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Polypropylene: 44 Years Young! The Challenge for the 21st Century
Abstract
Polypropylene is one of the most versatile and successful materials in the market because of the ever-increasing spectrum of polymer composition and properties which have originated from continuing breakthroughs in catalyst and process technology. Industrial polypropylene production is based on Ziegler-Natta supported catalysts. The success of MgCl2-supported catalysts is also due to the development of spherical catalysts with controlled particle size and porosity, used in bulk liquid monomer and gas-phase processes for production of a broad range of homo- and copolymers and multiphase polymer alloys via “Reactor Granule” technology. The most recent development is the discovery of MgCl2/TiCl4/diether catalysts which can give PP yields twice as high as those obtained with previous catalysts. The achievement of a better control of the polymer structure has resulted in an increased capability to tailor the products according to the performance requirements, both in the conversion technologies and in the application life cycle, as well as to extend their application to new market areas.
A new approach is demonstrated in which an Artificial Neural Network (ANN) was trained with first-principles data to predict the chain length, polydispersity (Đ) and adiabatic temperature for a zirconocene-catalyzed polymerization reaction. The ANN-generated data shows good agreement with the theoretical results, with an overall R2 of 0.9987. Using its significantly enhanced computational speed, the ANN was used to analyze the reaction space, providing insights into trends seen in molecular weight and Đ with various combinations of kinetic parameters, particularly pointing out regions of desirable and undesirable operation. The network was trained in reverse and used to generate reaction rate constants from chain length and Đ, enabling a new form of kinetic deduction for polymerization reactions. This training was used to derive potential rate constants for different catalysts reported in the literature. Overall, this data indicates that ANNs are a plausible tool for analyzing data from complex metallocene-catalyzed olefin polymerizations.
Polypropylene (PP) was compounded with submicron size silica filler particles (microsilica, μSi) up to 30 wt-%. In addition, three external compatibilizers, with characteristic functionalities, were studied to examine their influence in the mechanical properties of the PP/μSi composites. As a result, the modulus of the composite increased while the other tensile values deteriorated in correlation with increased filler concentration. The addition of an external compatibilizer reduced this deterioration, but the reduction was dependent on the type of the compatibilizer used. The influence of an acid functionalized compatibilizer was unsubstantial while the fluorosilane and the Lewis acidic phenylsilane functionalized polypropylenes acted as effective compatibilizers. In addition to examining the tensile properties, the toughness of the composites was evaluated as well. The microsilica filler was found to act as toughening agent since the Brittle-to-Ductile transition point of the composite increased by 2-3 orders of magnitude at high filler concentrations. However, this increase in the toughness was rapidly lost when an effective compatibilizer was used to bind the filler with the matrix. This observation was consistent with the common understanding of the filler toughening mechanism, where particle-matrix debonding is a prerequisite for facilitating the plastic stretch of the polymer ligaments between filler particles. In our case, however, the few filler aggregates in the polymer matrix also played a crucial role. While in uncompatibilized composites the filler aggregates remained passive (could not be seen at the fracture surface), the addition of an effective compatibilizer activated these aggregates to promote crack initiation and/or propagation. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers
The triple self-condensation of [2](4,7)indano[2]paracyclophan-5-one and 5-oxo-4,5,6,7-tetrahydro[2]paracyclo[2](1,4)naphthalenophane, as representative examples of fused cycloalkanonylparacyclophanes, promoted by titanium tetrachloride and triethylamine is here reported.
The possibility of using Rn
P(O)(CH2OR′)3—n
(R = alkyl, R′ = methyl or acyl, n = 0–2) polydentate phosphine oxides as external electron donors for the titanium-magnesium catalysts for isotactic polypropylene synthesis is demonstrated for the first time. The kinetics of propylene polymerization in liquid monomer at 70°C and the isotacticity and molecular-weight characteristics of the resulting polypropylene are studied as functions of the nature of the substituents at the phosphorus atoms in the external donor and the molar ratio of the cocatalyst AlEt3 to the external electron donor. Among the compounds examined, isoamyldi(methoxymethyl)phosphine oxide (R = iso-Am, R′ = Me, n = 1) is the most efficient. The isotacticity index of the polypropylene (PP) synthesized on the titanium-magnesium catalyst with this external donor is as high as 94–95%, and the activity of the catalyst (Cat) in the absence of hydrogen is 5.0–6.5 (kg PP) (g Cat)−1 h−1. With the optimum combination, the activity of this catalyst is ≈5 (kg PP) (g Cat)−1 h−1 and the isotacticity index is 94%. These parameters are close to those obtained for propylene polymerization in the absence of hydrogen on the same titanium-magnesium catalyst with phenyltriethoxysilane (external donor used in the industrial synthesis of PP): the activity is 5.6 (kg PP) (g Cat)−1 h−1, and the isotacticity index is 95%. The introduction of hydrogen into the reaction zone makes it possible to efficiently control the molecular weight of PP, increases the catalyst activity by a factor of 1.5–2.5, and somewhat decreases the isotacticity index (from 94 to 91–92%).
