Synthesis and characterization of branched water-soluble homopolymers and diblock copolymers using group transfer polymerization

Group transfer polymerization [GTP] was used to synthesize branched statistical copolymers by copolymerizing either 2-(dimethylamino)ethyl methacrylate) [DMA] or 2-(diethylamino)ethyl methacrylate) [DEA] with ethylene glycol dimethacrylate (EGDMA) in THF at 20 °C. Since GTP has reasonable “living” character, it allows good control over both the primary chain length and the molecular weight distribution compared to previous branched vinyl polymers synthesized using conventional radical polymerization. Using GTP allows a remarkably high proportion of EGDMA brancher to be copolymerized without causing macrogelation. This unexpected result is attributed to a significant amount of intramolecular cyclization occurring (in addition to intermolecular branching) in these syntheses. Branched diblock copolymers based on DMA and DEA were prepared by sequential monomer addition, with EGDMA being used to achieve branching in either the DMA block or the DEA block or in both blocks. The order of monomer addition was varied to examine whether branching affected the “living” character of the polymerization. There was some evidence for better blocking efficiencies if the first block was a linear homopolymer, rather than a branched block copolymer. 1H NMR spectroscopy indicated that very high comonomer conversions (>99%) were obtained in all cases. The branched diblock copolymers were characterized in terms of their block compositions and primary chain lengths using 1H NMR, and their molecular weight distributions were assessed by THF GPC using a light scattering detector to obtain absolute Mw values. The evolution of molecular weight with conversion was assessed by periodic sampling of the polymerizing solution. Dynamic light scattering (DLS) and surface tensiometry data obtained for dilute aqueous solutions of the branched block copolymers were compared to those obtained for a linear diblock copolymer. Similar surface tension profiles were obtained regardless of the block architecture, but DLS studies indicated that larger, more polydisperse micelles were obtained if the coronal block was branched.