RAFT-mediated Dispersion Polymerization Using Z-type Macro-RAFT Agents

Reversibleaddition-fragmentationchaintransfer(RAFT)-mediated polymerization-induced self-assembly(PISA)offers new opportunities for preparing block copolymer nano-objects with various morphologies at high solids.However,almost all RAFT-mediated PISA formulations are based on R-type macro-RAFT agents.In contrast,Z-RAFT-mediated PISA has rarely been reported.When preparing linear block copolymer nano-objects by R-RAFT-mediated PISA,the RAFT group will be located at the end of the block copolymer and therefore being embedded inside the nano-objects.When preparing linear block copolymer nano-objects by Z-RAFT-mediated PISA,the RAFT group will be located between the solvophobic chain and the solvophilic chain,making the RAFT group locating on the surface of block copolymer nano-objects,which is beneficial for further surface functionalization.For the preparation of star polymers by RAFT polymerization,the leaving group("R"group)of RAFT agent is connected with the multifunctional core,denoated as as the R-RAFT strategy.During R-RAFT-mediated polymerization,radical termination is likely to occur between two star polymers,leading to the formation of high-molecular-weight impurities.In contrast,when the multifuncted core is connected to the Z group of the RAFT agent,radical termination between two star polymers can be avoided during the Z-RAFT polymerization.In this thesis,Z-type macro-RAFT agents were introduced into PISA for the preparation of linear and star block copolymer nano-objects.Polymerization kinetics and morphological evolution process were studied in detail.Effects of reaction conditions on RAFT-mediated PISA were also studied in detail.The results of the study are as follow:1.RAFT-mediated PISA of styrene(St)at 70 ~oC mediated by multifunctional macro-RAFT agents is developed for the preparation of star block copolymer nano-objects.These results show that broad molecular weight distributions were observed when using R-type tetrafunctional macro-RAFT agents,and polydisperse porous spheres were obtained.In contrast,star block copolymers with narrow molecular weight distribution were obtained when using Z-type tetrafunctional macro-RAFT agents,and typical PISA-morphologies including spheres,worms and vesicles were obtained.The morphology of star block copolymer nano-objects can be further controlled by using a mixture of Z-type and R-type macro-RAFT agents.Finally,reversible vesicles with reversible thermo-responsive properties were prepared by further chain extension of thermo-responsive polymers.2.Poly(diacetone acrylamide)Z-type tetrafunctional macro-RAFT agents were employed to mediate photoinitiated RAFT dispersion polymerization of isobronyl acrylate(IBOA)for the preparation of uniform raspberry-like polymer particles.Effects of reaction conditions on the morphology of raspberry-like polymer particles were studied.GPC was utilized to monitor the evolution of polyme chains,providing important insights into the formation mechanism of raspberry-like polymer particles.The formation process of raspberry-like polymer particles was observed during the kinetic study,indicating that the poor RAFT control is important for the formation of raspberry-like polymer particles.Control experiments indicated that the use of PDAAM macro-RAFT agents,IBOA core-forming monomer,and photoinitiation are critical for the formation of raspberry-like polymer particles.3.An asymmetric difunctional RAFT agent was synthesized by linking a R-RAFT agent and a Z-RAFT agent.This asymmetric difunctional RAFT agent was used to mediate RAFT solution polymerization of N,N-dimethylacrylamide(DMA)in 1,4-dioxane.The evolution of molecular weight was studied systematically,indicating the occurrence of polymer chain rearrangement,and the final GPC curve contains three GPC peaks.The rearrangement process of polymer chains can be observed by monitoring the GPC traces at different times.Different monomers were also used,demonstrating the versatility of this phenomeon.Furthermore,monofunctional and symmetric difunctional RAFT agents were also used to mediate the polymerization,confirming using asymmetric difunctional RAFT agent was the key for the rearrangement of polymer chains.Finally,polymer nano-objects were prepared by RAFT-mediated PISA mediated by the obtained RAFT polymer(prepared by using the asymmetric difunctional RAFT agent).Due to the unique structure of this RAFT polymer,a certain number of RAFT reactive groups was located on the surface of polymer nano-objects,allowing the further surface functionalization.