (ethylene) concentration has a direct bearing on the polymerization kinetics, which influenced the activation energy (Ea) besides other parameters like catalyst/cocatalyst concentration, temperature, etc which were kept constant during the study. The role of the catalyst system in controlling the activation energy was also further exemplified by employing a different ZN catalyst system wherein higher activation energy was observed. This was ascribed to restricted activation pathways for the catalyst under the comparable experimental conditions employed. As soon as better activation pathways for the catalyst were enabled the activation energy dropped down remarkably. The Ea for the synthesis of ultra-high molecular weight polyethylene (UHMWPE) using traditional MgCl2 supported Ti catalyst was found to be 5–12 kcal/mol which compared well with the values obtained by other researchers using other similar catalyst systems for different ethylene polymerization processes. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
Research concerning polyolefin synthesis is often focused on the particular catalyst being used, and the importance of the support material is generally not fully appreciated. Only a few common inorganic carriers, such as SiO2 or MgCl2, are typically described in literature, with new developments in support materials rarely found. Acknowledging this lack in fundamental research on support materials, numerous cases in which the catalyst support, particularly organic nanoparticles, plays a critical role in the formation of polyolefins are described. Here, new organic supports for both Ziegler–Natta and metallocene catalysts are described under the same conditions used for inorganic supports. In these cases, similarly high activities can be achieved, while also offering additional features based on designed polymer architectures which inorganic supports cannot provide. These features produce polyolefin fibers and core–shell structures directly from the reactor. Due to the broad synthetic variety, nanoparticles can be optimized for morphology control of the polyolefin products, such as shape and bulk density. The introduction of nucleophilic groups can further improve the binding strength between the catalyst and the support. For the design of new support systems, many of the central concepts found in nanotechnology (e.g., size control, surface properties, or nanoparticle interactions) are extremely important, and it is surprising how much modern nanotechnology has to offer such a mature field as polyolefin synthesis.
Micrometer-sized spherical polyurethane supports with a narrow size distribution and adjustable porosity were synthesized via the nonaqueous emulsion polymerization technique. They can be directly used in metallocene-catalyzed olefin polymerization, preventing complex aggregation procedures that are typical for other organic and inorganic supports based on silica or latex particles. The reasonable catalytic activity and study of fragmentation behavior, using various optical techniques, demonstrates the potential applicability of the supports.
In this paper, we synthesized a highly-dispersed MgCl2 support using the Grignard reagent method, and loaded it with TiCl4 and VCl4 to prepare polyethylene catalysts with titanium and vanadium active centers. The two catalysts demonstrated narrow particle distribution. The polymerization data demonstrated that the two catalysts both had good activity. And GPC and 13C NMR data indicated that polymer produced with vanadium active centers has some good characteristics, a broader molecular weight distribution and higher degree of methyl branching compared with polymer produced by catalysts with titanium active centers. Also, we found that the molecular weight distribution can be controlled by changing the hydrogen pressure in polymerization; because of the dual active sites have different responses to hydrogen pressure. Also, through 13C NMR data, we found that most of the methyl branching occurred in the high molecular weight part. The existence of methyl branching in the high molecular weight part and broader molecular weight distribution of polyethylene can solve the contradiction of mechanical properties and processing properties.
1-Hexene homopolymerization and 1-hexene–ethylene copolymerization with TiCl4/MgCl2–Al(C2H5)3 catalyst were compared to investigate the effect of ethylene on the distribution of active centers. The polymerizations were quenched with cinnamoyl chloride, and the number of active centers ([C*]/[Ti]) was determined by measuring the cinnamoyl group labeled on the propagation chains. Both polymer samples were fractionated into 9–10 fractions according to molecular weight, and [C*]/[Ti] in each fraction was also determined. Adding small amount of ethylene in 1-hexene polymerization markedly increased the number of active centers that produce low molecular weight polymer. This phenomenon agrees with the mechanism suggesting the presence of Ti−CH(CH3)(CH2)3CH3 type dormant sites and their activation by ethylene. The kp value of the newly emerging active centers in the copolymerization system is much lower than that of the homopolymerization system. In the copolymerization system, the active centers producing polymer chains of the second highest molecular weight and isotacticity show the highest kp value, while those producing polymer of the highest molecular weight and isotacticity show only the second highest kp. On the other hand, the active centers producing polymer with lower–middle chain length show the lowest ethylene incorporation rate. These results disclose differences of catalytic properties between the multiple active center types and correlations between their different kinetic parameters, which may lead to new understanding of the active centers and polymerization mechanism.
