Cationic homopolymer model networks and star polymers: synthesis by group transfer polymerization and characterization of the aqueous degree of swelling

Abstract

A series of hydrophilic model networks based on homopolymers of a weak base were prepared using group transfer polymerization (GTP) and characterized in terms of their aqueous degree of swelling as a function of pH. This constitutes the first reported example of cationic hydrogels in which the length of segments between cross-links is kept constant. The synthesis involved the GTP of 2-(dimethylamino)ethyl methacrylate (DMAEMA, weak base monomer) in tetrahydrofuran (THF, solvent) using the bifunctional initiator, 1,4-bis(methoxytrimethylsiloxymethylene)-cyclohexane (MTSMC) and the subsequent in situ cross-linking with the addition of 8-fold mole excess with respect to the initiator of the difunctional monomer ethyleneglycol dimethacrylate (EGDMA). Four different model networks with degrees of polymerization (DP) of the linear segments between cross-links of 10, 20, 50 and 100 were prepared. The molecular weight (MW) and molecular weight distribution (MWD) of the linear segments were measured using gel permeation chromatography (GPC) in THF. The degree of swelling of all model networks was measured in water as a function of the solution pH. The conditions of gel synthesis were optimized with respect to cross-linker loading and monomer concentration using the results of a parallel study on the synthesis of star polymers of DMAEMA. The star polymers were also prepared by GTP of DMAEMA and linking (to stars) with EGDMA, but a monofunctional initiator, 1-methoxy-1-trimethylsiloxy-1-methyl-propene (MTS), was used rather than MTSMC. The star polymers were analyzed by GPC which revealed the percentage of free homopolymer relative to star polymer. The optimal conditions were identified as those under which the percentage of unlinked (free) homopolymer is minimized — the same conditions were adopted for the synthesis of the networks.