hydrogenation catalysis：催化加氢

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1、Polymerization13Polymerization&MetathesisPolymerizationisthereactionofanunsaturatedorganicreactant,typicallyaC=C,withitselfoverandoveragaintoproduceapolymerchain:Whenonlyafewalkenescoupletogethertomakeashortchain,werefertothatasoligomerization(oligomersareveryshortpolymers).Therearetypicallythreepar

2、tstomostpolymerizations:Initiation:generatingtheactivecatalystfromalessactivecatalystprecursorPropagation:thepolymerchaingrowthportionofthereactionthatoccursoverandoveragainPolymerization13Termination:areactionstepthatstopsthepolymerchaingrowthInagoodpolymerizationcatalysttheinitiationstepgenerates

3、asmuchactivecatalystaspossibleandthepropagationstepoccursasoftenaspossible,whiletheterminationstepisunlikelytohappen.Forearlytransitionmetalcatalyststheb-hydrideeliminationstepthatcancauseterminationisnotthatfavorable.ALivingCatalystistypicallyapolymerizationcatalystinwhichallthetransitionmetalcompl

4、exespresentarethesameandalreadyinanactivecatalyticstate(noinitiationstep).Whenthesubstrateisaddedallthecatalystmoleculesstartthepolymerchaingrowingstepsatthesametimeandsamerate.Thismeansthatatanypointthepolymerchainsgrowingoffeachandeverycatalystareallessentiallythesamelength.Thisleadstopolymersthat

5、haveaMw/Mnvalueverycloseto1.0.WhatisMw/Mn?TheaveragemolecularweightofapolymercanbedefinedbyMnandMw.Mnisthesimpleaverageoftotalmassofthechainsdividedbythenumberofchains.TheweightaveragemolecularweightMwisthesummationofthesquareofthemolecularweightsdividedbythesummationofthemolecularweightsofallthemol

6、eculespresent.InMwmore“weight”isprovidedtothetothehighermolecularweightpolymers,whileMntreatsallofthemthesame.ThebasisforMwisthatthelargermoleculescontributemoretothepropertiesofthepolymersotheyshouldhavemoreimportance.MwisalwaysgreaterthanMnandthenarrowerthedistribution,thecloserMnandMware.Theratio

7、ofMwtoMnisameasureofthedistributionofdifferentlengthpolymerchains.Thisratioisreferredtoasthedispersivity.Asthedistributionnarrows,thedispersivityapproachesaminimumvalueof1.0.Suchapolymerreferredtoasmo