Inert Nanogels Enhance the Photopolymerization Kinetics of MMA

Objectives: The photo-induced, free radical polymerization of monovinyl dental monomers such as methyl methacrylate (MMA) is plagued by slow reaction kinetics and lengthy reaction times. Currently, MMA is reacted with divinyl comonomers, prepolymerizied MMA (pMMA) or reactive additives (e.g. nanogels) to enhance reaction kinetics while forming uniform networks. This study aims to understand the role of reactive/inert additives in the photopolymerization kinetics of MMA and the formation of macroscopic networks.

Methods: Inert and reactive nanogels of 5 nm were prepared via solution polymerization of isobornyl methacrylate/urethane dimethacrylate with mercaptoethanol as chain-transfer agent. Inert nanogels are reacted via thiol-Michael Reaction with mercaptoethanol and trimethylamine(catalyst) to ensure no reactive groups remain while the reactive nanogels are methacrylated to introduce polymerizable groups on chain ends. Both nanogels were dispersed up to 50 wt% loading within MMA and photopolymerized (100 mW/cm², NIR spectroscopy).
Results: Inert and reactive nanogel characterization by GPC gave an average molecular weight (Mw) of 120,000g/mol and 135,000g/mol respectively. The low reactivity of MMA was maintained with 15% double bond conversion observed at 10 mts. The addition of the inert nanogels at 15wt% and 50 wt% achieved average conversions at 20 and 55 % respectively within 10 mts while the final conversion achieved at 60 mts were 85 and 100%, respectively. The reactive nanogels followed a similar conversion trend as the inert nanogels at ≤ 35 wt%, however, the 50 wt% reactive nanogel achieved 90% conversion within 10 mts
Conclusions: Improved kinetics observed with the addition of nanogels is achieved as nanogels swell and enhance the polymerization rate locally which in turn increase the viscosity and overall reaction kinetics. For reactive nanogels, the ability to form of co-continuous networks above the percolation threshold contributes to enhancing kinetics. Utilizing inert nanogels as additives can maintain the linearity of the parent network