| Since the concept ofLiving/Controlled Free Radical Polymerization(CLRP)was proposedin 1982,more than 30 years have passed.Enormousamountofpolymerizationmethods basedon the most successful three CLRP techniqueswhich are Nitroxide-mediated polymerization(NMP),atom transfer radical polymerization(ATRP)as well as reversible addition-fragmentation chain transfer polymerization(RAFT)have been developed just within the last 30 years.However,those methods are suffering from inherent drawbacks andthe limitation of practical applying.Therefore,the invention of new CLRP technique that is simpler,more efficient and compatible with traditional industrial processes is still in eager demand.In the privous study of our reaserch group,a new CLRP method named as cycloketyl radical mediated polymerization(CMP)was proposed,and improvedin thesubsequent reaserch that brought about the 9H-xanthene-9-thione(XT)-mediated polymerization.Research topics in this thesis inheritedthis technique.Furthermore,systematic experiments and discussionswith the intention ofameliorating the controllability of XT-mediated polymerization and revealing the mechanism were carried out.The main research items are as follows:1.A monomer vinyl acetate(VAc)which has a certain value in industrial application was researched to polymerize in the presence of XT in bulk and solution system.Theoretical molecular weight and molar ratio of initiator to XT was taken as the system variables in a set of parallel bulk polymerizations.Insolutionpolymerization,monomer concentration was designated as the object of investigation.After the investigation on polymerization behavior of VAc in the presence of XT,PVAc macromolecular initiator was synthesized to initiate styrene and methyl methacrylate.Experimental results show that XT loses its controllability over the polymerization of VAc.An induction period has been observed at the beginning of reaction,and the duration of this induction period was positive correlated with the amout of XT.In the bulk polymerizations with theoretical a Mn of 2000,and the ABVN/XT equaling 2/1 and 1.5/1,the induction period were 5 hours and 21 hours respectively.when that ratio shifted to 1.3/1,no polymeric product was obtained.As the induction ends,polymerization begins immediately,and the conversion increased very fast.Toluene were conducted to avoid gel effect.But,the solution polymerization behaviors were similar to that of bulk polymerization under the situation of ABVN/XT=1.5/1.The Conversions were about 22%in those solution polymerizations.TheMnandPDIincreasedwiththeelevatingofmonomerconcentration.However,the exsit of active groups on the resulting PVAc-XT molecules was provedviaNMRspectra.Reinitiationsof styrene(St)and methyl methacrylate(MMA)were realized by PVAc-XT macroinitiator in THF.The Mnof reinitiation products of St and MMA increased to64000 and 58000 respectively,comparing the Mnof PVAc-XT equaling 42000.Based on the results above,XT acted as an inhibitor in the polymerization of VAc,and consumed the primary radical.Polymeric product could only be obtained when the XT was vanished.Thought,XT dosen't have the effect of control over polymerization of VAc,the resulting PVAc-XT possesses initiating ability to other monomers.2.Aiming to improve the controllability of XT-mediated radical polymerization,the substituent effect on polymerization behaviors was investigated viashifting the electronic effect of substituent groups onXT's aromatic ring.Four thioketones with various substituents,termed 2,7-dinitro-9H-xanthene-9-thione(DNXT),2-trifluoromethyl-9H-xanthene-9-thione(TfXT),9H-xanthene-9-thione(XT),N-phenyl-acridine(N-PhAT),and 2,7-bis(dimethylamino)-9H-xanthen-9-one(BDMAXT),were successfully synthesized and characterized via 1H-NMR and 13C-NMR.Polymerizations of MMA in toluene in the presence of ABVN and equimolar ofthose thioketones(including XT)were brought into practice.Subsequently,DNXT with electro-withdrawing substituent groups made the Mn of the as-formed PMMA increase from 2700 to 17000,accompanying the PDI over 2.0.polymerization mediated by TfXT demonstrated a Mn growth from 8100 to 18000,and the PDI was below 2.0.The controllability of polymerizations mediated by thioketones connected with electro-donating groups was comparingly weak.Then,the factors of molar ratio of ABVN to thioketones,temperature and solvent were discussed detailedly in the polymerizationmediated by DNXT and TfXT.Those reactions followed first-order kinetics.And the conversion,reaction rate and initial Mn decreased with the elevating of thioketone's concentration.Elevated temperature could accelerate the polymerization andreduce the initial Mn as well as increase the growth speed of Mn with the rising conversion.Besides,polymerization of MMA in toluene and DMF exerted same behavior.However,DMSO brought about higher polymerization rate and conversion,lower Mn of product,and narrower PDI than that of polymerizations in toluene and DMF.Furthermore,TfXT-mediated polymerization of tBMA was also studied under the identical condition with MMA.In comparison to MMA,polymerization kinetics of tBMA deviated from first-order process,but still was under control as the Mn linearly increasing with conversion.A ESI-TOF-MSanalysis was conducted to characterize thepolymeric product of TfXT-mediated polymerization of MMA.Two groups of spectral lines related to the cross-termination product and its fragments generated by heterolytic cleavage of C-S bond were observed.Based on the experimental evidence,a mechanism was deduced as follows:first,the propagating chain radical is reversible captured by thioketone's thiocarbonyl,resulting in the formation of adduct radical;Second,anequilibrium of coupling and homolytic cleavagebetween the adduct radical and primary radical was established.Therefore,the concentration of propagating radical was reduced,leading to the polymerization becoming controlled.3.Based on the fact that TfXT mediated polymerization of MMA carried living characteristics,a "one-pot two-step" procedure was utilized to prepare a PMMA-b-(PtBMA-co-PMMA)block copolymer.In the situation of ABVN/TEXT equaling 1/2,Mn of the precursory PMMAand the consequent block copolymer was 11300 and 23900 respectively.Then this copolymer was hydrolyzed to form amphipathicPMMA-b-(PMAA-co-PMMA)block copolymer.Peak of solvated methoxyl of PMMA segments could be clearly observed on the 1H-NMR spectrum of the latex formed by PMMA-b-(PMAA-co-PMMA)block copolymer in D2O,that proving the livingness of propagating chain in the TEXT-mediated polymerization of MMA.Subsequently,DSL,SEM,and TEM were used to analyze the diameter and morphology of PMMA-b-(PMAA-co-PMMA)spherical particles in water and methanol.4.According to the mechanism of XT mediated polymerization proposed in previous experiments,three thioether small molecular initiators named as 9-benzyl-9-(phenylthio)-9H-xanthene(BnPTX),9-benzyl-9-(methylthio)-9H-xanthene(BnMTX),and methyl 2-methyl-2-(9-(phenylthio)-9H-xanthen-9-yl)propanoate(MIBPTX)were synthesized to simulate the as-formed cross-termination product in the XT-mediated polymerization.The synthetic route for those thioethers included four steps which were reduction,thioetherification,deprotonation andsubstitution.It is the first time for the synthesis of BnMTX and MIBPTX.The structures of those thioethers were characterized by 1H-NMR,13C-NMR,FTIR and UV.Consequently,those thioether small molecular initiators were induced in the bulk polymerization of St as thermal initiator.The initiation capability was confirmed as the polymeric product was detected at the very beginning of reaction.But,no controllability of the polymerization could be perceived.Afterwards,the synergistic effect of conventional thermal initiator ABVN and thioether small molecular initiators was proved existentin solution polymerization of St in THF.The results indicate that thioether small molecular initiators could mediate the radical polymerization,and make the Mn of polymeric product increase with monomer conversion. |
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