Reversible Addition-fragmentation-chain Transfer Grafting Polymerization From Silicon Wafer

The recent development of controlled/living radical polymerization(CLRP) provides a great opportunity in preparing well-controlled and well-defined polymer brushes from various surfaces.Reversible addition fragmentation transfer(RAFT) polymerization,based on a series of reversible degenerative chain transfer reactions between radicals and dormant species,has emerged as another major CLRP technique.In this work,RAFT grafting polymerizations from silicon wafer were investigated.Three types of RAFT-silane coupling agents, (3-(2-Dithiobenzoatepropionyl)propyl)dimethylmethoxysilane(RAFT1,with phenyl Z group),(3-(2-dithioisobutyratepropionyl)propyl)dimethylmethoxysilane (RAFT2,with isopropionyl Z group),and(3-(2,2'-(2-Dithiobenzoatepropionyl) methylpropionyl)propyl)dimethylmethoxysilane(RAFT3,with phenyl Z group) were synthesized.These RAFT agents were then immobilized onto silicon wafer through R group,and used to mediate the grafting polymerization from silicon wafer.The suface grafting polymerization of three types of monomer:styrene(St), butyl acrylate(BA)and methyl methacrylate(MMA)were carried out in the presence of "free" RAFT agent with the close reactivity to the immobilized RAFT agents,which was added to the monomer solution,respectively.Both XPS and ellipsometry confirmed the formation of grafting polymer layer.The grafting layer grew linearly with polymerization time and monomer conversion,indicating that the grafting polymerization should be "living"/controlled.It was found that Z group of RAFT agent,type of monomer,and grafting density all play significant role in determining the polymer thickness.Contrast to ATRP and NMP,the thickness of the grafting polymer layer seems independent of the molecular weight of the polymer formed in the solution among the different systems.The reason for this is the inhibition/retardation in polymerization rate,which is mostly often observed in a RAFT polymerization.The more severe the inhibition and retardation is,the thinner the thickness is,due to the termination between the intermediate radical on the surface and the propagation radical in the solution.A grafting polymerization model was developed to put more insights of this complex system.From comparing the model with experimental data,it is suggested that:(1)In the presence of the free RAFT agent,the radical on the surface could escape to the solution via RAFT reactions while the RAFT group density on the surface remained stable.Such,the radical density was reduced, lowering the thickness of the grafting polymer layer.(2)In the absence of the free RAFT agent,the RAFT groups on the surface would lose graducally to the solution. Fortunately,the losing rate seemed not so fast as expected,most likely due to the diffusion controlled reaction.A much higher thickness(～56nm)of the grafting layer could be achieved.