Silica–magnesium bisupport (SMB) was prepared by a sol–gel method for use as a support for metallocene and metallocene/Ziegler–Natta hybrid catalysts. SMB was treated with methylaluminoxane (MAO) prior to catalyst immobilization. The supported heterogeneous catalysts were applied to the ethylene copolymerization with 1-hexene. The h-copolymer (ethylene–hexene copolymer produced by metallocene/Ziegler–Natta hybrid catalyst) showed two melting points and broad molecular weight distribution. The differences in physical properties between h-copolymer and m-copolymer (ethylene–hexene copolymer produced by metallocene catalyst) could be explained by the differences in chemical composition and side chain distributions of the produced copolymers.
The influence of different aluminum alkyls (diethylaluminum chloride, triethylaluminum, triisobutylaluminum, tri-n-hexylaluminum, tri-n-octylaluminum and isoprenylaluminum) on the ethylene polymerization activity of a laboratory prepared high-activity SiO2/MgCl2-supported TiCl4 catalyst has been investigated. A slurry reactor (decane diluent) was used for measuring rates of polymerization. The average molar mass, the breadth of the molar mass distribution, the polymerization activity, and the shapes of the activity-time profiles, were strongly dependent on the nature of the aluminum alkyl. For several of the cocatalysts used, the catalytic activity approached a constant value after a certain amount of time under reaction conditions. In this constant activity region, a first-order dependence of the polymerization rate on the monomer concentration was found for all of the systems examined. However, the activation energy of the polymerization reaction was found to depend strongly on the type of cocatalyst which was used.
A mathematical model for the isothermal, slurry polymerization of ethylene using solid Ziegler-Natta catalysts is developed, accounting for the effect of gas-liquid mass transfer limitations on the overall rate and polymer properties, such as molecular weight and polydispersity index. The existence of micron-sized catalyst particles in the initial stages of polymerization influences the final polymer properties and also leads to the diffusion and reaction steps being in parallel rather than in series. The gas-liquid interfacial contribution to the polymer formation is shown to significantly enhance the value of the polydispersity index. Higbie's penetration theory has been satisfactorily employed to model the monomer absorption in the presence of growing polymer macroparticles. High polydispersities are shown to arise in the presence of gas-liquid mass transfer limitations, even when only one type of active catalyst site is considered, and when the intra-macroparticular diffusional resistance is low.
Supported titanium–magnesium catalysts (TMC) comprising isolated and clustered titanium ions in different oxidation states, which are obtained using titanium compounds of different composition (TiCl4, TiCl3•nDBE (DBE – dibutyl ether), [η6–BenzeneTiAl2Cl8]), were synthesized and tested in ethylene polymerization. The state of titanium ions was studied by the ESR method both for the procatalysts and after their interaction with triisobutilaluminum. For identification of ESR-silent Ti3+ ions and Ti2+ ions, special procedures of additional catalyst treatment with pyridine, water, and chloropentafluorobenzene were used to obtain Ti3+ ions that are observable in ESR spectra. In distinction to numerous earlier works performed with the TiCl4/MgCl2 catalyst comprising after the interaction with AlR3 the Ti3+ surface compounds both as isolated ions and clusters (ESR-silent), this work considers the [η6–BenzeneTiAl2Cl8]/MgCl2 catalyst (TMC-3) comprising mainly the isolated Ti2+ ions and a new catalyst TMC-4 obtained by treating the TMC-3 with chloropentafluorobenzene. This catalyst comprises only the isolated Ti3+ ions both before and after the interaction with triisobutylaluminum. It was shown that in spite of sharp distinctions between the catalysts under consideration concerning titanium oxidation state and the ratio of isolated Ti3+ ions to clustered ones, all these catalysts produce polyethylenes with similar molecular weights and molecular-weight distributions. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6362–6372, 2009
Me2Si(C5Me4)(N-tBu)TiCl2, (nBuCp)2ZrCl2, and Me2Si(C5Me4)(N-tBu)TiCl2/(nBuCp)2ZrCl2 catalyst systems were successfully immobilized on silica and applied to ethylene/hexene copolymerization. In the presence of 20 mL of hexene and 25 mg of butyloctyl magnesium in 400 mL of isobutane at 40 bar of ethylene, Me2Si(C5Me4)(N-tBu)TiCl2 immobilized catalyst afforded poly(ethylene-co-hexene) with high molecular weight ([η] = 12.41) and high comonomer content (%C6 = 2.8%), while (nBuCp)2ZrCl2-immobilized catalyst afforded polymers with relatively low molecular weight ([η] = 2.58) with low comonomer content (%C6 = 0.9%). Immobilized Me2Si(C5Me4)(N-tBu)TiCl2/(nBuCp)2ZrCl2 hybrid catalyst exhibited high and stable polymerization activity with time, affording polymers with pseudo-bimodal molecular weight distribution and clear inverse comonomer distribution (low comonomer content for low molecular weight polymer fraction and vice versa). The polymerization characteristics and rate profiles suggest that individual catalysts in the hybrid catalyst system are independent of each other. POLYM. ENG. SCI., 47:131–139, 2007. © 2007 Society of Plastics Engineers