The progresses obtained in both catalyst performances and process technologies have made the continuous expansion of PP properties possible. The discovery of MgCl2-supported catalysts and the development of their controlled spherical morphology produced a real revolution in the catalytic-system performances. A series of definitive improvements in catalyst chemistry and an impressive increase in polymer productivity were allowed by the total control of the “replication” phenomenon of polymer generated by the parent catalyst. The Reactor Granule Technology started up with relevant consequences in terms of development of new polymer properties for new applications and new markets, and of new economic and versatile process technologies with the best environmental impact. Through this technology, it is also possible to use a mixed catalysis combining the heterogeneous Ziegler-Natta catalysts with homogeneous catalysts such as radicalic initiators, metallocenes and single site, and this possibility results in a dramatic development of the material potentials and properties. The recently conceived “Multizone Circulating Reactor Technology” represents the ultimate frontier in the polymeric materials science. It will allow the uniform phase homogenization of dissimilar materials generating novel polymeric alloys with an enormously expanded property/performance envelope.
The nickel complexes are of special relevance to catalysis for ethylene oligomerization and polymerization. Beyond the famous
commercial SHOP process for ethylene oligomerization, among recent progress of nickel catalysts, various nickel complexes
containing different ligands such as the bidentate and tridentate ligands are of interest. In contrast to the importance of
hetereogeneous catalysis, the homogeneous catalyst is a small share for polyolefins, while the well-defined complexes affect
the microstructure of the resultant polyolefin. The nickel catalysts often perform ethylene activations for inner olefins
and the branched polyethylene with broad or bimodal molecular weight distribution. The catalytic behavior will be affected
by adaptation of ligands coordinating around the nickel center. In addition, the auxiliary ligand Ph3P can improve the catalytic activity by one order of magnitude, and its active center can be confirmed through isolating and
characterizing the reliable intermediate.
Metallocene-catalyzed polymerization of olefins and diolefins, including styrene and selected polar vinyl monomers, is an important new technology that in recent years has started to make significant inroads into the polyolefin marketplace. Metallocenes have broadened our knowledge of the mechanistic understanding of ZieglerNatta catalysis, stereospecific polymerization, and termination reactions. In contrast to classic ZieglerNatta catalysis, the polymerization takes place at a defined transition metal center, which allows precise control of the monomer insertion and other reaction steps. Many titanocenes, zirconocenes, and hafnocenes with various symmetries have been synthesized and found to give tailored polymers of totally different structures. Methylalumoxane or other bulky cocatalysts are the main causes for the high activities of the metallocene catalysts. Single-site catalysts have the capability of permitting the user to control polymer tacticity, molecular weight, and molecular weight distribution more efficiently than in the
The polyolefin industry is increasingly confronted with the challenge to meet restricting environmental regulations. In parallel, the environmentally conscious public opinion (customer!), now globally organized, demands even more stringent anticipative actions to prevent any short or long term damages inflicted upon environment. The gradual shift from high pressure high temperature radical polymerisation for production of polyethylene to silica supported CrO3< based Phillips catalysts and several generations of stepwise improved TiCl3 based Ziegler-Natta catalyst technology has led to highly efficient catalytic system accomplishing substantial improvement with respect to environmental issues. The development and progress of the last decade in single-site metallocene catalyst technology finally indicate that the advancing polyolefin industry has moved, in anticipation, towards even more modern technologies meeting the ultimate goal of employing clean processes that provide environmentally green products. (C) 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.
The historic development of polypropylene from its original discovery to the present is summarized, the economic impact of Natta's discovery is described and the main reasons for the exceptional development of polypropylene are given. The catalyst played an essential role in the successful development of the product; it is important that the principal research efforts were directed towards the study of the catalysts to achieve optimal product properties and process. On the basis of the possibilities of improving even further the process and developing new applicative opportunities for the product a very bright forecast for polypropylene is made.
In our research group, we have been investigating the mechanisms presi ding over the steric control in the Ziegler-Natta polymerizations of 1-alkenes to polymers of high or very high stereoregularity.
