Mechanistic Study on E ff ect of Electron Donors in Propylene Polymerization Using the Ziegler − Natta Catalyst - Guo et al. - 2021 - Un

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pubs.acs.org/JPCCArticleMechanisticStudyonEffectofElectronDonorsinPropylenePolymerizationUsingtheZiegler−NattaCatalystXingGuo,LiangCui,YisenWang,JianjunYi,JingwenSun,ZhenLiu,*andBopingLiu*CiteThis:J.Phys.Chem.C2021,125,8533−8542ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:TiCl4/MgCl2catalystsarestillthemainindustrialcatalystforisotacticpolypropyleneatpresent.However,themechanismofisotacticpolymerizationofpropylenehasnotbeenfullyunderstood.DFTcalculationsrevealedthatthebareTiactivesitewasregioselectiveandnonstereoselectiveintheabsenceofanelectrondonor.Theroleofelectrondonorethylbenzoate(EB),diethyl-2,3-diisobutylsuccinate(DiBS),cyclohexylmethyldimethox-ysilane(CMDMS),anddicyclopentyldimethoxysilane(DCPDMS)ontheactivesitewasinvestigated.ThepresenceofEB,DiBS,CMDMS,orDCPDMSaroundtheTiactivesitecanpromotetheactivityandretaintheregioselectivity.ItisworthnotingthatinthepresenceofEBandDCPDMS,thestereoselectivebehaviorcanbepromotedwiththeadvantageof1kcal/mol.ThecopresenceofAlEt2Clspeciesandexternaldonorscanincreaseboththestereoselectivityandregioselectivity.1.INTRODUCTIONdonorsontheactivesiteremainselusive,presumablyduetotheheterogeneityandmulticomponentnatureofthecatalyst.In1953,KarlZieglerfirstdiscoveredthatsomecertainFortunately,greateffortshavebeencarriedouttounderstandtransitionmetal(mainlyTi,V,andZr)compoundswithAl-alkylswereabletocatalyzethepolymerizationofalkenesundertheroleofelectrondonorsontheactivesitebyexperimental28−32relativelymildconditions,lowertemperatures,andlowerandtheoreticalmethods.Andonietal.observedapressuresthanthoseoftheradicalpolymerization.1Thenextstructuredirectingroleofelectrondonorsintheformation33,34year,GiulioNattausedasimilarcatalysissystemtoproduceofMgCl2crystalmorphology.Thepresenceof1,3-dietherstereoregularpolymers.2ThesefindingsmadeitpossibleforledtothepreferentialgrowthofMgClcrystalsalongthe2themassiveproductionofstereoregularpolymers.Therefore,(110)surface,whereasdiisobutylphthalateorethylbenzoateKarlZieglerandGiulioNattawerejointlyawardedtheNobelwaslessselectiveandallowedthegrowthofMgCl2crystalsPrizeinchemistryfortheiroutstandingcontributionsin1963.alongboth(110)and(104)surfaces.Furthermore,electronInaddition,thiscatalysissystemwasnamedtheZiegler−NattadonorsinfluencethedistributionandamountofTiCl4inthecatalyst.ultimatecatalyst.ItisgenerallybelievedthattheadsorptionofTheZiegler−NattacatalystisoneofthemostimportantmononuclearTiCl4ontheMgCl2(110)surfacewouldformDownloadedviaUNIVOFCALIFORNIASANTABARBARAonMay16,2021at06:44:21(UTC).Seehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.catalystsforindustrialpolyolefinproduction,whichmainlyatacticactivesites,andtheadsorptionofdinuclearTiClon283producespolyethyleneandisotacticpolypropylene.Nowa-theMgCl2(104)surfacewouldgenerateisotacticactivedays,atypicalsupportedZiegler−Nattacatalysissystemsites.6,35Busicoetal.andCorradinietal.proposedthattheconsistsoffourkeyelements:TiCl4(thecatalystprecursor),electrondonortendedtocoordinateonthemore-acidic(110)MgCl2(thesupport),electrondonors(Lewisbases),andsurface,therebypreventingTiClfromanchoringonthealkylaluminum(thecatalystactivator).4,5Theintroductionof4nonstereoselective(110)surfaceandinhibitingthenon-electrondonorsisagreatbreakthroughintheZiegler−Natta6,36,37stereospecificactivesites.Actually,experimentalstudiescatalysissystemforpropylenepolymerization.Electrondonorsremarkedamore“direct”effectoftheelectrondonorsonthecanbedividedintotwotypes:(i)theinternaldonor(addedactivesite,improvingthestereoselectivityofalreadyselectiveduringthecatalystpreparation),e.g.,ethylbenzoate,phthalate,6−16succinate,1,3-diether,malonates,etc.,and(ii)theexternaldonor(addedduringtheolefinpolymerization),e.g.,silane,Received:December18,2020alkoxysilane.17−23ThesuitableinternalandexternalelectronRevised:April6,2021donorpaircancontroltheregularityofpolypropyleneandPublished:April20,202124−27increasetheyieldofisotacticpolypropylenegreatly.Althoughthisfamouscatalystattractsmuchattentionfrombothacademiaandindustry,themechanismoftheelectron©2021AmericanChemicalSocietyhttps://doi.org/10.1021/acs.jpcc.0c112738533J.Phys.Chem.C2021,125,8533−8542

1TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlesitesandprobablytransformingthenonstereoselectiveactiveclustermethods.Thecoadsorptionmodesofthesefoursitesintostereoselectiveones.donorsonthetitaniumactivesitewerefirststudied.Then,theThedensityfunctionaltheory(DFT)isutilizedasaeffectsofthedonorsontheπ-complexformationandinsertionpowerfultooltoidentifythemechanismofthechemicalbehaviorofpropylenemonomerstotheTiactivesitewerereaction.Recently,Piovanoetal.observedtheexistenceofinvestigatedsystematically.Finally,theeffectsoftheTiCl4(EB)andTiCl4(EB)2complexeschemisorbedonthecopresenceofexternaldonorsandAlEt2ClspeciesontheMgCl2surfaces,andtheinternaldonorethylbenzoate(EB)propyleneinsertionwerediscussed.couldinduceacertainmobilityfortheTiCl4moleculethroughcombinationofFT-IRspectraofCOadsorptionandDFT2.COMPUTATIONALDETAILS38methods.Credendinoetal.studiedthecoordinationAllDFTcalculationsincludinggeometryoptimizationsandpropertiesofdimethylphthalate,9,9-bis(methoxymethyl)-frequencycalculationswereperformedusingtheGaussian09fluorene,andalkoxysilanetotheMgCl(110)and(104)472programpackage.39,40surfaces.ThesedonorsshoweddifferentmobilitiesontheGeometryoptimizationswerecarriedoutwithoutany(110)surface,andthedonorinterlayermigrationrequiredsymmetryconstraintbyusingtheB3LYPfunctionalindonordissociation.Kuklinetal.estimatedthestabilizationof48,49combinationwiththedef2-SVPbasissetforallatoms.theMgCl2surfacebymono-andbidentateelectrondonors,ThedispersioncorrectionsweretakenintoconsiderationwithsuchasEB,2,2-dimethyl1,3-dimethoxypropane,dimethyltheDFT-D3(zero-damping)method.Throughout,wehave41phthalate,anddimethylsuccinate.Boththeidealandemployedharmonicvibrationalfrequencycalculationstodefective(104)and(110)MgCl2surfacescouldbestabilizedconfirmthatthestructureshavebeenproperlyoptimized.byalldonors.Also,theelectrondonorsplayanimportantroleTheenergiesofallstructureshavebeenrefinedbycarryingoutintheactivationmechanismoftheZiegler−Nattacatalyst.single-pointenergycalculationsusingtheM06LfunctionalinKumawatetal.foundthatonthepureTiClsiteorwith504combinationwiththeTZVPbasissetforallatoms.dietherandEBcoordinatingneartheTisite,thealkylationofTheβ-MgCl2(110)surfacewasusedinthiswork.Infact,thetheTiIVClprecursortotheTiIIIClEtactivesiteproceeds42activesiteTi-alkylmaybeformedbytheadsorbedTispecies42easily.Notonlytheelectroniceffectbutalsothestericontheperfectsurfaceordefectivesurfacefollowedbythe9,51hindranceofthesubstitutedgroupofdonorshasaninfluencealkylationassistedbythecocatalystAlR3.TheTi-isobutylontheactivesite.Wondimagegnetal.observedthatbulkygroup,formedby1,2-insertionofapropylene,wasconfirmedsubstituentgroupsofalkoxysilaneR1R2Si(OMe)2couldtobethesmallestunitfordescribingthegrowing52significantlypromotethestereoselectivebehaviorofthepolypropylenechain.Thestructureofthebareactivesite(110)edgeactivesiteinthepresenceofphthalateandTiCl2iBu/MgCl2(110)isdepictedinFigure1.Alltheatomsin43increasethemolecularweightofthepolymer.However,onebulkysubstituentonR1orR2couldnotproducehighlyisotacticpolymers.Also,researchershaveconsideredtheinfluencethatthecocatalystalkylaluminummayhavehadontheexternaldonorsduringthepolymerization.Khatrietal.carriedoutatheoreticalstudyofthecomplexesformedbyexternaldonorsand44triethylaluminium(TEA).TheexistenceofweakinteractionsbetweenexternaldonorsandTEAthroughchargetransferhasbeenprovedbytheDFTandNBOstudies.TheAlR2ClmoleculeistheactivationreagentortheproductgeneratedfromtheactivationeventalongwiththeformationoftheTiactivesite.Credendinoetal.reportedthatthestronglyFigure1.StructureofthebareactivesiteTiCl2iBu/MgCl2(110).adsorbedAlEt3andAlEt2ClspeciescouldenhancethestereoselectivityoftheTiactivesiteonthestep-defected45(104)surface.ThesameresultswereobtainedbyFallahetal.,whentheAlMe3andAlMe2Clspeciescoordinatednearthethemodeloftheβ-MgCl2(110)surfacewerekeptfixed,exceptTiactivesiteonthe(110)surface.46SincetheexternaldonorforthetwoClatomsfromthesurface,whichhavedirectbondsalkoxysilaneisalwaysusedincombinationwiththecocatalystwiththeTicenter.AlltheotheratomswererelaxedforalltheAlR3duringthepolymerization,itisobviousthattheAlR2Clcalculations.speciescanaffectolefininsertioninthepresenceoftheFortheDiBS,CMDMS,andDCPDMSstudiedhere,theexternaldonors.conformationalanalysiswasperformedusingtheTinker53Consideringthattheelectrondonorsplayanimportantroleprogram.TheatomswereassignedtheappropriateatomintheZiegler−Nattacatalyst,itiscrucialtostudytheeffectoftypesbasedontheMMFFforcefielddefinition.Alltheinitialdonorsontheregio-andstereoselectivebehavioroftheactivestructuresobtainedatthemolecularmechaniclevelwerethensite.Inolefincoordinationpolymerization,theinsertionstepisfullyoptimizedusingtheGaussian09program(TablesS1,S2therate-determiningstep,andtherelativeenergyoftheintheSupportingInformation).Theglobalminimumvaluesoftransitionstateisthemeasureoftheregio-andstereo-EB,DiBS,CMDMS,andDCPDMSaregraphicallyshowninselectivity.Inthiswork,theinsertionstepfortheMgCl2-Figure2.supportedZiegler−NattacatalystwithEB,diethyl-2,3-ThedonoradsorptionenergyΔGadwascalculatedaccordingdiisobutylsuccinate(DiBS),cyclohexylmethyldimethoxysilaneto(CMDMS),anddicyclopentyldimethoxysilane(DCPDMS)00wasinvestigatedbydensityfunctionalcalculationsusingΔGGGGad=−−A/DAD8534https://doi.org/10.1021/acs.jpcc.0c11273J.Phys.Chem.C2021,125,8533−8542

2TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlecatalyst,withaCObondlengthof1.210ÅandC−Obondlengthof1.347Å.ThemoststableadsorptionstateofEBontheMgCl2(110)surfaceisamonodentateonewiththeOatomofthecarbonylgroupcoordinatedtotheMgatom,whichisadjacenttotheTiactivesiteornot.IthasbeenpreviouslyreportedthatbidentatestructuresofEBwithO−COboundtotheMgatomwerelessfavorablethanthemonodentate54one.Therefore,onlythemonodentatecoordinationwasconsideredinthiscase.Thenearest5-coordinatedMgatomontheleftsideoftheTiactivesitewasdefinedastheMg1site,andthe4-coordinatedMgatomlocatednexttotheMg1sitewasdefinedastheMg2site.TheoptimizedgeometryofEBbindingtotheMg1atom,Mg2atom,andTiatomontheTiCl2iBu/MgCl2(110)surfaceisshowninFigure3.AdsorptionoftheEBmoleculewasFigure2.Optimizedgeometryof(a)EB,(b)DiBS,(c)CMDMS,consideredinthe“planar”conformation,wherethearomaticand(d)DCPDMSelectrondonors.ringwasparalleltotheplainofthecarboxylgroup.TheadsorptionenergiesandinteratomicdistancesforEB,DiBS,CMDMS,andDCPDMSontheTiCl2iBu/MgCl2(110)whereGA/DistheGibbsfreeenergyoftheactivesiteabsorbedsurfacearelistedinTable1.bytheelectrondonor,andG0andG0aretheGibbsfreeADenergiesoftheisolatedactivesiteandtheisolatedelectronTable1.AdsorptionEnergies(kcal/mol)andInteratomicdonormolecule,respectively.TheadsorptionenergyΔGad<0DistancesforEB,DiBS,CMDMS,andDCPDMSAdsorbediftheadsorbatewasboundwiththesurface.ontheTiCl2iBu/MgCl2(110)SurfaceTheπ-complexformationenergyΔGπwascalculatedaccordingtodistance(Å)00coordinationmodeΔGadO1−MgO2−MgO−TiΔGGππ=−−complexGGM−AEBwhereGπ−complexistheGibbsfreeenergyoftheπ-complexEB−Mg1−25.72.025formedbytheisolatedpropylenemonomerandtheisolatedEB−Mg2−23.52.037activesite,andG0andG0aretheGibbsfreeenergiesoftheEB−Ti−14.92.025AMisolatedactivesiteandtheisolatedpropylenemonomer,DiBSrespectively.DiBS−Mg2−M−28.22.005TheapparentactivationenergyistheenergydifferenceDiBS−Mg2−C−42.82.0522.080betweenthetransitionstateandtheseparatedreactants.TheDiBS−Mg1(2)−40.92.0152.120apparentactivationenergyΔGAwascalculatedaccordingtoCMDMSCMDMS−Mg1−23.42.13900ΔGGGGAM=−−TSACMDMS−Mg2−33.02.1402.150whereGTSistheGibbsfreeenergyofthetransitionstate.CMDMS−Ti−15.22.130DCPDMS3.RESULTSANDDISCUSSIONDCPDMS−Mg1−18.82.142DCPDMS−Mg2−31.52.1542.1703.1.AdsorptionofElectronDonorsontheTiCl2iBu/DCPDMS−Ti−11.52.110MgCl2(110)Surface.Themonoester-typeelectrondonorEBisusedastheinternaldonorduringthepreparationoftheFigure3.StructuresofEBandDiBScoadsorptionaroundtheTiactivesite.8535https://doi.org/10.1021/acs.jpcc.0c11273J.Phys.Chem.C2021,125,8533−8542

3TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleTheadsorptionenergyfortheEB−Mg1modelis−25.7bridgemodegives−40.9kcal/moloftheadsorptionenergy.kcal/mol,whichhasanO−Mgbondlengthof2.025ÅandanEnergetically,themonodentateisthemostunfavorableO−Mg−Clangleof95.4°,closetoaCl−Mg−Clangleof86.8°adsorptionmode.Nevertheless,incontrasttothemonofunc-inMgCl2crystals.IntheEB−Mg1model,theMg1atomtotionalEB,theadsorptionofthebifunctionalDiBSismuchwhichtheLewisbaseiscoordinatedalsointeractswiththeClmorestable.atomsoftheTispecies.TheEB−Mg2modelislessfavorableThealkoxysilane-typeelectrondonorCMDMSandwithanadsorptionenergyof−23.5kcal/mol,showingtheDCPDMSareusedasexternaldonorsduringthepropylenedifferentaffinityofEBforthefive-andfour-coordinatedMgpolymerization.Thedistancebetweentheirtwooxygenatomsatoms.WhenEBbindstotheTiatom,theadsorptionenergyisabout2.7Å.Therefore,thealkoxysilane-typeelectrondonoroftheEB−Timodelis−14.9kcal/mol,higherthanEBcanonlyadsorbattheMgCl(110)surfacethroughthe2bindingtoMgatoms.TheactivesiteispoisonedintheEB−TimonodentateandchelatemodesowingtotheshortspacercaseduetothesaturatedTiatomwithoutanyvacantsiteforbetweenthecoordinatingoxygenatoms.Thepossiblepropylenemonomercoordination.adsorptionmodelsofCMDMSandDCPDMSontheThediester-typeinternaldonorsuccinateisusuallyusedinTiCl2iBu/MgCl2(110)surfaceareshowninFigure5.thepreparationofthesixth-generationZiegler−Nattacatalyst.ThemostpreferredadsorptionmodelforCMDMSandThedistancebetweenthetwocarbonyloxygenatomsintheDCPDMSelectrondonorsisthechelatemodeduetotheDiBSmoleculeisaround3.1Å.ThelargespacerbetweentheshortdistancebetweenthecoordinatingOatoms,thetwooxygenatomsconfersenoughflexibilitytotheDiBSCMDMS−Mg2model,andtheDCPDMS−Mg2model,withdonor,sothatitcanadsorbontheMgCl2(110)surfaceanadsorptionenergyofaround−32kcal/mol.Forboththroughvariouscoordinationmodes,i.e.,monodentate,externaldonors,theadsorptionenergiesdecreaseintheorderchelate,andbridge(Figure4).Thechelatemode,alsocalledmonodentatemodeontheTiatom

4TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleinsertionbehaviorofthepropylenemonomertotheTiactivesitewereinvestigatedsystematically.3.2.1.TheBareActiveSite.TheinsertionofpropylenefollowsthemodifiedCossee−Arlmanmechanism,whichisthecommonlyacceptedmechanismofolefincoordination56−58polymerization.Thismechanisminvolvestwosteps:(1)theolefinmoleculecoordinatestotheTi-alkylchain(theactivesite)toformaπ-complex;(2)then,theπ-complexundergoesinsertionviaafour-memberedtransitionstateassistedbyaremarkableα-agosticinteractiontoformanincreasedTi-alkylchain.TheinsertionofaprochiralpropylenemonomerintotheTi-alkylbondcanoccurviatwoways,1,2-insertion(alsocalledprimary)and2,1-insertion(alsocalledsecondary),withthere-andsi-enantiofaces.Therefore,therearefourpathwaysforthepropyleneinsertion:1,2-re,1,2-si,2,1-re,and2,1-si.Table2liststheπ-complexationenergy(ΔGπ),Figure6.FourpossiblepathwaysofpropyleneinsertionintothebareTi-iBugrowingchain(onlythereactionregionisshown).Table2.π-ComplexFormationEnergy(ΔGπ),theApparentActivationEnergy(ΔGA),andtheRelativeEnergyofaTransitionStatesonDifferentActiveSiteModelstheconformationassumedbythegrowingchainbefore/during37theformationofthenewC−Cbond.The(+)iBugrowinginsertiontypeΔGπΔGAΔΔGAchainwasonlyusedinthecaseof1,2-siinsertion,whiletheBare(−)iBugrowingchainwasusedinotherthreecases,to1,2-re−1.910.00.0minimizerepulsiveinteractionswiththemethylgroupofthe1,2-si−3.19.8−0.2monomer.ThemethylgroupatCβinthe(−)or(+)growing2,1-re−2.612.12.1chainswasonthefrontsideagainstcoordinatedpropylene.2,1-si−3.211.71.7Theinsertionwasassistedbytheα-agosticinteractionbetweenEBtheα-Hofthemethylenegroupandthetitaniumcenter.1,2-re−3.08.60.0Inthetransitionstate,oneCα−Hαbondwasstretchedwith1,2-si−4.010.01.4thecorrespondingCα−HαandTi−Hαbondlengthsof1.142,1-re−0.913.14.5and1.94Å,respectively.ThenewlyformedC−Cbondofthe2,1-si−3.110.31.7propylenemonomerinsertionintotheTi−CbondwasaroundDiBS2.30Å.1,2-re−4.67.80.0Therelativeenergy,representedasΔΔGA(Table2),isthe1,2-si−5.67.5−0.3energydifferencebetweenthetransitionstates(corresponding2,1-re−3.410.72.9‡to1,2-reinsertion).ΔGRegio,ameasureofregioselectivity,is2,1-si−5.59.31.5theenergydifferencebetweentheminimumof1,2-insertionCMDMS‡and2,1-insertion.ΔGStereo,ameasureofstereoselectivity,is1,2-re−4.08.40.0theenergydifferencebetween1,2-reinsertionand1,2-si1,2-si−4.78.70.3‡‡insertion.ThehighertheΔGRegioandtheΔGStereovalues,the2,1-re−3.310.11.7highertheregioselectivityandthestereoselectivity,respec-2,1-si−4.110.21.8‡tively.TheΔGStereovalueis0.2kcal/molindicatingthattheDCPDMSbareTiactivesiteontheMgCl2(110)surfaceisaspecificdue1,2-re−2.58.90.0totheinabilitytodistinguishthesi-andre-enantiofacesinthe1,2-si−4.010.21.3absenceofelectrondonors.2,1-re−2.611.52.6‡TheΔGRegiovalueis1.7kcal/mol,indicatingthatthebare2,1-si−3.210.81.9aTiactivesiteontheMgCl2(110)surfaceismoderatelyEnergiesareinkcal/mol.regioselectiveevenintheabsenceofelectrondonors,in9,52,59−61accordancewithothertheoreticalresults.Thus,1,2-theapparentactivationenergy(ΔGA),andtherelativeenergyinsertionoftenoccursduringthepropylenepolymerization.Inoftransitionstates.Theπ-complexformationenergyisintheaddition,after2,1-insertion,thesterichindranceofthemethylrangefrom−1.9to−3.2kcal/mol,whichindicatesthatthegroupontheα-Cofthegrowingpolymerchainwouldpreventcomplexationofthepropylenetothetitaniumcenterisstablethenextpropylenemonomerfromcoordinatingtotheactivethroughthefourconfigurations.TheΔGπvaluefor1,2-site.Both1,2-reinsertionand1,2-siinsertioncanoccurintheinsertionisslightlyhigherthanthatfor2,1-insertion,indicatingnonstereoselectiveTiactivesiteduringpolymerization,withthatthestericinterferencebetweenthemethylmoietyofthethepossibilityofthestereoerroroccurring.Therefore,itispropylenemonomerandthegrowingpolymerchainishigherimportanttointroducethethirdcomponenttocontrolthethantherepulsionbetweenthemethylgroupandtheMgCl2structureoftheactivesiteandincreasethestereoselectivityofsurface.theactivesite.Figure6showsthetransitionstatesofthefourinsertion3.2.2.TheActiveSitewithElectronDonors.Accordingtomodesofpropyleneinsertiononthebareactivesite.Corradinithecalculations,themonodentatemodeforEB(EB-Mg1)andproposedthetwochiralorientationsofthegrowingpolymerthechelatemodeforDiBS(DiBS-Mg2-C),CMDMSchainas(−)or(+)growingchains,whichdependedonlyon(CMDMS-Mg2),andDCPDMS(DCPDMS-Mg2)werethe8537https://doi.org/10.1021/acs.jpcc.0c11273J.Phys.Chem.C2021,125,8533−8542

5TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlestableadsorptionstructures.Thesemodelswereselectedinthissectiontoinvestigatetheeffectofelectrondonorsontheπ-complexationandinsertionofpropylenemonomerstotheTiactivesite.ThecalculatedGibbsfreeenergies(Table2)indicatethattheproperelectrondonorcanpromotetheactivityandstereoselectivityoftheTiactivesite.InthepresenceofanEBmoleculecoadsorbedatthenearestMg1atom,theΔGπvaluefor1,2-insertionslightlydecreasescomparedtothatforthebareactivesite,indicatingthattheelectrondonorstabilizedtheπ-complexthroughreinforcementintheelectrondensityoftheTiactivesiteviaO→Mg→Tielectron-donatingeffect.Nevertheless,inthecaseof2,1-reinsertion,theπ-complexationisnotfavoredowingtothesterichindrancebetweentheEBmoleculeandthemethylgroupofthemonomer.The1,2-reinsertionofpropyleneintotheTi−iBubondrequires8.6kcal/molofΔGA,being1.4kcal/mollowerthanthatintheabsenceofanelectrondonor.TheΔG‡valueincreasesfrom0.2to1.4kcal/mol,indicatingStereotheimprovementinstereoselectivityinthepresenceofEB,in62agreementwiththeexperimentalfindings.Thetransitionstatesforprimarypropyleneinsertion(1,2-reand1,2-si)aresketchedinFigure7.TheenhancedelectrondensityattheTiatompromotestheπ-complexationandtheactivationoftheCCdoublebondofthepropylenemonomerleadingtoaloweractivationenergy.Inthiscase,theactivesiteisstillregioselective.TheΔGπvalueswiththeDiBSmoleculearelowerthanthosewithoutandwithotherdonors,whichsuggeststheexceptionalstabilityofthediester-typeinternaldonorDiBStotheπ-complex.Furthermore,inthiscase,the1,2-insertiongivesthelowestΔGAvaluesamongallcases,whichindicatestheexcellentabilityofsuccinatetoincreasethecatalyticactivity.However,theactivationenergydifferencebetween1,2-reand1,2-siisonly0.3kcal/mol(Table2),slightlyhigherthanthesituationinthebareactivesite,indicatingthepoorabilitytodistinguishthesi-andre-enantiofacesinthepresenceFigure7.TopviewoftransitionstatesleadingtoprimarypropyleneoftheDiBSdonor.insertionintotheTi−iBubondinthecaseofEBandDiBS:(a)EB-Figure7illustratesthetransitionstatesforthe1,2-insertion1,2-re,(b)EB-1,2-si,(c)DiBS-1,2-re,and(d)DiBS-1,2-si.Colorcode:withtheDiBS.Apparently,thesterichindrancecausedbytheMgyellow,Clgreen,Tiviolet,Ored,Hwhite,Cofthedonorgray,Cofpropyleneorange,andCofthegrowingchainblue.isobutylgroupoftheDiBSdonoradsorbedbythechelatemodeissoweakthatthestereoselectivitycanbarelybepromoted.InanalogytotheEBdonor