In heterogenous olefin polymerization with Ziegler catalysts, the influence of monomer mass transport in the growing granule on polymer properties has been extensively modeled, but it has not been possible to clearly establish the importance of diffusion experimentally since the multisited nature of most Ziegler catalysts can produce similar effects. In this study, ethylene-propylene copolymers were made with single-sited metallocene catalysts by slurry polymerization in liquid monomers. These copolymers had a relatively narrow molecular weight distribution with a composition distribution (CD) broader than expected for a single-site catalyst. Data analysis indicates that mass-transfer limitations in the polymer particles are the most likely explanation for the observed results. For amorphous copolymers, a diffusion/reaction model could predict CD breadth in good agreement with experimental data, but for semicrystalline polymers the model was inaccurate. We postulate that model inadequacies are due to radial gradients in monomer diffusivity during polymerization which the model does not account for.
Living polymerization is of great utility both in understanding the mechanism of chain propagation reactions and in synthesizing well-defined block copolymers. In this review we deal with the development of soluble Ziegler-Natta catalysts for the living coordination polymerization of ethylene and -olefins and discuss the possibility of synthesis of new block copolymers.
The first example of living polyolefin with a uniform chain length was found in the low-temperature polymerization of propylene with the soluble catalyst composed of V (acac)3 and Al(C2H5)2 Cl. The mechanism of the living coordination polymerization is discussed on the basis of the kinetic and stereochemical data. Subsequently, some applications of living polypropylene are introduced to prepare tailormade polymers such as terminally functionalized polymers and block copolymers which exhibit new characteristic properties. Finally, new types of soluble Ziegler-Natta catalysts are briefly surveyed in connection with the synthesis of living polyolefins.
In conclusion, we indicate the future trends of research and development in the field of soluble Ziegler-Natta catalysts for the synthesis of living polyolefins and block copolymers.
In the present article, we review the state-of-the-art models for single particle olefin polymerization models. Special attention is paid to particle growth, polymerization rates, concentration and temperature radial profiles, polymer microstructure, and particle morphology. It is proposed that these models can be conveniently classified as polymer property and particle morphology models, according to their most important predictive abilities, even though particle morphology characteristics will influence polymer properties and vice versa. Currently, there is no single model that incorporates all of these modelling aspects into a unified formulation. A great deal of progress has been made toward understanding the relationship between the important phenomena involved in modelling single particle growth. It seems that the basic understanding on modelling polymer properties is quite substantial, but there is still significant contributions to be made when modelling the morphology evolution of these complex polymer particles. Some of these areas are discussed and suggestions for future development are made.
Polyolefins made with Ziegler–Natta catalysts have non‐uniform distributions of molecular weight (MWD) and chemical composition (CCD). The MWD is usually measured by high‐temperature gel permeation chromatography (GPC) and the CCD by either temperature rising elution fractionation (TREF) or crystallization analysis fractionation (CRYSTAF). A mathematical model is needed to quantify the information provided by these analytical techniques and to relate it to the presence of multiple site types on Ziegler–Natta catalysts. We developed a robust computer algorithm to deconvolute the MWD and CCD of polyolefins simultaneously using Flory's most probable distribution and the cumulative CCD component of Stockmayer's distribution, which includes the soluble fraction commonly present in linear low‐density polyethylene (LLDPE) resins and have applied this procedure for the first time to several industrial LLDPE resins. The deconvolution results are reproducible and consistent with theoretical expectations.
