L4-ionic chain polymer.pdf

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1、IonicChainPolymerizationCharacteristicsofIonicChainPolymerizationrelativetoRadicalChainPolymerization1.Thecounterionsarepresentinthereactionmediumtopreserveelectricalneutrality.2.Themechanismofpolymerizationmaybestronglyinfluencedbythecounterions,de

2、pendingonwhethertheyarestronglyorweaklyassociatedwiththepropagatingchain.3.Moremonomerspecific–usuallyproceedsonlyifthependantgroupcanstabilizetheactivecenter.4.Usuallymuchhigherrates–concentrationofactivechainismuchhigher(104–106times)thanfreeradica

3、lpolymerization.5.Largesolvationeffectonthepolymerizationrateandstereochemistry.6.Inprinciple,noterminationbetweentwoionicactivecenters.CationicPolymerization.CationicInitiatorsThepropagatingspeciesisacarbocation(=carboniumionorcarbeniumion)Initiat

4、ionisbroughtaboutbyadditionofelectrophiletoamonomermolecule+E++CH2=CR2ECH2CR2InitiationInitiator:①protonicacide.g.,HCIO,HSOHPO424,34②Lewisacid+protoniccocatalyst(water,protonorcationicsource)Lewisacids:BFAICITiCI,SnCl3344BF+HOH++(BFOH)-323AICI+HCI

5、H++(AICI)-34TiCI+CHOHH++(TiCIOCH)-4343H+A-+CH=CRRCHC+RRA-212312R=k[R+A-][M]:R+(cationicinitiator),A-(counterion)iiCationicpolymerization–Mechanism,Kinetics,Reactivity○Initiation•CarbocationicinitiationfollowsMarkovnickov’sruleTheadditionofthehydroge

6、n(intheformofaproton)toonecarbonatominthealkenecreatesapositivechargeontheothercarbon,formingacarbocationintermediate.Themoresubstitutedthecarbocation(themorebondsithastocarbonortoelectron-donatingsubstituents)themorestableitis,duetoinductionandhyper

7、conjugation.Themajorproductoftheadditionreactionwillbetheoneformedfromthemorestableintermediate.->morestablecarbocationintermediateisformedHCH(CH3)2CCH2>CH3CHCH2>H2CCH2H•para-substitutedstyrenesH2CCHH2CCHH2CCHH2CCH>>>OCHCH3HCl3ResonanceeffectInducing

8、effectInducingeffect(morestrongerthaninducing)(push)(pull)ortho-form:retardtheadditionbecauseofsterichindrancevinylethers:delocalizedintermediatecarbocationacceleratesreactionR'H2CCHORR'H2CCHORR'H2CCHOR○PropagationCR2slowfast+++CH2CR2+CH2CR2CH2①pi-