The states of ethyl benzoate (EB) and TiCl4 in the primary types of MgCl2-supported catalysts were studied by thermogravimetry/differential thermal analysis (TG-DTA) in combination with other methods. The results obtained by all experiments indicate that TiCl4 and EB in the MgCl2-supported high-yield catalysts interact only with MgCl2, leaving no vacant sites on TiCl4.
Due to the practical problems connected with polymer morphology, in both process viability and product processability, the morphology of the catalyst-polymer particle in a heterogeneous polymerization has been the object of great scientific effort at Montecatini and its successor since 1968. A model for the polymer growth mechanism, elaborated at the Ferrara laboratories in the late 1960s, was experimentally proven in the following years and has been adopted for the development of novel catalysts/processes/materials. More recently, thanks to the implementation of the fourth-generation catalyst system and its “reactor granule technology” approach, Montell has been able to produce novel polyolefin-based materials in spherical form with the broadest property ranges. In addition, the spherical-form polymers are utilizable without any pelletization operation, avoiding the related costs and improving polymer final quality. This paper describes the development and characterization of these polymers.
13C NMR analysis of the chain-end distribution of poly(propylenes) prepared using the highly active catalyst system MgCl2/TiCl4/diether—AlEt3 has revealed particularly high proportions of butyl chain-ends in polymers prepared at relatively low hydrogen pressures. This indicates that the high sensitivity of this type of catalyst to hydrogen, both with respect to catalyst activity and polymer molecular weight, can largely be ascribed to chain transfer following regioirregular (2,1-) insertion, such an insertion leading to a species having low activity in chain propagation. Isotactic stereoregularity increases with increasing hydrogen pressure, indicating that a stereoirregular insertion may also slow down the chain propagation, again leading to chain transfer and resulting in the conversion of a potential chain defect into an isobutyl chain-end. Analysis of highly isotactic polymer fractions isolated via temperature rising elution fractionation revealed the presence of both butyl and isobutyl chain-ends, indicating that even the most highly stereo-specific sites in MgCl2-supported catalysts are not totally regiospecific.
The role of Lewis bases in MgCl2 supported catalysts for olefin polymerization is a subject of continuous interest and discussion in order to obtain more and more active and stereospecific catalysts and to explain their stereoregulating mechanism.
Through molecular calculation and conformational analysis it was possible to identify chelating diethers that have the correct oxygen-oxygen distance necessary to tightly coordinate with the Mg ions of the support, even in the presence of other strong Lewis acids, and unable to give secondary reactions with TiCl4, AlR3, Ti-C and Ti-H bonds. The use of these donors has allowed the synthesis of catalytic systems that are both highly active and stereospecific even in the absence of external donors. Kinetic data of propylene polymerization with these catalyst systems are reported.
The importance of the distance between the donor atoms in bifunctional Lewis bases has been proved also in the case of new classes of internal donors.
Molecular modelling studies have enabled us to formulate models of active sites, located on some corners of MgCl2 crystallites, whose chirality is induced by the presence of a donor molecule in their environment. These models could explain, at least in part, the exceptional increase of isotactic polymer productivity observed for stereospecific catalyst systems, containing only the internal donor, with respect to catalysts lacking the Lewis base and could account for the influence of the donor on the molecular properties of the obtained polymers.
New models for steric environment of Ti isospecific polymerization sites on MgCl2 microcrystals are presented. They directly involve the presence of a donor molecule in order to obtain chiral activable Ti atoms otherwise belonging to isolated adsorbed TiCl4 molecules or Ti2Cl8 dimers which are lacking the required symmetry. The most important steric features of donor molecules have been obtained through structure-activity relationships and molecular comparisons, while their adsorption on MgCl2 faces lateral to (001) has been studied through a conformational analysis approach.
The development of Ziegler-Natta catalysts for propylene polymerization up to the high activity supported catalysts is reviewed.Special attention is paid to the structure of the support.By quenching the polymerization with radioactive carbon monoxide, the number of active centres and the propagation rate constant (Kp) have been determined for propylene polymerization in the presence of a highly active catalyst.The higher activity of this catalyst in comparison with TiCl3 based catalysts appears to be mainly due to a higher number of active centres; however higher values of kp are also observed.
