Solution [2 + 2] photopolymerization of biomass-derived nonrigid monomers enabled by energy transfer catalysis

The [2 + 2] photopolymerization has been known for more than fifty years and widely applied in many fields. However, this process was typically conducted in solid state, while the corresponding [2 + 2] photopolymerization of simple non-rigid diolefinic monomers were rarely achieved in solution under visible light, owing to the lack of monomer preassembly and low/no absorption of visible light. In fact, the [2 + 2] photopolymerization of simple biscinnamate monomers remains an unsolved problem. Here, we demonstrate that energy transfer catalysis could overcome the low efficiency in the intersystem crossing to triplet states of monomers, and enable the [2 + 2] photopolymerization of biomass-derived biscinnamate monomers in solution for the first time. As no preassembly is required, this solution polymerization protocol is applicable to biscinnamate monomers with different linker structures, and allows copolymerization between different biscinnamate monomers to regulate polymer mechanical properties. A series of cyclobutane-imbedded polyesters (Mw ranged from 25.3 kDa to 61.3 kDa) become accessible, which shown excellent solubility in organic solvents and good processability, in sharp contrast to the properties of the biscinnamate polymers obtained before via the solid state photopolymerization methods.