Controlled/ "Living" Free Radical Photopolymerization Of Methyl Methacrylate In Microemulsion

Microemulsions are thermodynamically stable, isotropic, optically transparent dispersionsmade up of two liquids, incompatible with each other, stabilized by surfactants and cosurfactants.Recently, increasing attention has been paid to it by academic scholars. In this paper,Controlled/living photopolymerization of methyl methacrylate(MMA) mediated by 4-hydroxy-2,2,6,6-tetramethyl-piperidinyl-1-oxyl(HTEMPO) and 1,2,2,6,6-pentamethyl-4-piperidinol(PMP)have been performed in anionic and cationic microemulsions. The mechanism and kinetic rule ofcontrolled free radical photopolymerization of MMA in microemulsion was further studied, andthe porous polymer materials from microemulsions were analyzed. The main work andconclusions (were listed) as following:(1) The phase diagrams of MMA/n-butanol/SDS-H₂O system, MMA/AA/SDS-H₂O system andMMA/CTAB/H₂O system were described by visual and conductivity method. The variation inphase's structure of microemulsion was measured by conductometer, the results showed that theconductivity of microemulsion changed with increasing the aqueous phase, and indicated that thesystem transfered from water in oil (W/O) microemulsion through bicontinuous microemulsionto oil in water (O/W) microemulsion. The particles analysis showed that there was no change innumber orders between the partical sizes of MMA microemulsion and latex particals afterphotopolymerization.(2) After selecting experiment conditions, the kinetic influence of MMA microemulsionphotopolymerization in O/W and W/O emulsion systems, mediated by HTEMPO andPMP/HTEMPO, was studied using Irgacure184 as an initiator, SDS as a surfactant, n-pentanoland n-butanol as a cosurfanctant at ambient temperature. The results were shown that obtainedthe MMA homopolymers (in the O/W microemulsion) molecular weight were arsy-varsy, whichwas characterized by GPC, when using n-pentanol as a cosurfanctant, although thepolydispersity was narrow, the number-average molecular weight (M_n) was lower. When usingn-butanol as a cosurfanctant, plots of M_n vs. conversion and Ln([M₀]/[M]) vs. time were almostliner respectively, but the polydispersity were higher (1.61～1.79), indicating that HTEMPObrought controlling effect to the photopolymerization whereas the result was not ideal. It was found that the M_n of the MMA homopolymers increased linearly and relativelynarrow molecular weight distributions(PDI0]/[M]) vs. time were liner respectively, and obtained the MMAhomopolymers exhibited relatively narrow molecular weight distributions (1.23～1.36), evenuntil the conversion was up. This showed that HTEMPO combined with PMP used as a mediatorwas evidently available.When CTAB was used as an emulsifier in the O/W microemulsion of MMA withoutcoemulsifier, the conversion vs. time curve was abnormal. The main reason was that MMAconcentration was too low and the rate of chain propagation was restrained soon after thepolymerization. However, it was found that obtained plots of conversion vs. time andLn([M]₀/[M]) vs. time were linear respectively, and the smallest value of polydispersity indexwas 1.21 in MMA/CTAB-H₂O/toluene/n-butanol system. The polydispersity was low (＜1.37)even under high conversion. So it was controlled by HTEMPO.(3) Based on stable polymerization, the influence of aqueous phase concentration, differentemulsifier and coemulsifier on the microporous morphology of the obtained products frommicroemulsions polymerization were analyzed with environmental scanning electron microscope.The results showed that there was a certain correspondence in morphology between precursormicroemulsions and final products. Inverse microemulsion percolating systems yielded polymermaterials with a closed-cell porous structure, and some of the holes began to crack and connect.Open-cell porous polymers were obtained by the polymerization of microemulsions with abicontinuous structure.