Acrylate/methacrylate Comonomer Combinations to Control Nanogel Structural Development

Objective: This study examines the relative consumption of acrylate/methacrylate comonomers during polymerizations to yield nanogels from mono-/di-functional combinations of (meth)acrylate monomers with the aim to provide enhanced nanogel structural control.

Method: A mixed acrylate/methacrylate monomer was prepared by reaction of hydroxyethyl acrylate (HEA) with 2-isocyanatoethyl methacrylate (IEM). The desired product (HEAIEM) was confirmed by ¹H NMR spectroscopy. Isobornyl methacrylate (IBMA) or isobornyl acrylate (IBA) was copolymerized in toluene with ethoxylated bisphenol-A-dimethacrylate (BisEMA), ethoxylated bisphenol-A-diacrylate (BisEA) or HEAIEM at an 80/20 mole ratio with 5 mol% mercaptoethanol as chain transfer agent.  The thermal polymerizations were initiated with 2,2-azobisisobutyronitrile at 80°C. Aliquots were taken at 15 min intervals for conversion (mid-IR) and compositional (NMR) analysis up to macrogelation.  Intermediate and final nanogel polymer samples were isolated by precipitation from hexane followed by characterization of molecular weight/hydrodynamic radius by triple-detection size exclusion chromatography.

Result:

The methacrylate group in HEAIEM is consumed faster than the other bonds in IBMA/HEAIEM and IBA/HEAIEM. These reach similar overall conversions but IBA/HEAIEM reaches its maximum conversion in less time with full conversion of IBA and the methacrylate of HEAIEM.  IBMA/BisEMA has higher conversion and size than IBA/BisEMA. The diacrylate is preferentially consumed in IBMA/BisEA and IBA/BisEA.

Material	Overall conversion/time	Mw (Da)	Rh (nm)	Relative Consumption (%)
IBMA/BisEMA	84.2% 105 min	550,000	11.5	62.5:39.1
IBMA/BisEA	65.7% 135 min	54,100	4.2	12.9:20.8
IBMA/HEAIEM	77.5% 120min	69,400	4.1	22.3:31.5:62.8
IBA/BisEMA	55.0% 45 min	48,200	4.4	45:47
IBA/BisEA	72.7% 60 min	284,500	7.3	11.5:16.8
IBA/HEAIEM	74.5% 75 min	239,500	5.5	100:51.5:100

Conclusion: Reactions using IBMA were significantly slower than with IBA. With the exception of IBA/BisEMA, the reactions are similar in conversion and molecular weight is larger with IBA. This work demonstrates an ability to impose control over nanogel branching density at various stages of the reaction process.