Influence of syndiotactic propylene units on the rheological parameters of poly(ethylene-propylene) copolymers

The plateau modulus has been measured for a trio of amorphous poly(ethylene-propylene) copolymers where the mole fraction of ethylene ranged from 0.57 to 0.67. These materials were prepared by using a living catalyst system involving bis(salicylaldiminato)titanium activated by methylaluminoxane. This catalyst with propylene yields highly syndiotactic (rr = 0.98; rrrr = 0.96) homopolymer. s-Polypropylene is known to have a plateau modulus and unperturbed chain dimension that are markedly larger than the values shared by the atactic and isotactic counterparts. The enhancement in the unperturbed chain dimension translates into an entanglement molecular weight for s-polypropylene that is ~Vs that of the atactic and isotactic counterparts. Thus, within the confines of the packing length notion, these empirical observations raise the question as to whether the rheological characteristics of the s-polypropylene will carry over into the random copolymer format. The copolymers with s-polypropylene segments yield (at 348 K) larger entanglement densities (∼0.39/nm ³) than the corresponding conventional materials (∼0.23/nm ³). These findings demonstrate that ethylene/propylene copolymers can be prepared so as to provide a high-performance elastomeric amorphous polyolefin with a low glass transition temperature (∼215 K).