Synthesis and rheological properties of long-chain-branched isotactic polypropylenes prepared by copolymerization of propylene and nonconjugated dienes

Abstract

Long-chain-branched isotactic polypropylenes (LCBed PP) were synthesized by copolymerizing propylene with a small amount of nonconjugated α,ω-diene (1,9-decadiene or 1,7-octadiene) using the catalyst system of rac-Me2Si(2-MeBenz[e]Ind)2ZrCl 2(MBI)/MMAO. In this approach, the LCB structures were introduced by the incorporation of in situ generated macromonomers with pendant 1-octenyl or 1-hexenyl groups during the polymerization. A detailed study on the effects of diene concentration on polymer properties was conducted. Polymer chain microstructures were characterized by 13C NMR, GPCV, and DSC. In the propylene/1,9-decadiene copolymerization, a series of LCBed polymer samples with the long-chain-branch density (LCBD) of up to 0.53 branch structures per 1000 carbons were produced with the diene concentrations of 0.177-3.54 mmol/L at 40 and 25 °C. A diene concentration of 35.4 mmol/L yielded cross-linked polymer gels. In the copolymerization of propylene and 1,7-octadiene, in addition to a small fraction of LCB structures produced, a cyclic seven-member ring structure was observed due to the cycloaddition of 1,7-octadiene. The cyclization significantly decreased the LCBD in the polymers. A small-amplitude oscillatory shear flow measurement was conducted to evaluate the rheological properties of the LCBed polymers. Compared to the linear samples prepared at the same polymerization conditions, the LCBed polymers exhibited enhanced low-frequency complex viscosity, improved shear-thinning, increased dynamic moduli, and reduced phase angle. The samples also showed thermorheological complexity and enhanced activation energy at low frequencies. These particular properties are related to the LCB in the polymers and become more significant with the increase of LCBD. The LCBed polypropylenes were also blended with their counterpart linear samples and demonstrated the improvement of rheological properties.