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Specialist Periodical Reports provide systematic and detailed review coverage of progress in the major areas of chemical research. Written by experts in their specialist fields the series creates a unique service for the active research chemist, supplying regular critical in-depth accounts of progress in particular areas of chemistry. For over 80 years the Royal Society of Chemistry and its predecessor, the Chemical Society, have been publishing reports charting developments in chemistry, which originally took the form of Annual Reports. However, by 1967 the whole spectrum of chemistry could no longer be contained within one volume and the series Specialist Periodical Reports was born. The Annual Reports themselves still existed but were divided into two, and subsequently three, volumes covering Inorganic, Organic and Physical Chemistry. For more general coverage of the highlights in chemistry they remain a 'must'. Since that time the SPR series has altered according to the fluctuating degree of activity in various fields of chemistry. Some titles have remained unchanged, while others have altered their emphasis along with their titles; some have been combined under a new name whereas others have had to be discontinued. The current list of Specialist Periodical Reports can be seen on the inside flap of this volume.
Over two decades ago, !he term characterisation covered just those techniques which measured the properties of polymers in solution in order to determine molecular weight and size. The discoveries of stereoregular polymers and polymer crystals created the need for new and advanced techniques for characterising chain structures and bulk properties. Further demands for new and improved characterisation methods for bulk polymers have resulted from the recent development and exploitation of multi phase polymeric systems, such as polymer blends, block and graft copolymers, and polymer composites. Today, therefore, characterisation is a very important part of polymer science. The polymer chemist must know the chain length, chain microstructure and chain conformation of the polymers he or she has prepared, i. e. the determination of molecular properties. The scientist involved in exploiting polymers in such applications as plastics, elastomers, fibres, surface coatings and adhesives must be informed on the morphology and physical and mechanical behaviour of his or her products, i. e. the determination of bulk and surface properties and their dependence on molecular properties. The techniques required for these determinations now cover an extremely wide field. Our aim has been to review a number of techniques critically and in sufficient depth so that the present state and future potential of each technique may be judged by the reader. Three criteria were used in the selection of techniques. First, we wished to present new methods which have been developed actively in the polymer field during the past five years.
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This chapter examines the relationship between the structure of organometallic compounds and their ability to polymerize vinyl monomers and olefins. The chapter identifies methods of synthesis for polymerization catalysts that are more selective and more active than the systems currently available. The π-complexes are the best known and are among the most stable transition metal alkyl compounds and include U (πC8H8)2, Cr (πC6H6)2, and Ti( πC5H5)2. In these compounds, the bonding electrons are delocalized over all the carbon atoms in the ring giving a symmetrical molecule in which the metal–carbon bond distances are identical. Wilke's π-ally1 compounds are members of this class. In this molecule, each allyl group occupies the transposition with respect to each other, all three carbon atoms of the allyl group are equidistant from the metal atom, and each allyl group formally occupies two coordinate positions.
This chapter discusses the scope of Ziegler catalysis; stereoselectivity, kinetics, and mechanism of Ziegler catalysis; homogeneous Ziegler–Natta catalysts; and side reactions in homogeneous catalysts. Ziegler catalysis involves rapid polymerization of ethylene and α-ole-fins with the aid of catalysts based on transition-element compounds, normally formed by the reaction of a transition-element halide or alkoxide or alkyl or aryl derivative with a main-group element alkyl or alkyl halide. There are thousands of patents involving every combination of pure or mixed main-group alkyls with transition-element compounds, each claiming advantages. The result of the early work led to the development of “second-generation” Ziegler catalysts. Polymers produced with unmodified Ziegler catalysts showed extremely high molecular weight and broad distribution, and in some cases, there was evidence for “living polymer.” All homogeneous catalyst systems for ethylene polymerization become heterogeneous when polyethylene is formed. On using vanadium-based homogeneous catalysts, polymers consisting of syndiotactic stereo blocks and stereo-irregular blocks are obtained. Very high stereoselectivity is observed for racemic 4-methyl- 1-hexene and racemic 3,7-dimethyl-1-octene, where the asymmetric carbon atom is in the α-position relative to the double bond. Stereoselectivity is caused by the chirality of the catalytically active center, and not by chiral atoms in the growing chain. It must be concluded from the results that reactions take place, which change the number of active sites present, due to the different behavior of the polymers in solution. Study of these new catalysts is intensive. After a short induction period, the activity of polymerization increases as a function of the monomer concentration.
