Crystalline Isotactic Polyterpenoid from Citronellal: A Viscous Bottlebrush Polymer with Order–Disorder Transitions

Abstract Most terpene-derived polymers reported to date are amorphous resins, elastomers, or cross-linked networks. Here we report a crystalline isotactic polyterpenoid, poly(4,8-dimethyl-1,7-nonadiene) (PDMND), synthesized from naturally abundant citronellal using a pyridylamido hafnium catalyst. Temperature-dependent WAXD reveals that PDMND adopts an isotactic-polypropylene-like (3/1) helical backbone with an axial repeat of ca. 6.6 Å. Unlike conventional semicrystalline polyolefins, the densely populated, nonlinear terpenoid side groups prohibit chain folding and promote the packing of fully extended bottlebrush-like rigid rods. DSC, WAXD, FTIR, and rheological analyses reveal reversible order–disorder transitions from an ordered cylindrical-rod crystal to a side-chain-disordered phase and further to a nematic-like crystalline state before melting. This work expands terpene-derived polymers from amorphous materials to crystalline stereoregular polyolefins with unusual bottlebrush-rod packing.