Counit Inclusion in Hydrogenated Polynorbornene Copolymer Crystals

Model crystallizable copolymers of norbornene and two 5-alkylnorbornenes were synthesized to investigate the extent and consequences of defect inclusion into hydrogenated polynorbornene (hPN) crystals. Living ring-opening metathesis polymerization yielded narrow-distribution polymers of targeted molecular weights, with modest down-chain compositional gradients controllable through the polymerization conversion; hydrogenation yielded semicrystalline copolymers. When the comonomer was 5-methylnorbornene (MeN), extensive inclusion of MeN units into the hPN crystal was observed; the copolymers showed substantial crystallinities even above 30 mol % MeN, and the dependence of the melting point <i>T</i>_m on crystal thickness followed that for hPN homopolymer. By contrast, when the comonomer was 5-hexylnorbornene, the more usual case of strong exclusion of the counits from the crystal was observed. hPN shows a transition between two crystal polymorphs below <i>T</i>_m, at a temperature <i>T</i>_cc; comonomer incorporation reduces <i>T</i>_cc more rapidly than it reduces <i>T</i>_m, expanding the region over which the high-temperature rotationally disordered polymorph is stable and providing insight into the dependence of the free energy for the two polymorphs on crystal thickness