Copolymerization of propylene with Si-containing alpha,omega-diolefins: how steric hindrance of diolefins affects long chain branch formation

Using the pyridylamido-hafnium/[Ph3C][B(C6F5)(4)]/(AlBu3)-Bu-i catalytic system, a series of long chain branched isotactic polypropylenes (LCBPPs) were in situ synthesized by copolymerization of propylene with Si-containing monomers 4,4-dimethyl-4-sila-1,6-heptadiene (DMS), 4-methyl-4-phenyl-4-sila-1,6-heptadiene (MPS), 4,4-diphenyl-4-sila-1,6-heptadiene (DPS), or 4-methyl-4-vinyl-4-sila-1,6-heptadiene (MVS). The effects of the substituent groups on their copolymerization behavior and topological structure were further investigated. The substituent groups decreased the cyclization tendency of the diallylsilane, which increased the probability of insertion of the pendant vinyl groups into another polymer chain. Thus, LCBPPs could be easily prepared under mild conditions by exquisitely controlling the comonomer structure and polymerization conditions such as the initial concentration of the comonomer, the dosage of the chain transfer agent (CTA) and the reaction time. The presence of LCB structure was confirmed by characterization using SEC equipped with triple detectors, small-amplitude dynamic rheological measurements and uniaxial extension. The introduction of LCBs endowed PP with improved rheological behavior and melt strength, which made the resultant LCBPPs possess potential applications in thermal processes such as blow molding and foaming.