Radical chemistry, Artykuły naukowe, Polimery i ich analiza
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//-->Fate of Initiator Radicals• Radical reactionsRecombination in solvent cageRecombination in mediaReaction with polymer radicals (kt)Reaction with initiator (MIH)Radical abstraction from polymer chainsReaction with solvent or inhibitorH3CCH3CNRadical Initiators• Azo InitiatorsNH3CNCNCH3CNNNazobisisobutyronitrile, AIBNTd = 50-70°C• Chain initiation,Ri= 2 f kd [I]R+XkiRXCN1,1'-azobis(1-cyclohexanenitrile)80-100°C, High organicsolubilityHOOCCH2CH2-H3CNH3CN• Efficiency factor, f = 0.1 - 0.9CH2CH2COOHCNCN4,4'-azobis(4-cyanovaleric acid)water soluble at pH > 7Decomposition of Azo Initiators• 2- bond cleavage to liberate nitrogenH3CCH3CNNH3CNCNCH3heator lightH3CCH3C2CN+Peroxy Initiators• High temperature initiatorsN2OCOOHCOORCOOR• Cage Recombination ---Side reaction- irreversible couplingCH3of succinonitrile radicals, efficiency decreases at highconversionHCHCCHCCN33+CCH3H3C3• Moderate temperature initiatorsOCH3NCNH3CCCH3Hydroperoxides Dialkyl PeroxidesTd = 155-175 C100-135 COPeresters110-130 CCNCH3CH3• orC CN NCOORCH3CCCH3CH3CCCH3CH3ODiacyl PeroxidesTd = 35-80 COO SO OS OOOPersulfates50-90 CNNPeroxy Initiators• Low temperature initiators, 35-60 CCOOOOOO2RCOOODecomposition of Peroxy Initiators• 1-bond cleavage processOROOOORORRRheat2ROO- CO2Peroxycarbonates• Redox initiation 0-5 COO SO OS O+++ FeOOOOOO S O+S OO+Fe+++OO• If R = aryl, acyl radical initiates–= alkyl, CO2lost before initiation occursReaction of Benzoyloxy radicals with styreneOPhOOOP hC O2-C H2CH80%Phheat2 PhC O2Chain Transfer• Hydrogen transfer to growing polymer chainRHP CHXX+RSHktrRX+P CHRSHHXStyPhC O2-C O2StyStyH2CPhC O2HCCH26%• Reinitiation of growing chain using transferredradicalRS+XkaRSXkpPh1%CH14 %P hC O2Effect of Chain Transfer on Rpand DPRelativerateconstantskp.>> ktrka~ kpkp<< ktrka~ kpkp>> ktrka< kpkp<< ktrka<< kpType of effectNormalTelomerizationRetardationInhibitionEffect onRpNoneNoneDecreaseLargedecreaseEffect on DPDecreaseLargedecreaseDecreaseLargedecreaseControl by Chain Transfer• Add chain transfer processes to terminationprocessesk [M.]2k [SH][M.] k [M][M.] k [I][M.]1DP=t+tr+tm+tikp[M.][M]• Assume chain transfer to monomer and initiator are.2.ktr[SH]k [M.]1small= kt[M.]+ ktr[SH][M ] =t+DPkp[M ] [M]kp[M]kp[M]1=DP1[SH]+ Ctr [M]DPo• Where Ctr is the chain transfer constantCommon Chain Transfer AgentsTransfer agentTolueneDi-n-butyldisulfideCarbontetrabromiden-butylmercaptanStyrene,Ctr x 1040.1252422,000210,000Vinyl AcetateCtr x 10421.610,000390,000480,000Additional Chain Transfer Processes• Chain transfer to monomer, Ctr x 104– Ethylene, 0.4- 4.0; Styrene, 0.3-0.6– Vinyl acetate, 1.75-2.8– Vinyl chloride, 10.8-16– Allyl systems, 50-100• Chain transfer to polymer– Polyethylene– Vinyl acetate– Vinyl chloride--branchingTransfer to Polymer• Polyethylene branching• Long branchesHHktrkaMInhibition of Radical Polymerization• Must stop oxygen- and carbon centeredradicalsR + O2ROOHROO ;ROROO + RHOHROOH + R+• Short branchesH HMHHHMM• Oxygen centered radicals stopped by hydrogentransfer• Carbon centered radicals stopped by addition• Inhibitor should not add to, abstract from or otherwisereach with monomer or solvent• Inhibitors should not undergo self reaction orunimolecular decomposition• Inhibitors must react rapidly with the propagatingand/or initiator derived radicals to terminate polymerchainsTrapping Oxygen Centered RadicalsOHROBHTOROOO+ ROHTrapping carbon centered radicals• Carbon centered radicals stopped by additionto oxygen or carbonOBenzoquinoneOORORHORR+ORHOOROHOR.ROOORTypical InhibitorsOHOHOHOHStable Radical InhibitorsO2NN NO2NDiphenylpicrylhydrazyl, DPPHNO2NOTEMPOOCH3Monomethylhydroquinone, MEHQ3,5-ditert-butyl catecholClClOBenzoquinoneOOClFeCl3CuCl2ClOChloranilOHHQN NOONNTriphenylverdazylO2SGalvanoxyl
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