The kinetics of the ethylene polymerization catalyzed by the Cp2TiEtCl/AlEtCl2 system in toluene were investigated. Hydrolysis of AlEtCl2 in benzene was studied and the resulting oxyaluminium compounds were used as cocatalysts together with Cp2TiEtCl as catalyst for ethylene polymerization. A strong enhancement of the cocatalytic activity for the formation of oxyaluminium compounds was observed, accompanied by an increase of molecular weight. The catalyst efficiencies were found to be rather high in both the Cp2TiEtCl/AlEtCl2 and the Cp2TiEtCl/(AlEtCl2 + H2O) system. Transfer reactions are suppressed under the applied experimental conditions, but termination reactions, leading to non-stationary kinetic patterns, may be assumed for both systems.
Ethylene polymerization, catalyzed by Cp2TiEtCl or Cp2TiMeCl and cocatalyzed by oxyaluminium compounds, formed via hydrolysis of AlEtCl2, AtEt2Cl, AlEt3, AlMeCl2, AlMe2Cl, AlMe3, and AlEt(OBu)Cl, was studied. The cocatalytic activity of oxyaluminium compounds was found to decrease with increasing number of alkyl substituents in the corresponding alanes. The only inactive cocatalyst from the oxyaluminium compounds studied is the one formed via hydrolysis of AlEt(OBu)Cl. Oxyaluminium compounds from methylalanes are substantially less active than those from ethylalanes. If the same aloxane cocatalyst is employed, the catalytic activities of both Cp2TiEtCl and Cp2TiMeCl are similar. The molecular weight distributions of polyethylene are narrow for the systems Cp2TiRCl/(oxyaluminium compounds from AlEtCl2); fairly broader distributions were found for systems Cp2TiRCl/(oxyaluminium compounds from AlMeCl2). Systems with oxyaluminium compounds from dialkyl- or trialkylalanes yield polyethylene with distinctly bimodal distribution curves. The mechanism of ethylene polymerization catalyzed by titanocene systems cocatalyzed by oxyaluminium compounds is discussed.
When for a given polymer a weight distribution formula is known or may be supposed, the experimental data obtained from its fractionation and from the viscometry of its fractions can be plotted, with a convenient choice of the coordinate system, in diagrams constituted by straight lines (or by lines tending toward linearity) as long as the relationship between the log of the intrinsic viscosities and the log of the degrees of polymerization of homogeneous fractions of the same polymer is also linear (or tending toward linearity). The straight-line diagrams found by Wesslau in the case of low pressure polyethylene are a particular case of this kind of plot. The Wesslau, the exponential, and the Beasley distribution formulas have been considered, and curves and tabulated values are given to be used when the fractionation and viscometry data of polymers having these distributions are to be expressed in those coordinate systems in which they are represented by straight lines (or by lines tending toward linearity). Certin analytical features of the Wesslau distribution, compared with the other distributions, may suggest some considerations about the mechanism of synthesis of polymers similar, from this point of view, to low pressure polyethylene.
The molecular weight distribution of several polypropylene samples was studied by the column elution method. With the exception of a lower molecular weight sample, the molecular weight distribution curves were found to be well described by a revised log-normal function presented by DAVIS et al. The molecular weights Mw and Mn observed by direct measurements for the whole polymers were in good agreement with the corresponding ones calculated from their molecular weight distribution curves according to this function. Moreover, no anomalous relation was found between the solution viscosity and the molecular weight for several unfractionated polymers and their high molecular weight fractions. These results reveal that there exists no long chain branch even in the high molecular weight region, and that the approximation of the distribution pattern by this revised lognormal function sufficiently characterizes polypropylene samples. The values of Mw/Mn of all samples studied were within a narrow range from 6 to 9, and no distinct dependency of the molecular weight distribution on the average molecular weight was observed.
A method is described which allows the individual characterization (Mw, Mn) of the two ethylene polymers formed simultaneously on Ti(III) and Ti(IV) sites of a Ziegler catalyst system. The method is based on the evaluation of the CH3 or CHD group distributions in fractionated polymers prepared in the absence or the presence of deuterium, respectively.Eine Methode wird beschrieben, die eine individuelle Charakterisierung (MW, Mn) der beiden Äthylenpolymeren erlaubt, welche gleichzeitig an Ti(III)- und Ti(IV)-Zentren eines Ziegler-Katalysatorsystems gebildet werden. Die Methode basiert auf der Auswertung der Verteilung von CH3- oder CHD-Gruppen in fraktionierten Polymeren, welche in Abwesenheit bzw. in Gegenwart von Deuterium hergestellt wurden.
