Microgel formation during early stages of photopolymerization leads to heterogeneous polymer networks which significantly limits material properties. We have explored controlled radical photopolymerization, including reversible addition-fragmentation chain transfer (RAFT) agents and nitroxide-mediated photopolymerization (NMP2) initiators, in cross-linked resins to mediate network formation and ultimate thermo-mechanical properties. RAFT agent addition has shown a significant impact on polymerization behavior and ultimate polymer properties in model systems. When comparing to conventional free radical polymerization, reaction rate is slightly decreased with RAFT containing formulations due to the formation of RAFT-adduct radicals during chain propagation that enable radical deactivation and reinitiation. RAFT-modified films exhibit narrower half width of tan delta peaks, indicating more homogeneous networks. In addition, when compared to thiol chain transfer agents, RAFT containing polymer networks enable further network rearrangement through the adaptive nature of bond formation and breaking, leading to 2-fold increase in toughness. Additionally, we have designed and synthesized a series of NMP2 initiators with different chromophores to tune light absorbance characteristics and material properties. In linear polymeric systems, incorporating NMP2 initiators leads to controlled molecular weight evolution and low polydispersity. When incorporating specific NMP2 into cross-linked resins, similar polymerization kinetics are achieved to those using conventional photoinitiators. However, gelation is delayed, and significantly decreased shrinkage stress is observed. NMP2 polymerized cross-linked polymer films also exhibit enhanced flexibility and increased toughness.

Controlled radical photopolymerization and photoinitiation to direct network structure

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