Novel Heterodiels-Alder(HDA)-Cascade RAFT Polymerization Reaction System For Branched/Cross-Linked Polymers

The hetero Diels-Alder(HDA)-cascade RAFT polymerization reaction(RAFT-HDA)is efficient and fast with mild reaction conditions,allowing for fast and reliable linkage of different functionalized building blocks.Due to the limited variety of dienophiles and the complexity of dienophore preparation,the development studies related to this reaction are somewhat limited.In this paper,we explore both diene and dienophiles: on the one hand,we explore new dienophores to expand new RAFT-HDA systems and,at the same time,develop a simple and efficient method to prepare covalently cross-linked stable microgels with tunable properties;on the other hand,we explore new dienophores to expand the variety of monomers available for RAFT-HDA systems.A new platform for the preparation of hyperbranched polymers,RAFTHDA-ABx,was constructed.Furan rings,which are highly reactive and present in a variety of commercial monomers,were selected for investigation as new dienophores.Bifunctional dienophile(S)was constructed by preparing doubleterminated phosphoryl disulfide mediated styrene(St)polymerization,and polyfunctional diene building block(SF)was constructed by RAFT polymerization mediated copolymerization of furfuryl methacrylate(FMA)with St.A series of tests including NMR and gel permeation chromatography(SEC)were performed to characterize the feasibility of the furan ring as a new diene in the RAFT-HDA reaction.The amphiphilic random cross-linked network was constructed by the HDA reaction using the styrene building block(S)as a dienophile and hydrophobic segment,the copolymer of polyethylene glycol monomethyl ether acrylate(OEGMA)and FMA as a diene building block and hydrophilic segment,then be further assembled to form microgels.Dynamic light scattering(DLS)and transmission electron microscopy(TEM)results confirmed microgel particle sizes in the range of several hundred nanometers to microns.Due to the controlled nature of RAFT polymerization,the effects of single-molecule chain functional group degree(f),furan/dithioester ratio(r),polystyrene chain segment length(D),and water content on the microgel system were investigated.Besides the dynamic nature of the RAFT-HDA reaction confers force responsiveness to the microgels.It should be noted that although the microgels are basically spherical in shape,some special morphologies may occur due to the rigidity of the hydrophobic styrene segments and are worthy of further study.Based on the above work,the types of dienophile RAFT reagents(Boron)suitable for the RAFT-HDA reaction are further expanded.Preparing dienophilic building blocks(M)by polymerization of methyl acrylate(MA)mediated by a new type of disulfide containing pyrazole group(Boron type RAFT reagent),polyfunctional diene building blocks(MF)by copolymerization of FMA and MA mediated by RAFT polymerization.The comb-type polymer was obtained by HDA reaction between the two building blocks,and the characterization results verified the availability of such dithioesters in the RAFT-HDA system.Subsequently,this system was extended to different types of monomers(St,N,N-dimethylacrylamide(DMA),methyl methacrylate(MMA))and a series of different monomer-based comb polymers were prepared to verify the versatility of these RAFT reagents for different monomers in the RAFT-HDA reaction.Then,the ABx macromolecules were constructed by the FMA polymerization mediated by such pyrazole-based dithioesters,and the hyperbranched polymers were innovatively prepared by the HDA reaction between the dithioester group A and the furan ring group B.The structure and properties of the hyperbranched polymers were characterized accordingly,which proved the feasibility of this method and developed a new platform for the simple and efficient construction of hyperbranched polymers.