Unexpected Reaction Of Inert Vinylic Groups During RAFT(mini) Emulsion Polymerization And Microphase Separated Latex Film Preparation

Generally, structured particles compose of polymers with complementary properties. So, thelatex is termed as a system with balanced film formation property and mechanical strength.Structured latex particles are usually prepared by multistage emulsion polymerizations. Thebasic understanding of the mechanism controlling phase separation and latex morphology hasbeen extensively reviewed. However, contradictory results are often found in the literaturesbecause latex particle morphology is complicatedly influenced by an interplay of thermodynamicas well as kinetic parameters during the polymerization process. On the other hand, upon filmformation, the uncertainty of particle morphology results in an uncertain phase distribution in thefilms. The adjustment of the morphology of phase separation in films and therefore the filmmechanical properties are often arbitrary and based on trial-and-error. Considering this point, weproposed a feasible method of preparation latex film with stable morphology. The film-formationsystem is based on block copolymer latex particles with vinyl groups-enriched surface. Duringits film forming process, the “thiol-ene” click reaction (TEC) confined at the colloidal interfacewas initiated in a controlled manner, which led to the connection of polymer chains in differentparticles. So, a latex film with stable morphology can be attained, whose microphase separationstructure is built up based on the particle structure.At first, two kinds of amphiphilic macroRAFT agents with cyclohexenyl groups and alkenylgroups are prepared, respectively. These inert vinylic groups are all confined between thehydrophilic and hydrophobic blocks. Then, the propagation selectivity towards styrene (St)relative to inert vinylic group are compared among RAFT solution, RAFT mini-emulsion andRAFT emulsion polymerizations. Based on the findings, two-stage RAFT (mini-)emulsionpolymerizations are employed to prepare polybutyl acrylate-polystyrene (PBA-b-PSt) diblockcopolymer latex particles with cyclohexenyl group-enriched surface. Furthermore, these particlesare observed to have “shell-core” morphology. During the latex film formation process, the TECis initiated at scheduled stage. It leads to the film with spherical morphology as well as improvedmechanical strength. The following points are concluded:(1) Both RAFT solution and miniemulsion polymerizations exhibited superb selectivity toward St relative to the cyclohexenyl vinylic unit. The resulting polymers retained quantitativependent cyclohexenyl functionalities, predictable molecular weights together with narrowedmolecular weight distributions. A different case was found during RAFT emulsionpolymerization. Copolymerization between cyclohexenyl group and St occurred during thenucleation stage, leading to consumption of cyclohexenyl groups, positive deviations ofmolecular weights from the theoretical values and slightly broadened molecular weightdistributions.(2) Compared with cyclohexenyl groups, alkenyl vinylic bonds have higher reactivity. Thealkenyl vinylic bonds were retained only during low conversion stage of RAFT solutionpolymerization. In contrast, in RAFT mini-emulsion and RAFT emulsion polymerization, thealkenyl vinylic bonds copolymerized with St since the very beginning of reaction. It causedpositively deviated molecular weights and slightly broadened molecular weight distributions.(3) It is further confirmed by purposely designed experiments that both the consumption ofcyclohexenyl groups and alkenyl vinylic bonds is a combined result of their chemical structureand the heterogeneous characteristics of (mini-)emulsion.(4) The enrichment of cyclohexenyl groups on the particle surface facilitated their TECreactions, which is demonstrated by their high conversion as much as80%.(5) The PBA-b-PSt “shell-core” latex can be prepared by RAFT batch-semibatch two-stage(mini-)emulsion polymerization. After TEC reactions, the resulting latex films have stablemicrophase separation structure as well as improved mechanical properties. As a result of thesurface enrichment of the cyclohexenyl group, TEC reactions attain high efficiency, which bearsthe primary responsibilities for the higher mechanical performance found for the film formedfrom mini-emulison polymerization than their emulsion reaction counterparts.