Control of Intermolecular Cross-Linking Reaction in Free-Radical Cross-Linking Monovinyl/Divinyl Copolymerizations by the Aid of Amphiphilic Nature of Primary Polymer Chains and Cross-Link Units with Opposite Polarities

The control of the intermolecular cross-linking reaction in free-radical cross-linking monovinyl/divinyl copolymerizations resulting in amphiphilic network polymers was attempted by the aid of amphiphilic nature of primary polymer chains and cross-link units with opposite polarities. First, benzyl methacrylate (BzMA)/tricosaethylene glycol dimethacrylate (PEGDMA-23) (95/5) copolymerizations in 1,4-dioxane (DO) and DO/H2O (85/15) mixed solvent were investigated. The assembly of short primary polymer chains induced the restricted motion of propagating polymer radical, leading to a suppressed increment of molecular weight with conversion. Second, 2-hydroxyethyl methacrylate (HEMA)/henicosapropylene glycol dimethacrylate (PPGDMA-21) (95/5) copolymerizations in DO, DO/H2O (80/20) mixture, and DO/H2O (70/30) mixture were explored. The assembly of nonpolar cross-link units led to an enhanced occurrence of intermolecular cross-linking reaction. Moreover, the assembly of polar primary polymer chains consisting of poly(HEMA) in DO suppressed the cross-linking reaction as is the case with BzMA/PEGDMA-23 (95/5) copolymerizations. Then, the solvent effect on gelation was generalized by using various solvents. Especially, our attention was focused on acetonitrile (ACN) and N,N-dimethylformamide (DMF) as solvents because the resulting gel in HEMA/PPGDMA-21 (95/5) copolymerization swelled in DMF but shrank in ACN despite almost the same solubility parameters for both solvents. Thus, both primary polymer chains and cross-link units of amphiphilic prepolymers would not assemble in DMF, whereas primary polymer chains having hydroxyl groups would assemble in ACN. The assembly of short primary polymer chains in ACN led to a suppressed increment of molecular weight with conversion as a reflection of the restricted motion of propagating polymer radical. Conclusively, the assembly of short primary polymer chains suppressed the occurrence of intermolecular cross-linking reaction, whereas the assembly of long cross-link units promoted it.