Kinetic studies of the copolymerization of ethylene with norbornene by ansa-zirconocene/methylaluminoxane catalysts: Evidence of a long-lasting 'quasi-living' initial period

摘要

Copolymers of ethylene (E) with norbornene (N) were synthesized using the catalysts rac-Et(Ind)(2)ZrCl2/MAO (1), 90%racl10%meso-Et(4,7-Me(2)Ind(2)ZrCl(2)/MAO (3). Catalyst activity, molar mass (MM), and copolymer composition were studied as a function of time. The polymers showed an unusually narrow molar mass distribution (MMD) and a significant increase of their MM with time for up to one hour, suggesting a "quasi-living" polymerization at 30 degreesC. The experimental data were fitted to kinetic equations and the propagation and transfer reactions were described in quantitative terms. Norborne greatly depressed the propagation rate, along with the chain transfer rate. The more sterically hindered catalyst of the series showed lower propagation and chain transfer turnover frequency than 1 and yielded polymers with a low (2) to a very low (3) norbornene content. The presence of norbornene in solution seemed to be one of the main factors responsible for the observed "qausi-living" character of the copolymerization, probably due to coordination of norbornene to the active site. Time-resolved kinetic studies also allowed for the calculation of the fraction of active metal centers, ranging from 56% (3) and 66-68% (1) to 94% (2) of the total zirconium present, depending on catalyst structure. [References: 39]