Objectives: Compare the effects of thermal, photo, photo-thermal, redox, and iniferter-based nanogel syntheses in terms of curing efficiency.
Methods: Nanogels were synthesized from isobornyl methacrylate, urethane dimethacrylate (30:70 molar ratio) and dodecanethiol as chain transfer agent (15mol%). Free-radical polymerization was carried out in solution (80wt% toluene). Initiation systems examined (n=3) include: photoinitiator, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (0.1-5wt%); thermal initiator, 2,2-azobis(2-methylpropionitrile) (AIBN, 1wt%); redox, benzoyl peroxide/N,N-dimethyl-p-toluidine (1.0/1.5wt%); and photo-iniferter, tetraethylthiuram disulfide (1wt%), which was used in conjunction with 2,2-dimethoxy-2-phenylacetophenone. Light-activation was conducted with a mercury arc lamp (320-500nm) at an irradiance of 600mW/cm2 for photo and iniferter samples. Heated polymerizations were carried out at 70ºC for thermal and either 45 or 70ºC for photothermal reactions. All trials were carried to full conversion using real-time near-infrared FTIR to monitor reaction progress.
Results: Thermal polymerization with AIBN required 180min to achieve 60.0±3.3% conversion while photopolymerization at 23, 45 and 70ºC provided conversion values of 95.7±0.4, 96.7±0.2 and 97.0±0.3 % in 18.5, 18.5 and 13min, respectively. A 4h 23ºC redox polymerization produced limited nanogel conversion but starting the reaction at 40ºC and adding a second batch of redox initiator after 30min gave 91% conversion at 50min. The room temperature photoiniferter-induced reaction took more than 2h to reach 89% conversion but notably, the reaction demonstrated living-radical character by successful incorporation of several consecutive batches of monomer to yield nanogels of increasing particle size.
Conclusions: Photo or photothermal polymerization protocols applied to nanogels provide substantial synthetic efficiencies while iniferters appear to offer unique surface-initiated structures.
Supported by: NIH/NIDCR 5R21DE018354