HOMO AND COPOLYMERIZATION OF OLEFINS AND CYCLO-OLEFINS BY HOMOGENEOUS METALLOCENE AND HALF-METALLOCENE CATALYSTS

Propene copolymers by homogeneous metallocenes form a new class of thermoplastic elastomers with lower melting points, lower glass transition temperatures and higher impact strength than stereoregular polypropene (PP). The copolymerization of propene with ethene is a common way of tailoring the density and crystallinity of propene based polymers and thus to obtain a material that is more elastic than iPP or sPP homopolymer. The introduction of metallocenes in copolymerization reactions has also created new opportunities for the synthesis of olefin copolymers with uniform compositions and random sequence distributions and with tailored microstructures and properties. The molecular weight control is an important issue for accessing the practical production of polymeric materials. Indeed, both for Cs-symmetric and C2-symmetric catalysts it was found that the Mw of ethene-propene copolymers are in general lower than those of propene homopolymers. Incorporation of a substituent at the 3 position of the cyclopentadienyl ring of bridged (C5H4)(C13H8)MCl2 (M = Ti, Zr, Hf) with Cs symmetry induces desymmetrization of the metallocene to C1 symmetry. The microstructure of propene homopolymers using C1-symmetric metallocenes greatly depends on the nature and hindrance of the substituent on Cp group. When the substituent is a Me or i-Pr group, a hemi-isotactic or atactic polypropene is obtained; when the substituent is the bulkier t-Bu substituent, isotactic polypropene is achieved. These systems are amenable to study fine details of the polymerization mechanism. The mechanism which leads to the isotactic polypropene with C1-symmetric Me2C (3 ter butyl C5H3)(C13H8)MCl2 has been a topic of debate. For this reason a number of complex C1-symmetric metallocenes have been synthesized and studied recently. The aim of this part of the project on propene polymerization catalysis with C1-symmetric fluorenyl-containing metallocenes bearing one or two t Bu substituents at the 3 position of Cp or at 3 position of the fluorenyl groups. The main objective of this investigation was: to assess the impact of these changes on the polymerization mechanism and on catalytic performances for obtaining isotactic polypropene with high molar masses; to identify among the C1 symmetric catalysts those most promising for obtaining copolymers with high molar masses, as well as to give a rationale to such possibility. The effect of one or two ter-butyl substitutions on the catalytic activity, on the stereoerrors, and on molar masses in conditions which tend to approach those used in industrial processes was evaluated. Thus, C1-symmetric metallocenes were subjected to MAO-cocatalyzed propene homopolymerizations at different temperatures and monomer concentrations. Stereoerrors were investigated by 13C-NMR analysis. Observed polymer tacticity has been compared to that predicted by three statistical models: the first is enantiomorphic site control, which is predicted by the site epimerization mechanism, since it employs a single site with enantioselectivity α. The second is an alternating model that is generally applicable to a catalyst that regularly alternates insertions between a perfectly stereoselective site (α = 1) and a variably stereo selective site having a stereoselectivity equal to β. The third is an alternating model that is applicable to a catalyst that regularly alternates insertions between two variably stereoselective sites. The stereoselectivity of one site is α, and the stereoselectivity of the other site is β. Both alternating models assume that no site epimerization is occurring. An analysis of chain end groups by 1H-NMR was performed in order to understand the role of substituents on polymer molar masses. Finally, the industrial synthesis of homo- and copolymers by homogeneous catalysts has to be achieved by supported systems in liquid propene because homogeneous processes present serious drawbacks such as reactor fouling as well as morphological issues. Thus, we have made efforts, at a laboratory scale, to study the variables that need to be controlled in order to produce reproducible polymers. The second part of the thesis concerned about cyclo-olefin copolymers. COP are a family of plastic materials derived from the polymerization of strained-ring monomers, with linear olefins, which generally exhibit unique physical and mechanical properties that differ from those of polyolefins with acyclic structures. Ethene(E)-co-Norbornene(N) copolymers are the most versatile and studied copolymers, because of their important end uses. (E-co-N)s are amorphous materials with a wide range of glass transition temperatures, ranging from r.t. to 220 °C, depending on polymer composition, the higher the norbornene content, the higher is the percentage of norbornene diads and triads, and the higher is the Tg; they show excellent transparency and high refractive index because of their rigid bicyclic monomer units and high chemical resistance and are suitable for many applications such as coatings for high-capacity CDs and DVDs, for lenses, medical equipments, blisters, toner binder, and packaging. A promising approach to further expand the properties of COCs is the introduction of a third monomer like a linear α−olefin such as 1- octene or the substitution of the bicyclic norbornane unit with a cyclopentane unit by 1,5-hexadiene cyclopolymerization. The presence of a long linear α-olefin or that of a cyclopentane unit into E-N copolymer backbone could increase the flexibility of the copolymer chain and improve the commercial applicability of the material, while maintaining a high transparency and the other characteristic properties of E-N copolymers. The catalytic performances of nonbridged half-titanocenes initially in E/N copolymerizations, then in E/N/HED terpolymerizations, and in E/N/O terpolymerizations, were investigated. Priority was to synthesize terpolymers that meet industrial requirements; therefore the synthesis have been conducted at conditions as close as possible to those used in industrial plants, that is temperature: 60 °C and 4 bar ethene pressure. For all the three half-titanocenes, the addition of 1,5-hexadiene seems to negatively affect the activity, that results to be much lower with respect to the activity in E/N copolymerizations. The terpolymers have been characterized by 13C-NMR spectroscopy; the main objective was to determine the polymer composition in terms of norbornene and 1,5-hexadiene content. Especially at high norbornene content, the terpolymers obtained give rise to particularly complicated spectra and the peak assignment is very difficult; the chemical shifts of norbornene cyclic units result to be fairly similar to those ascribed to cyclopentane units. When the N content in the terpolymers is high, an overlapping between E/N backbone resonances and the major part of signals attributed to cyclopentane units, as well as the shifting of some other peaks, is reasonable to happen. As a consequence, to give unambiguous assignment of the CP structure peaks, a preliminary study of 13C-NMR spectra of N/HED and E/HED copolymers as function of feed composition has been performed. Since the introduction of a termonomer with a linear alkyl chain may allow to modulate the COC properties the synthesis of poly(E-ter-N-ter-O) by nonbridged half titanocenes was investigated. Terpolymerizations were carried out in a lab-scale autoclave under conditions as close as possible to those used in industrial plants, that is at high ethene pressure (4 bar) and high polymerization temperature (60 °C). The terpolymers obtained were characterized by 13C-NMR to determine the polymer compositions, by SEC to determine the molar masses and molar mass distributions and by DSC to determine the glass transition temperatures. The preliminary data obtained are reported. The terpolymers have been synthesized at quite high N content to achieve synthesis of terpolymers with at least 30 N mol %, and thus Tgs higher than 70 °C. [N]/[E] was fixed to 4/1 and just one 1- octene composition has been investigated. Very low amount of poly(E-ter-N-ter-O) has been obtained with all the catalytic systems. Under these experimental conditions the addition of 1- octene lowers notably the activity, showing depressed values for all the catalytic systems. All the terpolymers prepared have been characterized by 13C-NMR spectroscopy to determine the molar composition; the spectra exhibit the presence of signals characteristic of 1-octene units, even if, at high N content, an overlapping between E-N backbone resonances and 1-octene signals is revealed.