Polymerization Kinetics in MAPO-based BisGMA, UDMA or BisEMA Resins

Objectives: The use of the photoinitiator Lucirin-TPO in model resin composites is known to increase degree of conversion (DC), improve mechanical properties and reduced monomer elution compared with materials using camphorquinone. The aim here was to investigate the potential of alternative monomers as replacements for BisGMA to control polymerization kinetics and obtain higher degree of conversion.
Methods: Three resin series were prepared: BisGMA/TegDMA, UDMA/TegDMA or BisEMA/TegDMA (100, 75, 50 or 25 mol% of base monomer, 12 different resins). TPO was introduced at 0.42 wt%. Light-curing was carried out using an AURA light engine (Lumencor, USA) with a narrow output specific to TPO (395-415 nm). Kinetics of polymerization were followed in real-time by means of FT-NIR spectroscopy. 12 x 2 mm samples were irradiated for 1 s at 1000 mW/cm² (n = 3). DC were determined after 2 minutes of monitoring and rates of conversion (R_p) were determined as the first derivative of DC.
Results: UDMA and BisEMA-based resins showed the highest DC (77 and 75%, respectively). However UDMA-based resins displayed the highest polymerization rates while Bis-EMA-based resins exhibited the lowest (3x lower R_p^max), polymerising in a steady-state like fashion. The BisGMA-based resins presented the lowest DC (68%). For such short exposure time (1s), DCs decreased with greater TegDMA content and most markedly for the BisEMA resins. At 75 mol% TegDMA, each base-resin mixture converted to similar levels (approx. 40 %). BisGMA, UDMA and BisEMA-based resins presented varied DCs ranges depending on the base monomer content (15-68, 40-77 and 38-75%, respectively).
Conclusions: For 1 s irradiation time, the polymerization of some MAPO-resins reached high DC, although with different kinetic profiles. In terms of DC and Rp, UDMA may be an appropriate alternative to BisGMA, whilst the use of BisEMA led to lower Rp values.