Investigation of the effects of hydrogen and different external donors on the stereoregularity and chain-end distribution of polypropylene prepared using MgCl2/TiCl4/phthalate ester-AlEt3-alkoxysilane catalyst systems has not only confirmed the importance of regiospecificity in relation to hydrogen activation but has also indicated a significant effect of stereospecificity. Alkoxysilanes giving high isospecificity also give high molecular weight polypropylene. Polymer stereoregularity also increases with increasing hydrogen concentration in polymerization. These effects indicate that not only regioirregular but also stereoiregular insertion slows down chain propagation. In each case the probability of chain transfer with hydrogen is higher than that following a stereoregular insertion, and in each case chain transfer with hydrogen leads to the regeneration of isospecific propagation.
The discovery (1968) of the high yield Ziegler-Natta catalysts based on active MgCl2 was the beginning of a scientific and industrial revolution that has brought about the creation of superactive, isospecific, spheriform fourth generation catalytic systems. The rationalization of the polymer/catalyst replication phenomenon and the understanding of the catalyst “architecture” effects on polymer shape and morphology has led to the exploitation of the “Reactor Granule Technology”. This has made the generation of a broad range of homo, copolymers and multiphase alloys (Catalloy) possible by synthesis, most of which having a previously unobtainable spectrum of performance (Refs. 1,2,3). The reactor granule technology concept has also been the basis for the achievement of a family of polyolefin/non polyolefin alloys with engineering properties.
More recently, the reactor granule approach has been extended so as to couple the advantages of both heterogeneous and homogeneous metallocene catalysts (mixed catalysis), thus allowing the synthesis of a very new family of “in situ” polyolefin alloys.
The new third generation Ziegler-Natta catalysts give extremely high yield and produce highly stereoregular polypropylene of controlled morphology and size. The role of Lewis bases and MgCl2 is underlined and their mechanisms of action is discussed in the light of the most significant experimental data. Furthermore, technological and economical advantages are emphasized.Die neuen Ziegler-Natta-Katalysatoren der sog. dritten Generation erlauben mit hohen Ausbeuten die Herstellung von hochstereoregulärem Polypropylen mit kontrollierbaren morphologischen Eigenschaften und Korngrößen. Die Roue von Lewis-Basen und von MgCl2 wird unterstrichen, und ihre Wirkungsmechanismen im Hinblick auf signifikante experimentelle Daten werden diskutiert. Weiterhin werden die technologischen und wirtschaftlichen Vorteile betont.
Les propriétés exceptionnelles d'une nouvelle classe d'hydrocarbures macromoléculaires à structure linéaire obtenus par polymérisation d'α-oléfines sont décrites. La haute température de fusion, la cristallinité élevée, la faible solubilité, les propriétés mécaniques spéciales de ces polymères sont attribuées à une régularité particulière de structure due à l'existence, dans chaque macromolécule, de longues successions ordonnées d'atomes de carbone asymétriques ayant la même configuration stérique. Pour ce type particuler d'ordonnement d'atomes de carbone asymétriques dans des macromolécules linéaires, on propose le terme “isotaxique.”
The exceptional properties of a new class of linear polymeric hydrocarbons, obtained by the polymerization of α-olefins, are described. The high melting points, the high degree of crystallinity, the low solubility, and the special mechanical properties of these polymers are attributed to a particular regularity of structure, due to the existence, in each macromolecule, of a long sequence of asymetric carbon atoms, all having the same steric configuration. It is suggested that this special type of order of asymetric carbon atoms in linear macromolecules be called “isotactical”.
New heterophasic copolymers with high rubber content have been synthesized by combining heterogeneous Ti-based catalysts with
metallocenes. The molecular, morphological and mechanical features of these materials were evaluated: it was pointed out that
the new copolymers, having more than 60Wt.% of rubber, perform as reinforced elastomers whereas traditional ones show a more
plastomeric behaviour. This suggests that new polyolefin materials can augment rather than compete with the traditional ones.
The year 2000 challenges plastic producers to prepare polymers that: (1) originate from unique, easily-recyclable base-materials, (2) cover a large application-property range, and (3) are obtained by means of environmental-friendly, catalyst systems using low-energy consuming industrial processes.In pursuing these targets for more than 30 years, Montecatini research has developed fourth-generation Ziegler-Natta superactive catalysts and advanced process technologies, emphasizing both economic production processes and environmental protection. With a combination of these advanced technologies, Montecatini is able to produce, directly from the reactor, a broad property range of polyolefinic-based polymers and alloys in spherical form, with a soft-porous morphology and a lower crystalline structure and thermal capacity in comparison with that of the hard, conventional pellet. In addition, the spherical form polymers thus obtained are utilizable without any pelletization operation, which is expensive and causes polymer overheating, often impairing the finished part quality.
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