Design Rules for Thermally Depolymerizable Polybutadienes

The vast majority of commodity plastics are based on all-carbon backbones. This creates a baseline for processing and mechanical properties that has been difficult to emulate with recyclable polymers that are based on heteroatomic linkages. Nevertheless, we have recently shown that polymers with all-carbon backbones based on a butadiene core exhibit high thermal depolymerization yields due to relatively weak C-C bonds. Here, we report a systematic investigation of the thermally-cleavable C-C design principle using density functional theory (DFT) calculations to characterize the bond dissociation energies (BDE) of 116 repeat units selected to elucidate the relative roles of resonance delocalization, hyperconjugation, and sterics in tuning the intermonomer C-C BDE. The results show that the BDE can be routinely tuned down to less than half the value of a typical alkane with $\beta$ unsaturations and lone-pair donors providing a larger impact on BDE than steric substitutions and hyperconjugation. Several new polymers were experimentally synthesized and tested for depolymerization yield based on these design rules. Among these, the remarkably simple 2,5-dimethyl-2,4-hexadiene repeat unit that is a close relative to isoprene is found to exhibit facile depolymerization at high temperatures with minimal side-product formation. These results demonstrate that a practically inexhaustible design space exists for thermally depolymerizable polymers based on relatively weak C-C bonds.