Solution [2 + 2] photopolymerization of biomass-derived nonrigid biscinnamate monomers enabled by energy transfer catalysis†
[2 + 2] photopolymerization has been known for more than fifty years and widely applied in many fields. However, this process is typically conducted in the solid state, while the corresponding [2 + 2] photopolymerization of simple non-rigid diolefinic monomers has been 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 the 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) became accessible, which showed excellent solubility in organic solvents and good processability, in sharp contrast to the properties of the biscinnamate polymers obtained before via solid state photopolymerization methods.