Counit Inclusion in Hydrogenated Polynorbornene Copolymer Crystals

Crystallization in poly(A-\textit{co}-B) random copolymers, where homopolymer A is crystalline but B is not, is dictated by the degree to which crystals of A can include B units. Typically, B units are strongly excluded from the A crystals, drastically reducing the degree of crystallinity w$_{c}$ and crystal thickness t$_{c}$ even at modest comonomer contents. However, in some cases, B units can be incorporated into the crystals as defects, significantly diminishing the counits' impact on w$_{c}$ and t$_{c}$. The extent and consequences of counit inclusion have been investigated in hydrogenated polynorbornene (hPN) with alkylnorbornene counits, synthesized by living ring-opening metathesis polymerization followed by hydrogenation. In the case of 5-hexylnorbornene (HxN) counits, a steep decline in w$_{c}$ and t$_{c}$ with counit content is found, indicative of strong exclusion. In contrast, when the counits are 5-methylnorbornene (MeN), extensive inclusion of MeN units into the crystals is observed. hP(N-\textit{co}-MeN) copolymers maintain appreciable crystallinity above 30 mol{\%} MeN, and the dependence of the melting point T$_{m}$ on t$_{c}$ tracks that of the hPN homopolymer. Four times as much MeN as HxN (molar basis) is required to produce a comparable drop in w$_{c}$. Therefore, copolymerization with MeN can be used to tune T$_{m}$ without drastically reducing w$_{c}$. Additionally, hPN exhibits a polymorphic transition to a rotationally disordered (RD) crystal at temperature T$_{cc\, }$\textless T$_{m}$. Incorporation of comonomers increases the disparity between T$_{m}$ and T$_{cc}$, indicating that thin crystals stabilize the RD phase.