AB diblock copolymers via RAFT-mediated miniemulsion polymerization

[摘要] The Reversible addition fragmentation chain transfer (RAFT) technique is a robust andversatile technique that enables the synthesis of polymers of controlled molecularweight and polydispersity. The application of the RAFT technique in heterogeneousaqueous media has attracted great interest in academics and industry due to it beingmore environmentally friendly, besides its other advantages. To date, the synthesis ofwell-defined high molecular weight polymers via the RAFT process under industriallyrelevant conditions still remains a challenge for polymer chemists.The study addresses the application of the RAFT process in heterogeneous media,namely in miniemulsion polymerization, for the synthesis of AB diblock copolymers ofn-butyl methacrylate and styrene.AB diblock copolymers of high molecular weight were successfully prepared via a twostepmethod. In the first step, a dithiobenzoate monofunctional RAFT agent was used inbulk polymerization with the first monomer, n-butyl methacrylate. After thepolymerization, the majority of the polymer chains contained the thiocarbonyl-thioRAFT agent functionality, which makes the chains potentially active for chainextension. The polymeric RAFT agent (also referred to as the starting block) obtained inthe first step was chain extended in the second step, in miniemulsion, upon furtheraddition of fresh initiator and the second monomer, styrene.The effects of the initiator/RAFT agent concentration ratio on the miniemulsion systemswere investigated. The miniemulsion systems used for the high molecular weight ABdiblock copolymers exhibited living features despite the high polydispersity indices.Kinetic results showed an increase in the rate of polymerization throughout thepolymerization. Size exclusion chromatography (SEC) results indicated significantbroadening in the molecular weight distributions and a steep increase in thepolydispersity during the polymerization. It was concluded that the broad molecularweight distributions and steep increase in the polydispersity was not only related to theinitiator concentration but possibly due to other factors such as inhomogeneity in the miniemulsion system and a transition in the kinetic behavior during the polymerization.Secondary particle formation emerged from kinetic data and transmission electronmicroscopy (TEM) results, but this were not supported by the SEC results.The effect of the use of a water-soluble initiator on the miniemulsion system was alsoinvestigated. Results indicated a similar behavioral pattern as observed in the AIBNinitiatedsystems, and not much improvement in terms of the molecular weightdistributions and polydispersity was seen.The effect of the molecular weight of the diblock copolymers on the miniemulsionsystem was investigated. Poly(n-butyl methacrylate)-b-poly(styrene) diblockcopolymers of lower molecular weight were synthesized via the two-step process.Kinetic results indicated a similar behavioral trend as to that of the high molecularweight diblock copolymers synthesized, however SEC chromatograms showednarrower molecular weight distributions and low polydispersity indices.

[发布机构] Stellenbosch University