Lower Critical Ordering Transition of an All-Hydrocarbon Polynorbornene Diblock Copolymer

A new diblock copolymer chemistry, hydrogenated poly(n-hexyl norbornene)-block-poly(cyclohexyl norbornene), is demonstrated to undergo a lower critical ordering transition (LCOT) upon heating. Diblock copolymers exhibiting LCOT behavior are rare, and to our knowledge, the system described in this work is the first all-hydrocarbon species to do so. Furthermore, the tendency toward demixing for this polymer at high temperatures does not arise from a large free volume mismatch between its components, the mechanism most commonly invoked to explain an LCOT in block copolymers or a lower critical solution temperature in polymer blends. We compare the LCOT-type polymer with a homologous family of other norbornene-based polymers exhibiting the more common upper critical ordering transition and demonstrate that the mismatch in thermal expansion coefficients between blocks, which is related to the mismatch in free volumes, does not dictate the type of phase behavior.