The polymerisation of ethylene was carried out in the presence of (C5H5)2TiCl2/(C2H5)2AlCl in benzene (I) and of TiCl4/(C2H5)AlCl2 in chlorobenzene (II). In the initial state of the polymerisation with I oligomers with a narrow molecular weight distribution are formed. According to their IR and NMR spectra they were identified as predominantly saturated linear hydrocarbons. The effect of a substitution of the cyclopentadienyl rings by R = CH3 and C2H5 was studied as well as the influence of aging particularly on the molecular weight distribution. The catalytic system II yields predominantly unsaturated branched oligomers with broad molecular weight distribution. The ligand effect on the structure and on the molecular weight of the polymers obtained is discussed.Äthylen wurde in Gegenwart von (C5H5)2TiCl2/(C2H5)2AlCl in Benzol (I) und von TiCl4/(C2H5)AlCl2 in Chlorbenzol (II) polymerisiert. Im Anfangsbereich der Polymerisation werden mit dem Katalysator I Oligomere mit enger Molekulargewichtsverteilung erhalten. Aufgrund ihrer IR- und NMR-Spektren handelt es sich dabei um gesättigte lineare Kohlenwasserstoffe. Der Effekt einer Substitution der Cyclopentadienylringe mit R = CH3 und C2H5 sowie der Einfluß der Alterung des Katalysators wurde insbesondere in bezug auf die Molekulargewichtsverteilung der erhaltenen Oligomeren untersucht.Das Katalysatorsystem II liefert vorwiegend ungesättigte verzweigte Oligomere mit breiter Molekulargewichtsverteilung. Der Einfluß der Liganden im Katalysatorkomplex auf die Struktur und auf das Molekulargewicht der Polymeren wird diskutiert.
Vanadium tetrachloride combined with alkylaluminium could be shown to produce two types of polypropylenes, the syndiotactic and the isotactic ones, depending on both the kind of alkylaluminium component and polymerization temperature. It was found that the syndiotactic polypropylenes always have narrow molecular weight distributions (1,5≤M̄w/M̄n≤2,5), whereas the isotactic polypropylenes always have wide distributions (16≤M̄w/M̄n≤19), suggesting that the isotactic polymerizations are not induced by homogeneous but by heterogeneous catalysis. With the aid of ESR analysis for catalyst systems it was concluded that the syndiotactic polymerizations take place in a soluble complex of trivalent vanadium, whereas the isotactic polymerizations proceed on the insoluble vanadium dichlorides formed from the catalyst mixture.
It is shown that the compositional inhomogeneity of ethylene-propylene copolymers obtained with the given systems is explained by the presence in the latter of both soluble and insoluble complexes, with different activities in copolymerization of ethylene and propylene. Copolymers with a narrow compositional distribution are formed from the soluble catalyst, and copolymers with a broad distribution from the insoluble catalyst. Further inhomogeneity arises as a result of precipitation from solution of the part of the copolymer richer in ethylene and subsequent chain propagation within this copolymer in bulk. When the complex system is prepared beforehand in the absence of monomers, at Al:V ratios of 9:1 and 18:1, copolymers of homogeneous composition are formed, because almost all of the catalyst is soluble in the hydrocarbons.
MWD of linear PE samples synthesized using various catalytic systems was determined. Correlation were derived between parameters of MWD and basic characteristic properties of melts (spin-spin relaxation time, index characterizing the extent to which the reduction of cross-sectional magnetization differs from the calculated exponential value with relaxation time, melt index and wetting index), which enable MWD parameters of linear PE to be determined without solution.
In polymerization of ethylene in the presence of alkoxy-(aroxy)titanium trichloride-tri-isobutylaluminium catalytic systems, the introduction of the RO-groups in the Ti4+ compound has the effect of causing rapid stabilization of the active centres and the appearance of a long period of polymerization, at a constant rate with the simple kinetic relationship wpol = kp·[catalyst]·[C2H4]. The catalytic activity of the systems is dependent on the nature of the RO substitunt in the ROTiCl3: : p-BrC6H4O>C6H5O>C12H25O>C4H9O. The ROTiCl3R3Al catalytic systems have a number of characteristic properties, from which the conclusion may be drawn that the RO groups function as “micro-carriers” of active centres.
This paper deals with molecular weight regulation and molecular weight distribution in ethylene polymerization, using a highly active Ziegler-Natta catalyst. The molecular weight regulation can be described by equations derived in a previous paper. The development of the molecular weight distribution with reaction time showed that there must exist active sites with an overall propagation rate constant of at least 2.9 × 103 dm3/mol sec at 85°C. This value is higher by a factor of approximately 40 than the value determined by kinetic experiments.
