Polymerization mechanism of conjugated dienes in the presence of Ziegler-Natta type catalysts: Theoretical study of butadiene and isoprene polymerization with CpTiCl3-MAO initiator

The mechanism of butadiene and isoprene polymerization in the presence of CpTiCl3-MAO has been investigated by means of ab initio computations. According to experimental evidence, the catalytically active species are assumed to be [CpTi-P](+) organotitanium cations (P = growing polymer chain). The results show that (i) in the active species, P is coordinated to Ti both with the pi allyl group of its ending unit and with the pi bond of the penultimate unit; (ii) the cis eta(4) coordination of an incoming monomer to the active species requires the breakage of the latter interaction and the change of the allyl coordination mode from eta(3) to eta(1) That significant rearrangement of P is predicted to be the rate-determining step of the whole propagation reaction and is predicted to be much easier when the ending unit of P is butenyl rather than 2-methylbutenyl. That finding accounts both for the large difference observed in the homopolymerization rates of butadiene and isoprene and for the fact that in copolymerizations the two monomers exhibit almost the same reactivity.