Methyl Methacrylate Polymerization Mediated By Diarvlnitroxides

Controlled/living radical polymerization(CLRP)has become an easy method toprepare well-defined(co)polyrmers with predictable molecular weight,narrow molecularweight distribution and end group functionalities.These properties are required to obtainmaterials with complex constrmction.Among the various techniques currently used forCLRP,nitroxide-mediated polymerization(NMP)was the first one to be developed and hasattracted a renewal of interest because of its simplicity,since in most cases it requires onlyadding the suitable nitroxide or alkoxyamine to the polymerization system.Neverthelessone of the main reproached that is still made to NMP is related to the difficulty to controlthe methacrylate derivatives.This was explained firstly by the occurrence of ?-hydrogentransfer from the propagating radical to the nitroxide and secondly by the too highactivation-deactivation equilibrium radicals.Up to now,most of the nitroxides studied in NMP are dialkyl or alkyl-arylnitroxides,while few studies were devoted to the use of diarylnitroxides in NMP.This dissertationpresented the CLRP of methyl methacrylate(MMA)mediated by two diarylnitroxideswhich could be easily synthesized by oxidation of their corresponding amines in high yield.The radical delocalization on the phenyl ring would decrease the electron density one theoxygen and avoid hydrogen transfer side reaction.In a preliminary study,diphenylnitroxide(DPN)was synthesized and used as acontrol agent to mediate the polymerization of MMA.Under both bimolecular(DPN/azobisisobutyronitrile(AIBN)pair)and unimolecular(DPN-based alkoxyamineinitiator)initiating system,the kinetic plot was pseudo-first-order and the number-averagemolecular weight(Mn)increased linearly with conversion up to about 50%.But theprepared polymer showed a bimodal molecular weight distribution and the PDI was higherthan 1.5,which was due to the low thermal stability of DPN,Considering the poor controllability of DPN over MMA polymerization,4,4,-dimethoxydiphen-yl nitroxide(DMDPN)was then used as a control agent.Thepresence of methoxy groups in para position of phenyl rings makes DMDPN much morestable than its parent radical DPN.Under the bimolecular(DMDPN/AIBN pair)initiatingsystem,the molecular weight distribution was unimodal and a linear increase of Mn withconversion was observed up to 60%,despite the PDI was still higher than 1.5.Greatimprovement was achieved when a unimolecular initiating system(DMDPN-basedalkoxyamine initiator)was introduced.The Mn increased linearly with conversion up to about 65%,the PDI was 1.2?1.4 all through the polymerization and the living chainfraction was more than 86%.~1H NMR results indicated that DMDPN was attached to endof the polymer chains and the side reaction of ?-H transfer from the propagatingmacroradicals to the free nitroxide was neglected.The living character of the preparedPMMA was also confirmed by chain extension from the prepared PMMA macroinitiatorwith MMA or styrene(St).The dissociation rate constants(kd)and recombination rateconstants(kc)of the DMDPN-based alkoxyamine initiator and the PMMA-DMDPNmacromolecular were determined by electron spin resonance(ESR).Due to thepenultimate unit effect,a 6-times increase of kd between the alkoxyamine initiator andmacromolecular was observed.According to Fischer's diagram,it was found that the kdand kc value of the DMDPN/MMA system at 110? were suitable for the NMP of MMA.Though DMDPN was capable of controlling the polymerization of MMA,thepolymerization rate was very slow.30 h was required to reach a high conversion.Toaccelerate the polymerization rate,several compounds were introduced into thepolymerization system.A long half-life time radical initiator,dicumyl peroxide(DCP),wasfirst used as the accelerator.The more the amount of DCP used,the faster thepolymerization rate,but the PDIs were higher than 1.5 and there was a large discrepancybetween the theoretical and calculated molecular weights.When camphorsulfonic acid(CSA)was used,a dramatic increase in polymerization rate was observed,but thepolymerization process was not "living" as the over consuming of DMDPN by CSA.Whenthree strong polar compounds,diethyl malonate(DEM),acetylacetone(AAT)andmalonitrile(MN)were used,the stronger the polarity of the compounds,the better theaccelerating effect.A conversion of 45%was achieved in 3 h at 110 ? and[MN]/[alkoxyamine]molar ratio of 3:1,and the PDI was even lower than that without anadditive.The polymerization of St mediated by DMDPN was investigated.Compared with theDMDPN/MMA system,the polymerization rate of St was quite slow and a higherpolymerization temperature was needed.At 130?,the kinetic plot was pseudo-first-orderand the molecular weight distribution was unimodal.A linear increase of Mn withconversion was observed up to 65%.But the PDI of the prepared PS was relative high(PDI=1.5-2.5)due to incompatible kd and kc value for the DMDPN/St system.