The stereospecificity of the polymerization of propene under the influence of α-TiCl3 and alkyl aluminum compounds is explained. The explanation rests exclusively on quantum chemical principles and the crystal chemistry of transition metal trichloride layer structures, subjects described in two preceding papers.The active centers formed by alkylation of titanium ions at those faces of the surface where chlorine vacancies are available are shown to possess the asymmetry necessary for the appropriate orientation of the α olefin in the π complex with the exposed titanium.The nonequivalence of the crystallographic sites of vacancy and alkyl group at the active center ensures that the growing alkyl group will move back to its original position after each incorporation of a new monomer.Thus the polymerization consists of a sequence of sterically identical steps which leads to isotacticity of the product.
A fractional poisoning method has shown that all groups of Ziegler catalyst centres, both stereospecific and non-stereospecific, have a similar distribution of propagation rate constants. The most regular and highly molecular isotactic fraction is quite uniform by stereoregularity. Most of the poison-resistable part of active centres in δTiCl3-AlEt3 systems have properties similar to those of initial system TiCl3-AlEt2Cl, which may be caused by different adsorption abilities of co-catalysts AlEt3 and AlEt2Cl at TiCl3 surface. Values np and have been estimated for δTiCl3(TAC)-AlEt3 system.
Experimental results obtained when studying the kinetics of ethylene polymerization in toluene, as well as data on fractionating polyethylene, are analysed and compared with the results of theoretical consideration of molecular weight distribution on the basis of the proposed kinetic scheme.The constants of the reactions of propagation, initiation, transfer and termination of polymer chains are evaluated and compared with literature data.
A novel method for determining the polymerization mechanism and the kinetic rate constants from the molecular weight distribution is proposed. The particular criterion function used as basis for parameter adjustment is
where θ is the vector of dependent variables, y(r, θ) is the theoretical molecular weight distribution for the assumed polymerization mechanism, and yE(r) is the experimental molecular weight distribution which is a function of the chain length r. A form of the gradient method of optimization was used to solve the criterion function. The proposed method is particularly powerful since the whole molecular weight distribution is utilized.
Es wurde das Trennvermögen einer einfachen Kolonnenfraktionierung ohne Temperaturgradienten untersucht. Polyäthylen (Marlex 6002) mit einemM
w
/M
n
-Verhältnis von 11,3 wurde in sechs Fraktionen aufgetrennt, deren Verteilungsbreiten durch Refraktionierung bestimmt wurden. Für die mittleren Fraktionen wurdenM
w
/M
n
-Werte unterhalb 1,2 erhalten, wogegen die erste und die beiden letzten Fraktionen etwas breitere Verteilungen aufwiesen.
The addition of alkylchloroaluminium derivatives to ZrR4(R = allyl or benzyl) produces active catalysts for the preparation of C4–C20α-olefins from ethylene.
The chemisorption of (CH3)2AlCl on the crystal surfaces of the α- and β-modifications of TiCl3 have been calculated by the “atom-atom” potentials method. The reactive centres were found to have the same composition for 80% α-TiCl3 and 30% β-TiCl3; their amounts diminished when the temperature was raised. These results agree with those given in the literature about the yields of stereospecific α-olefin polymers over the catalytic systems containing the α- and β-forms of TiCl3.
In this paper a comparison is made of the molecular weights and molecular structure of polyethylene obtained by polymerization with catalysts based on titanium chlorides, deposited on a carrier or in bulk. The effect of polymerization conditions on the molecular weight of polyethylene prepared by polymerization with a highly active titanium-magnesium catalyst comprising titanium tetrachloride deposted on magnesium chloride, is investigated, and the rate constants of the chain transfer reactions are calculated from the results obtained.
Dependences of the molecular weight distribution and stereochemical regulation of the polypropylenes produced with VCl4–AlEt2Cl catalyst on the polymerization temperature were examined. The molecular weight distributions of the polymers obtained at temperatures below −40°C were unimodal and narrow (Mw/Mn ≤ 2). The molecular weight distributions obtained at higher temperatures (above −21°C) were bimodal with one narrow distribution and one wide one (Mw/Mn > 2), and the polymer fraction of the wide distribution increased with the polymerization temperature. The fractional amount of (CH2)2 groups in the polymers, which corresponds to tail-to-tail linkage of two propylene units, increased to a maximum at −21°C followed by a gradual decrease with the polymerization temperature. The production of isotactic polymers was confirmed at temperatures above −21°C. From these data, it is concluded that only the homogeneous form of the catalyst system is responsible for the polymerization at temperatures below about −21°C while the heterogeneous form appears and catalyzes the polymerization together with the homogeneous one at temperatures above −21°C.
