Giuliano Cecchin’s scientific contributions

Three procatalysts of the MgCl2/TiCl4 type, differing mainly in their morphological characteristics, were investigated in ethylene polymerization and ethylene-1-butene copolymerization. Apparently, hydrogen has an intrinsic and general deactivating effect but it can also play an activating effect in homopolymerization. This peculiarity was found to be related to a catastrophic breakage of the polymer/catalyst particles during growth and thus to the exposure of new active centers. In this case the kinetic profiles are irregular and characterized by one or more secondary peaks which reflect the moment when this morphology-driven rate-enhancement effect takes place. In general, the prepolymerization of the procatalysts with propylene tends to slightly enhance homopolymerization rate, to slow down copolymerization rate and to stabilize the morphology of the growing polymer particles, thus preventing the occurrence of the irregular kinetic profiles observed during homopolymerization in the presence of hydrogen. The behavior of the procatalysts investigated was found to depend on the distribution of their pore size rather than the absolute values of their porosity. Likely this is due to an easier diffusion of the monomer and a more regular and homogeneous growth of the polymer within larger as opposed to smaller pores. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

MgCl2 supported Ziegler-Natta catalysts for the polymerization of olefins have had recently a spectacular success in simplyfling polymerization processes and improving polymer quality; a rich literature is available reporting various data and interpretations of the mechanisms involved in this type of catalysis. The authors' aim was to review thoroughly and critically the scientific and patent literature. The characteristics of the components and their interaction are particularly borne in mind in view of the catalysts' activity and selectivity. Other items concern structure and concentration of the active centers, polymerization kinetics and the role of the catalyst in determining polymer morphology. Finally, industrial applications are outlined. Particular attention is payed throughout to the stereospecific catalysts for polypropylene. Areas where further research is necessary for future progress and improvement are pointed out.

The most advanced catalysts, based on MgCl2-supported TiCl4 and electron donors, are able to provide polypropylenes with an isotacticity higher than 99%. This, together with the continuous progress made in understanding and exploiting the role of electron donors in controlling polymer MW and MWD, has led to polypropylene products having an unprecedented level of stiffness or stiffness/impact balance. On the other hand, other potential fields of application exist where rigidity is not required and, actually, the key property is softness rather than stiffness. As a matter of fact, it has clearly been established that soft polypropylenes can be more attractive from the business standpoint than their stiff counterparts. Generally, these materials are multiphase copolymers obtained via sequential gas-phase copolymerization of propylene and ethylene-propylene mixtures using the morphology-controlled conventional MgCl2-TiCl4 catalysts based on the couple phthalatesilane as internal and external donors. This communication deals with a new class of donors that can be used either as external donors in combination with phthalates (A) or as internal/external donors (B). When combined with the MgCl2-TiCl4 systems, both donors substantially improve the flexibility and softness of the resulting soft materials while maintaining the operability window of the Catalloy process. This is due to the particular microstructure of the relevant building blocks: the presence of a controlled concentration of stereodefects in the homopolymer fraction, and good comonomer distribution in the copolymer fraction. As compared with the conventional products, the new ones show comparable or better flexibility when the rubber phase is relatively rich in ethylene. This likely opens the door for these products to enter the demanding thermoplastic elastomers (TPE) application field.

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.

A historical survey about the development of Ziegler-Natta catalysts for polymerization of propylene is given. After having employed catalysts based on titaniumtrichloride for several years, in the 70's catalysts of the so-called second generation were introduced, which are more effective and yield products of higher crystallinity. Catalysts of the so-called third generation and their morphological properties are extensively dealt with. These catalysts give high yields and highly stereoregular polymers and enable simplification of polypropylene technology and energy saving. In the future polypropylene manufacturing will probably consist only of the polymerization steps with no need for centrifuging, extruding or another post-treatment.Es wird ein historischer Überblick der Entwicklung von Ziegler-Natta-Katalysatoren für die Propylenpolymerisation gegeben. Nach dem jahrelangen Einsatz von Titantrichlorid wurden in den 70er Jahren neue Katalysatoren der sogenannten zweiten Generation eingeführt, die schneller wirken und zu kristallineren Produkten führen. Ausführlich wird auf die neuere Entwicklung zu Katalysatoren der „dritten Generation”, speziell auf ihre morphologischen Eigenschaften eingegangen. Diese Katalysatoren ergeben sehr hohe Ausbeuten, sterisch sehr regelmßige Polymeren und eröffnen Möglichkeiten zur Vereinfachung der Polypropylentechnologie und Energieeinsparung. In Zukunft wird die Polypropylenherstellung nur aus der eigentlichen Polymerisationsstufe bestehen, und es wird keinerlei Nachbehandlung wie Zentrifugieren oder Extrudieren erforderlich sein.

