Polymer, 2017, vol 111pp. 115-122
Three iso-specific organometallic catalysts for ethylene and propylene copolymerization with varying r1r2 parameters, i.e., the products of ethylene and propylene reactivity ratios, were utilized to synthesize equal-molecular-weight propylene-based elastomers (PBEs) of alternating, random, and blocky backbone sequences. To compensate for the variations in catalyst iso-specificity and thus to maintain a constant amount of 50% isotactic propylene trimer concentration, the ethylene content of PBEs was varied between 11 wt % and 16 wt %. Although all three PBEs have an equal amount of crystallizable sequences, the high-C2 (high-ethylene) blocky PBE was found to have the highest crystallinity and crystallization rate, while the low-C2 alternating PBE possesses the lowest crystallinity and rate. This suggests that the backbone sequence distribution of a PBE affects its rate and degree of crystallization. The polypropylene (PP) crystallite width and perfection were enlarged and improved in the order of random, alternating, and blocky PBE, as measured by crystalline peak widths using the wide-angle X-ray scattering (WAXS) method. In addition, their crystals, cross-hatched lamellae web and embryonic axialites, which are the assembly of PP crystallites, are largest in the alternating PBE, followed by the random PBE as indicated by bimodal atomic force microscopy (AFM). A limited crystallite assembly was noted in the blocky PBE. Increasing crystallite assembly may have led to lowering the crosslink density (less crosslinks) and elasticity erosion. As a result of the wider and more perfect PP crystallites and the lack of assembly of these crystallites in the blocky PBE, it may have the highest crosslink density and strongest crosslinks and, hence, the best elasticity, as measured by set, hysteresis and retractive force, among the three, followed by the random PBE, and then the alternating PBE.