Soluble ethylene polymerization catalysts derived from (π-C5H5)2Ti(R)Cl and R ′AlCl2, where R = methyl or phenyl and R ′ = methyl or ethyl, were studied both by polymerization kinetics at 0°C and by diagnostic experiments. It was found that the first insertion of ethylene into the TiR bond is difficult when R = methyl or phenyl, and for this reason these catalysts show a different overall behavior than when R = ethyl or higher alkyl.
Procedures and an apparatus for the elution fractionation of polypropylenes are described. The isotactic polymer is precipitated onto a sand column from p-xylene solution by cooling slowly from 126°C. to room temperature. After removal of the solvent and the atactic fraction soluble at room temperature, the column is heated to 156°C. Fractions are eluted with mixtures of a high boiling hydrocarbon fraction as solvent and 10% ethylene glycol in 2-butoxyethanol as nonsolvent. The dissolving power of the eluents is periodically increased to fractionate the polymer. Upward solvent flow is used. Recovery is usually quantitative, and good reproducibility is demonstrated. Polymer degradation during fractionation has been reduced to generally less than 5% by suitable techniques. Molecular weight distribution curves for three samples are shown. Good selectivity is established by the fact that the ratio of the molecular weight of the highest to that of the lowest fraction is usually in the range of 50 to 200. A description of the fractionation of atactic polypropylene is given also. The atactic material was used to show the effect of depositing the polymer selectively or randomly on the surface of the support as well as the effect of diffusion on fractionation efficiency.
The influence of preparative conditions on the molecular weight and stereoregularity distributions of polypropylene was investigated. The stereoregularity distribution is narrowed by using a highly stereospecific catalyst, by decreasing the polymerization temperature, and for the three-component catalyst by keeping the mole proportion of the electron-donating third component at 0.5. The molecular weight distribution can be narrowed by using a highly stereospecific catalyst, a high monomer concentration, and a high polymerization temperature, and by having a lower conversion, particularly at low monomer concentration. The possibility of long-chain branching in polypropylene was indicated by data from the fractionation of tritium-labeled polymers.
thylen reagiert bereits bei −70°C mit den Methyltitan(IV)‐verbindungen CH 3 TiCl 3 , CH 3 Ti(O‐ iso ‐C 3 H 7 )Cl 2 und CH 3 Ti(O‐ tert ‐C 4 H 9 )Cl 2 zu Alkyltitan(IV)‐verbindungen mit Alkylresten von C 3 bis C 33 . Beim CH 3 TiCl 3 ist diese Aufbaureaktion noch von einer Verdrängungsreaktion begleitet, die α‐Olefine liefert.
Systeme mit Methyltitan(III)‐verbindungen (durch Reduktion der entsprechenden Titan(IV)‐verbindungen mit Hg[Si(CH 3 ) 3 ] 2 erstmalig erhalten) polymerisieren Äthylen dagegen unter den gleichen Bedingungen, selbst in kleinen Katalysatorkonzentrationen, rasch zu hochmolekularen Polyäthylenen.
The soluble V(acac)3-Al(C2H5)2Cl system has been found to polymerize propene to give a syndiotactic "living" polymer having narrow molecular weight distribution (M̄w/M̄n = 1.05-1.20). A kinetic study proved that the polymerization of propene proceeds without any detectable chain termination and transfer reactions at temperatures below -65 °C. The stereoregulation energy for the syndiotactic specific chain propagation was evaluated by 13C NMR analysis of the polymers produced.
A mechanism for the polymerization of olefins is proposed. An essentially octahedrally coordinated ion of a transition element with empty or nearly empty t2g orbitals carrying in its coordination sphere one alkyl group and having one vacant octahedral position is supposed to be the active center. In this model the monomeric olefin is coordinated in the vacant position through a “π bond.”It is shown that theoretically one may expect the transition metal-to-carbon bond to become more susceptible to radical breaking at the very moment the π bond between metal ion and olefin is formed. Based on this theoretical argument, the proposed mechanism: involves to a first approximation an electronic rearrangement and only small nuclear displacements. This mechanism may account for the low activation energy of the propagation step.It further relates catalytic activity to the ionization potential for a d electron of the transition metal and the nature of the negative ions used in the transition metal compound.It also provides a basis for the explanation of the formation of isotactic polymers when special solid catalyst systems are used.