Preparation of zwitterionic microspheres of PDMAEMA-b-PMAA by RAFT dispersion polymerization in alcohol, their pH-sensitivity in water, and self-assembly in KCl solution

Zwitterionic microspheres of poly2-(dimethylamino) ethyl methacrylate (PDMAEMA) block polymethacrylic acid (MAA) were prepared by RAFT dispersion polymerization in alcohols. 4-Cyano-4-(phenylcarbonothioylthio) pentanoic acid (CPDB) was used as the RAFT agent. 2,2′-Azobis(2,4-dimethyl) valeronitrile (V65) was used as the initiator, and PDMAEMA-CPDB was used as the macro-chain transfer agent (CTA). 1HNMR was applied to determine the polymerization degrees of both PDMAEMA-CTA and PDMAEMA-b-PMAA. SEM, dynamic light scattering (DLS), and small-angle X-ray diffraction (SAX) were employed to characterize the microspheres. It was observed that the polarity of alcohols impacted the partition of MAA and V65 between the mini-droplets of PDMAEMA-CTA/MAA complexes and the continuous phase, thereby affected on the diameters of microspheres, the total conversion of MAA, and the polymerization degree of PMAA in the copolymers. For example, in methanol, the smallest diameter and polymerization degree of PMAA in the block copolymers were obtained, whereas the total conversion of MAA of the RAFT polymerization system was the highest. Most of RAFT polymerization of MAA occurred nearby PDMAEMA-CTA chains; thus, PDMAEMA-CTA acted as a template to regulate the length of PMAA in the copolymers. pH-sensitive characteristics of microspheres were dependent to the extra length of PDMAEMA-CTA, i.e., the difference of lengths between two blocks. When the length of PMAA was too short, e.g., PDMAEMA600-b-PMAA87, or the length of PMAA was closed to that of PDMAEMA, e.g., PDMAEMA79-b-PMAA93 and PDMAEMA102-b-PMAA131, the amphoteric characteristics of microspheres disappeared. The self-assembly behavior of diblock copolymers in KCl alcohol-water solution was also determined by the extra length of PDMAEMA. Microspheres turned into nanofibers or nano-ribbons, but for PDMAEMA600-b-PMAA87, the microspheres maintained their original spheric morphology. These results indicated that the charge-screening effects of salts played the key roles in the self-assembly.