Ziegler‐Natta catalysts, discovered in the years 1953–1954, account today for a production volume of ∼65 million tons of polyolefins, including mainly polyethylene and polypropylene products. Since their first discovery, the development of Ziegler‐Natta catalysts has been relentless, and their evolution is the result of the exploitation, starting from the mid‐1960s, of four major breakthroughs: the active form of MgCl 2 ; the stereoregulating effect of electron donors (isotactic polypropylene); the chemical route to the active form of MgCl 2 ; and, finally, the control of the morphology of the catalyst/polymer particles. Thus, the most advanced Ziegler‐Natta catalysts are today prepared starting from controlled morphology MgCl 2 , or its precursors, TiCl 4 , or other titanium derivatives and, especially for isotactic polypropylene, electron donors. With respect to the first‐generation of Ziegler‐Natta catalysts based on TiCl 3 , MgCl 2 ‐based catalysts are not only much more efficient in terms of productivity ( both increased number of active centers and value of their propagation constant), but they also offer an unprecedented level of customization to meet any process (slurry, bulk, or gas‐phase) or product requirements (polyethylene, polypropylene, and their copolymers). Actually, MgCl 2 ‐supported catalysts are complex but also versatile systems whose architecture (particle shape, size, size distribution, and porosity) and performances (activity, hydrogen response, stereocontrol, and capability to tune polymer MW and MWD) can be directed acting on both the morphology of the MgCl 2 precursors and the nature of the electron donors. Many mechanistic aspects of the Ziegler‐Natta catalysis have been elucidated, eg, the mechanism of polymer particle growth, the role of prepolymerization, the activating effect of hydrogen in propylene polymerization, and the stereoblock nature of isotactic polypropylene. On the other hand, many other aspects are still open to debate or, at least, still need clarification; eg, the exact nature and number of active centers and their mechanism of deactivation, the intimate mechanism of action of electron donors, the difference between the species that are active in ethylene or propylene polymerization, and the comonomer effect in ethylene copolymerization. Thus, there is still a lot of room for further investigation and improvements in Ziegler‐Natta catalysts. A big step forward in this field would be the generation of catalysts combining the economics and morphological features of MgCl 2 ‐based systems with the peculiarities of single‐site systems.

Continuous fine tuning of both chemistry and architecture of heterogeneous, isospecific MgCl2/TiCl4 catalyst systems for propene polymerization has led to a significant improvement in their performances in terms of activity, stereoselectivity, and capability to control both molecular parameters and morphology of the resulting polymers. As a result, a number of improved/innovative propene-based products has recently been developed that clearly outperform the previously available materials. In particular, a family of reactor-grade polymers has been synthesized including homopolymers and heterophasic copolymers offering, respectively, an unprecedented relationship between fluidity and stiffness and between stiffness and impact strenght, and supersoft polypropene alloys that could not have been achieved even via mechanical blending.

A morphological investigation was carried out on spherical catalysts based on MgCl2-supported TiCl4 and related nascent polymer particles, including both homopolypropene and polypropene-based multiphase copolymers. Transmission and scanning electron microscopy show that both catalyst and polymer grains display a dual morphological texture consisting, respectively, of microparticles and subparticles (catalyst) and microglobules and subglobules (polymer). The morphology of sequential, multiphase copolymers indicates that the second monomer, either ethylene or 1-butene, or monomer mixture, ethylene/propene, polymerizes around the pre-formed subglobules of the homopolypropene matrix. Based on the experimental results, a model/mechanism of polypropene growth has been proposed that entails the features of both a dual grain and a polymeric flow system: the monomer polymerizes at the surface of catalyst microparticles forming a polymer shell (microglobule) around each of them; polymer microglobules form larger agglomerates (subglobules) which, as polymerization goes on, tend to behave as individual polymeric flow units: catalyst microparticles undergo further fragmentation and tend to be convected from the bulk to the surface of polymer subglobules, where they sustain the reaction.

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.