Alternating Amphiphilic Multiblock Copolymers: Controlled Synthesis via RAFT Polymerization and Aqueous Solution Characterization

Four linear amphiphilic multiblock copolymers based on 2-(dimethylamino)ethyl methacrylate (DMAEMA, hydrophilic) and methyl methacrylate (MMA, hydrophobic), bearing from two to five blocks, were synthesized using reversible addition-fragmentation chain transfer polymerization and stepwise monomer additions. The degree of polymerization of each hydrophilic polyDMAEMA block was similar to 50, while that of the hydrophobic polyMMA blocks was ca. 20. The stepwise monomer addition procedure secured that each higher multiblock was composed of its immediately lower homologue plus exactly one extra block. The molecular weights (MW) of the synthesized (co)polymers, as measured by gel permeation chromatography (GPC), were close to the theoretically expected. GPC also indicated rather narrow molecular weight distributions which, however, broadened with the number of blocks. The compositions of the copolymers, as determined by proton nuclear magnetic resonance spectroscopy, were also close to the expected values. The cloud points of the copolymers. Measured by turbidimetry, Were found to increase With the number of blocks. The sizes of the micelles formed by the copolymers in aqueous solution were characterized using dynamic light Scattering and small-angle neutron scattering. The determined values of the hydrodynamic radii, the radii of gyration, rind the aggregation numbers of the micelles increased slightly with the block number from the triblock to the pentablock copolymer, implying extensive folding of the chains of the tetrablock and the pentablock copolymers in their micelles. The aggregation number of the tetrablock copolymer was approximately one-half that of the diblock copolymer whose unimer M W was one-half that of the tetrablock, suggesting that the micellar cores of these two types of micelles were composed of the same number of hydrophobic units.