A New Semicrystalline Polymer from Renewable Source: Poly-1,4-(3-methylene-cyclopentene) as an Example of Three-Dimensional Topotactic Transition Triggered by Temperature

In search for biobased alternatives to fossil-based polymers, a synthetic approach is developed that allows the polymerization of a terpene-derived monomer, 3-methylene-cyclopentene (MCP), to poly-1,4-MCP (pMCP) with a high yield and elevated molecular mass. The so-obtained pMCP is regioregular, has an endocyclic double bond/monomer in a fixed cis configuration, and is characterized by random enchainment of monomers with the tertiary C atom in R and S configurations. Yet, it crystallizes from the melt with chains in a nearly extended conformation (average chain periodicity c ≈ 0.96 nm) in the monoclinic system. Upon heating, the low-temperature crystalline (LTC) form transforms, before melting, in a disordered, high-temperature crystalline form (HTC), characterized by the same extended conformation of the LTC form, and orthorhombic crystals of larger apparent coherence length, through a three-dimensional topotactic transition. This transition entails that close neighboring coherence domains of the LTC form rearrange themselves, forming domains of large size not only along the lattice directions of the unit cell perpendicular to the chain axes but also parallel to the chain axes. This unique temperature-driven LTC → HTC topotactic transition is reversible and, upon cooling, the large HTC domains break, forming smaller domains of the LTC form.