Thermo-induced formation of unimolecular and multimolecular micelles from novel double hydrophilic multiblock copolymers of N,N-dimethylacrylamide and N-isopropylacrylamide

Two novel double hydrophilic multiblock copolymers of N,N-dimethylacrylamide and N-isopropylacrylamide, m-PDMA(p)-PNIPAM(q), With varying degrees of polymerization (DPs) for PDMA and PNIPAM sequences (p and q) were synthesized via consecutive reversible addition-fragmentation chain transfer (RAFT) polymerizations using polytrithiocarbonate (1) as the chain transfer agent (Scheme 1), where PDMA is poly(N,N-dimethylacrylamide) and PNIPAM is poly(N-isopropylacrylamide). The DPs of PDMA and PNIPAM sequences were determined by H-1 NMR, and the block numbers, i.e., number of PDMA(p)-PNIPAM(q) sequences (n), were obtained by comparing the molecular weights of multiblock copolymers to that of cleaved products as determined by gel permeation chromatography (GPC). m-PDMA(42)-PNIPAM(37) and m-PDMA(105)-PNIPAM(106),06 multiblock copolymers possess number-average molecular weights (M-n) of 4.62 x 104 and 9.53 x 104, respectively, and the polydispersities (M-w/M-j are typically around 1.5. Block numbers of the obtained-multiblock copolymers are ca. 4, which are considerably lower than the numbers of trithiocarbonate moieties per chain of 1 (similar to 20) and m-PDMA(p) precursors (similar to 6-7). PDMA homopolymer is water soluble to 100 degrees C, while PNIPAM has been well known to exhibit a lower critical solution temperature (LCST) at ca. 32 degrees C. In aqueous solution, m-PDMA(42)-PNIPAM(37) and m-PDMA(105)-PNIPAM(106) multiblock copolymers molecularly dissolve at room temperature, and their thermo-induced collapse and aggregation properties were characterized in detail by a combination of optical transmittance, fluorescence probe measurements, laser light scattering (LLS), and micro-differential scanning calorimetry (micro-DSC). It was found that chain lengths of PDMA and PNIPAM sequences exert dramatic effects on their aggregation behavior. m-PDMA(105)-PNIPAM(106) multiblock copolymer behaves as protein-like polymers and exhibits intramolecular collapse upon heating, forming unimolecular flower-like micelles above the thermal phase transition temperature. On the other hand, m-PDMA(42)-PNIPAM(37) multiblock copolymer exhibits collapse and intermolecular aggregation, forming associated rnultimolecular micelles at elevated temperatures. The intriguing aggregation behavior of this novel type of double hydrophilic multiblock copolymers argues well for their potential applications in many fields such as biornaterials and biomedicines.