Efficient multiblock star polymer synthesis from photo-induced copper-mediated polymerization with up to 21 arms†
Photo-induced copper-mediated polymerization (photoCMP) is employed for the synthesis of multiarm-multiblock star copolymers. Based on a core-first approach, star polymers with four, six and twenty-one arms have been synthesized. Due to the high efficiency of the photoCMP process with respect to the reaction rate and end group functionality, each arm could be chain extended multiple times, allowing for up to 8 sequential polymerizations on the four-arm star and up to 4 blocks on the 21-arm star. Relatively high dilutions of the reaction mixtures and premature stopping of the polymerizations between 50–70% monomer conversion must be maintained in order to avoid the occurrence of star–star coupling reactions. For the optimization of the reactions, microreactor polymerizations were used to achieve economic and fast screening of the reactions. Flow reactors show improved control over the polymerizations, and also are beneficial in order to handle large amounts of solvents, which are required to synthesize the multiblock star polymers in significant quantities. By switching between an n-alkyl acrylate and tert-butyl acrylate (that can be hydrolyzed to acrylic acid segments after polymerization) in the chain extensions, hierarchically structured star polymers are obtained in which hydrophilic and hydrophobic blocks are alternated. These materials show pH responsiveness and complex self-assembly in the aqueous phase depending on the number of arms and the length of the individual blocks.