Two novel double hydrophilic multiblock copolymers of N,N-dimethylacrylamide and N-isopropylacrylamide, m-PDMAp-PNIPAMq, 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 1H NMR, and the block numbers, i.e., number of PDMAp-PNIPAMq 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-PDMA42-PNIPAM37 and m-PDMA105-PNIPAM106 multiblock copolymers possess number-average molecular weights (Mn) of 4.62x10(4) and 9.53x10(4), respectively, and the polydispersities (Mw/Mn) 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 (approximately 20) and m-PDMAp precursors (approximately 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-PDMA42-PNIPAM37 and m-PDMA105-PNIPAM106 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-PDMA105-PNIPAM106 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-PDMA42-PNIPAM37 multiblock copolymer exhibits collapse and intermolecular aggregation, forming associated multimolecular 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 biomaterials and biomedicines.

A thermally sensitive ultralong multiblock copolymer, [poly(ethylene oxide) 23 -b-poly(N-isopropylacrylamide) 124 ] 750 (M w ) 1.78 × 10 7 g/mol and M z /M w ) 1.49), was prepared using the oxidative coupling of two mercapto groups at the two ends of triblock PNIPAM 62 -b-PEO 23 -b-PNIPAM 62 (M n,PEO ) 1.0 × 10 3 g/mol and M n,PNIPAM ) 1.4 × 10 4 g/mol) copolymer chains. The folding of individual multiblock copolymer chains in an extremely dilute solution (10 -6 g/mL) was studied by laser light scattering (LLS). Moreover, the association of multiblock and triblock copolymer chains in relatively concentrated aqueous solutions (10 -3 g/mL) was also comparatively studied by a combination of LLS, fluorescence spectrometry, and microcalorimetry. We found that in the single-chain folding process the average radius of gyration (〈R g 〉) remains a constant in one heatingand-cooling cycle, but the average hydrodynamic radius (〈R h 〉) decreases as the solution temperature increases. Our result reveals that the single-chain folding undergoes two stages at ∼32 and ∼40 °C, presumably due to the successive contraction of thermally sensitive PNIPAM segments in the middle around each hydrophobic S-S coupling point and near the hydrophilic PEO block. Each PNIPAM block collapses into a small globule (bead) stabilized by the two attached PEO blocks on the chain backbone, a string-bead conformation, which makes the chain thicker and more extended. The association of multiblock chains also undergoes similar two stages to form stable mesoglobules during the heating. In contrast, the triblock chains associate at ∼30 °C to form polymeric micelles.