On the Mechanism of Propagation and Chain Transfer

In the course of our polymerization studies the performance of these new catalysts in the copolymerization of norbomenes with a-olefms such as ethylene, 1-hexene and 1-decene was investigated. The primary reason for exploring this area was to enable copolymerization of norbomenes with ethylene (and other a-olefins) 

In the case of naked nickel , rather than being incorporated as co-monomers these olefins drastically reduced the molecular weight of the resulting polynorbor-nene. This dramatic effect on the weight average molecular weight is plotted in Fig. 4.11. 

A first glance at Fig. 4.11 suggests that the effect of olefins on polymer MW is catastrophic, since at 30 mol% 1-decene the products are in the trimer-hexamer range (M, 1000) however, further study revealed that the control of MW is very precise and controllable. Indeed, the addition of controlled levels of 1-decene (or other a-ole-fins) proved to be a very reproducible way to achieve any desired MW from ohgo-mers up to very high MW polymers. This is illustrated in Fig. 4.12, where it can be seen how precisely the molecular weight can be controlled, and that similar molecular weights result whether the studied polymerization is a homopoiymerization of norbornene itself or a copolymerization with alkylnorbornenes. 

Having elucidated fhe end group structure for norbornene polymerized in the presence of ethylene, a catalytic cycle showing initiation, propagation, and chain transfer can be constructed (Fig. 4.14). 

The extension of fhis novel chain transfer event to higher molecular weight a-olefins has enabled us, and ofhers, to make unique poly(norbornene) block copolymers [44]. Examples of fhe diversity of this chain transfer chemistry are given in Fig. 4.15 where chain transfer to efhylene, allylglycidyl ether, isobutylene, cyclo- 

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Novel data on the composition of active centers of Ziegler-Natta catalysts and on the mechanism of propagation and chain transfer reactions are reviewed. These data are derived from the following trends in the study of the mechanism of catalytic polymerization a) determination of the number of active centers (mainly with the use of radioactive CO as a tag) b) analysis of the microstructure of polymers with the use of C-NMR c) analysis of specific features of highly active supported catalysts d) quantum-chemical calculation of the electronic structure of active centers and their reactions. 

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