RAFT-synthesized Diblock and Triblock Copolymers: Thermally-Induced Supramolecular Assembly in Aqueous Media

This review highlights recent advances in the synthesis of functional, temperature-responsive, water-soluble block copolymers, including particular focus on the results obtained by employing reversible addition-fragmentation chain transfer ( RAFT) polymerization. The applicability of the RAFT process for the polymerization of functional monomers under a diverse range of experimental conditions has facilitated the synthesis of water-soluble (co) polymers that were previously inaccessible. Unprecedented control afforded by RAFT in homogeneous aqueous media allows well-defined polymeric systems to be prepared without stringent purification techniques and under increasingly ``green\u27\u27 conditions while maintaining the ability to tailor many of the macromolecular characteristics ( molecular weight, chain topology, copolymer composition, functionality, etc.) that affect self-assembly in solution. Block copolymer formation and postpolymerization modification utilizing crosslinking and copper-catalyzed azide-alkyne click\u27\u27 chemistry are described, with attention being paid to their ability to control copolymer structure for subsequent self-assembly in response to changes in temperature

Aqueous RAFT Synthesis of Micelle-Forming Amphiphilic Block Copolymers Containing N-Acryloylvaline. Dual Mode, Temperature/pH Responsiveness, and Locking of Micelle Structure Through Interpolyelectrolyte Complexation

Temperature- and pH-responsive, micelle-forming, amphiphilic block copolymers were prepared from N,N-dimethylacrylamide (DMA), N-isopropylacrylamide (NIPAM), and N-acryloylvaline (AVAL) utilizing aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization. A series of block copolymers were synthesized by employing DMA as a macro-chain transfer agent to mediate the statistical copolymerization of NIPAM with AVAL. Structural organization and solution behavior were investigated utilizing dynamic light scattering, two-dimensional NMR spectroscopy, and transmission electron microscopy. It has been demonstrated that the critical micellization temperature for the block polymers can be tuned to range from approximate to 10 to 36 degrees C by adjusting the solution pH. Micelles with apparent hydrodynamic diameters from 45 to 86 nm are formed between pH 2 and 5. Above pH 5, a sufficient number of the AVAL units are deprotonated which prevents micellization. The extent of pH and temperature changes on the apparent hydrodynamic diameters have been illustrated via 3-D plots. Significantly, micelles assembled within a specified range of pH and temperature can be locked by interpolyelectrolyte complexation of anionic AVAL segments with those of a cationic polymer, in this case a RAFT-generated poly([ar-vinylbenzyl]trimethylammonium chloride) (PVBTAC). When the temperature is lowered to room temperature, the polymeric micelles remain locked in their multimeric structures which remain dispersed in water. Addition of 0.3 M NaCl to the aqueous solution results in dissociation of the complexes into the respective water-soluble components

Chiroptical Properties of Homopolymers and Block Copolymers Synthesized From the Enantiomeric Monomers \u3ci\u3eN\u3c/i\u3e-acryloyl-L-alanine and \u3ci\u3eN\u3c/i\u3e-acryloyl-D-alanine Using Aqueous RAFT Polymerization

Chiral homo- and block copolymers based on the enantiomeric monomers N-acryloyl-l-alanine (ALAL) and N-acryloyl-d-alanine (ADAL) were prepared directly in water using controlled reversible addition–fragmentation chain transfer (RAFT) polymerization. The polymerization of the chiral monomers proceeded in a controlled fashion producing the respective homopolymers, block copolymers, and a statistical copolymer with targeted molecular weights and narrow molecular weight distributions. The chiroptical activity of these biomimetic polymers and their analogous model compounds was investigated using circular dichroism (CD). P(ALAL) and P(ADAL) were shown to be optically active exhibiting mirror image CD spectra. In addition, statistical and enantiomeric block copolymers prepared at 1:1 stochiometric ratios exhibited virtually no optical activity