On the ethylene-norbornene copolymerization mechanism

Results of our studies on polymerization kinetics and tests of Copolymerization statistical models of ethylene-norbornene (E-N) copolymers obtained on the basis of microstructures determined by C NMR analysis are reported, Ethylene-norbornene (E-N) copolymers were synthesized by catalytic systems composed of racemic isospecific metallocenes, i-Pr[(3Pr -Cp)(Flu)]ZrCl or a constrained geometry catalyst (CGC) and methylaluminoxane. Polymerization kinetics revealed that E-N Copolymerization is quasi living under standard polymerization conditions. Calculations of the number of active sites and of chain propagation and chain transfer turnover frequencies indicate that the metal is mainly in the Mt-N* state, while the Mt-E* state contributes more to transfer and propagation rates. The first-order and the second-order Markov statistics have been tested by using the complete tetrad distribution obtained from C NMR analysis of copolymer microstructures. The root-mean-square deviations between experimental and calculated tetrads demonstrate that penultimate (second-order Markov) effects play a decisive role in E-N copolymerizations. Results show clues for more complex effects depending on the catalyst geometry in copolymers obtained at high N/E feed ratios. Comonomer concentration was shown to have a strong influence on copolymer microstructure and copolymer properties. The copolymer microstructure of alternating isotactic copolymers obtained with i-Pr[(3Pr-Cp)(Flu)]ZrCl have been described at pentad level. Second-order Markov statistics better describes also the microstrucure of these copolymers.