,theactivesiteretainsregioselectivityinthissituation.propyleneinsertionincreases,withaΔG‡valueof1.3StereoItiswell-knownthattheexternaldonorhasasignificantkcal/mol.TheDCPDMSexhibitsahigherΔG‡valuethanStereo63,64effectontheisotacticityofthefinalpolymerproduct.ThetheCMDMS;thissupportsthefactthatitcanpromotethePPmostlyusedalkoxysilane-typedonorCMDMSandDCPDMSproductionwithhigherisotacticity,whichisconsistentwithadsorbontheMgCl(110)surfacethroughthestablechelate652theexperimentalfindings.Furthermore,withprimarymode.FromtheenergydatashowninTable2,itisobservedpropyleneinsertionremainingfavoredforbothCMDMSandthatCMDMSstabilizestheπ-complexformationwiththeDCPDMS,theΔG‡valueisalwaysabove1.7kcal/mol.Regioadvantageofaround1kcal/mol.ThelowerΔGπvalueforThelargesterichindranceofthecyclopentylgroupinCMDMSascomparedtoDCPDMSrevealsthattheπ-complexDCPDMShasanimportantinfluenceontheresistancetocanbebetterstabilizedbythecyclohexylgroupascomparedtotheconfigurationwiththemethylgroupclosetothethecyclopentylgroup.ThelowΔG‡valueof0.3kcal/molStereocyclopentylgroupinthe1,2-insertion.ThepresenceofaalsoindicatesthattheenantiofacesareindistinguishableinthehighlyhinderedcyclopentylgroupinhibitstheinsertionofthecaseofCMDMS.si-enantiofaceinthe1,2-insertionandimprovesthestereo-Nevertheless,thelowestactivationenergyofCMDMS(8.4selectivityoftheactivesite.Figure8reportsthetransitionkcal/mol)islowerthanthatofDCPDMS(8.9kcal/mol).ThestatesforprimarypropyleneinsertionofCMDMSandresultsagreewellwiththecalculationsofalkoxysilane-typeDCPDMS.electrondonorsontheedgesitebyWondimagegnetal.,who3.3.TheRoleofAlEt2ClSpecies.TheAlR2Clmoleculeisobtainedthelowestactivationenergyof13.0kcal/molfortheactivationreagentortheproductreleasedbytheactivation43CMDMSand16.1kcal/molforDCPDMS.reaction,whichcanadsorbontheMgatomnearbytheTiConcerningthecaseofDCPDMSbindingtothesurfaceinactivesite.TheAlEt2Clmoleculewasselectedinthissectiontothevicinityneartheactivesite,thestereoselectivityofprimaryinvestigatetheimpactofthecopresenceofexternaldonorsand8538https://doi.org/10.1021/acs.jpcc.0c11273J.Phys.Chem.C2021,125,8533−8542

6TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticlebenoticedthattheΔG‡values,theactivationenergyStereodifferencebetween1,2-reand1,2-siinsertion,increasesignificantlyinbothcasesofCMDMSandDCPDMStogetherwithAlEtCl.Inaddition,theΔG‡valuesincreaseslightly.2RegioInshort,thestereoselectivityandregioselectivitycanbeenhancedbytherestrictedstericenvironmentresultedfromthecopresenceofAlEt2Clandtheexternaldonor.AcomparisonoftheapparentactivationenergiesandΔG‡valuesfordifferentdonorcatalystsystemsisprovidedStereoinFigure9.ItisclearthattheactivityofthecatalystcanbeFigure9.BargraphforthecomparativestudyofapparentactivationenergiesandΔG‡valuesfordifferentdonorcatalystsystem.StereopromotedinthepresenceofdonorsandtheΔG‡valuesStereocanbeincreasedinthecaseofEBandDCPDMS.TheFigure8.Topviewoftransitionstatesleadingtoprimarypropylenecoadsorptionoftheelectrondonor,likeEBandDCPDMS,insertionintotheTi−iBubondinthecaseofCMDMSandconvertstheaspecificTiactivesiteintoanisospecificonebyDCPDMS:(a)CMDMS-1,2-re,(b)CMDMS-1,2-si,(c)DCPDMS-stericallycontrollingtheorientationofthegrowingchainatthe1,2-re,and(d)DCPDMS-1,2-si.Colorcode:Sipurple,andothersastransitionstate.mentionedabove.Inshort,theproperelectrondonorcoadsorbedonthetheAlEt2Clmoleculeontheπ-complexationandinsertionTiCl2iBu/MgCl2(110)surfacecanyieldamoderatelystereo-selectivesite.ThisfindingisinagreementwithDFTbehaviorofpropylenetotheTiactivesite.66,67calculationsofKumawatetal.andVankaetal.WithTheAlEt2ClmoleculecanbestronglyadsorbedontheproximityoftheTiactivesitetogetherwiththeexternaldonorregardtothecopresenceofAlEt2Clspeciesandtheexternaldonor,thestereoselectivitycanbepromotedbytheincreasedviatheCl−MgbondandAl−Clbond(FigureS1inthesterichindrancearoundtheTiactivesite.SupportingInformation).Therelatedenergydatafortheπ-Busicoetal.proposedathree-sitemodelforZiegler−NattacomplexationandinsertioneventsaresummarizedinTable3.Clearly,theexistenceoftheAlEtClspeciesdisfavorsthecatalystswherethebulkyligandL1,2(Clordonor)22+coordinatingtotheadjacentunsaturatedMgcorrespondsformationoftheπ-complexcomparedtoothercases.Itshould68totheisotacticpropagation.Correaetal.hadattemptedtorationalizetheinfluenceofelectrondonorsontheTiactivesiteTable3.π-ComplexFormationEnergy(ΔGπ),theApparentusingquadrantrepresentation,wheregrayorwhitequadrantsActivationEnergy(ΔGA),andtheRelativeEnergyof9representregionsstericallyoccupiedbythedonorsornot.InTransitionStatesontheTiActiveSitewiththeCopresenceathiswork,thebareTiactivesite(Scheme1a),theTiactivesiteofExternalDonorsandAlEt2ClSpecieswithoneadsorbeddonor(Scheme1b),andtheTiactivesiteinsertiontypeΔGπΔGAΔΔGAwithoneadsorbedexternaldonorandoneAlEt2ClmoleculeCMDMS+AlEtCl(Scheme1c)weresketchedinthequadrantrepresentationof21,2-re−1.19.90.0Scheme1.1,2-si−0.913.83.9FromScheme1a,itisobviousthatthe1,2-insertionchain2,1-re−2.212.12.2orientationtendstobeoppositetothemethylgroupof2,1-si−1.212.82.9propylenetoreducestericinterferencebetweenthegrowingDCPDMS+AlEt2Clpolymerchainandmonomer.Moreover,inthecaseof2,1-1,2-re−0.99.40.0insertion,thechainorientationisofnosensesincethemethyl1,2-si−1.912.02.6groupofpropylenedoesnotinterferewiththegrowingchain.2,1-re−0.312.63.2Themainstericrepulsionisbetweenthemethylmoietyand2,1-si−1.112.93.5theMgCl2surface.Afteradonorcoordinating,thestericregionaroundthemonomercanleadtoanincreaseintheenergyaEnergiesareinkcal/mol.differencebetweenre-andsi-enantiofaces,thusincreasingthe8539https://doi.org/10.1021/acs.jpcc.0c11273J.Phys.Chem.C2021,125,8533−8542

7TheJournalofPhysicalChemistryCpubs.acs.org/JPCCArticleScheme1.QuadrantRepresentationof(a)BareTiActiveSite,(b)TiActiveSitewithOneAdsorbedDonor,and(c)TiActiveSitewithOneAdsorbedDonorandOneAlEt2ClMoleculestereoselectivityoftheTiactivesite,especiallyforthelargespeciesandexternaldonorscanincreaseboththestereo-DCPDMSmoleculeandmono-estertypeEBmolecule.selectivityandregioselectivity.ConsideringthecopresenceofAlEt2Clspeciesandexternaldonors,thesterichindranceofonepieceofregionaroundthe■ASSOCIATEDCONTENTmonomeroccupiedbytheAlEt2Clmoleculefurtherincreases*sıSupportingInformationtheenergydifferencebetweenre-andsi-enantiofaces.TheTheSupportingInformationisavailablefreeofchargeatactivationenergydataofthetransitionstate(Tables2and3)https://pubs.acs.org/doi/10.1021/acs.jpcc.0c11273.areinagreementwiththesefindings.Structureoftheco-presenceofCMDMSandAlEt2ClaroundtheTiactivesite;energiesofvariousconforma-4.CONCLUSIONStionalCMDMSandDCPDMS;andCartesiancoor-DFTcalculationshavebeenperformedtosystematicallydinatesofthestructures(PDF)investigatetheeffectofelectrondonorsontheactivesite.ThecoadsorptionperformanceofEB,DiBS,CMDMS,and■DCPDMSontheactivesiteisstudied.ThemostprobableAUTHORINFORMATIONdonorcoadsorptionstructuresareobtained,andtheeffectofCorrespondingAuthorsthedonorontheregio-andstereoselectivebehavioroftheZhenLiu−SchoolofChemicalEngineering,EastChinaactivesiteisdiscussed.UniversityofScienceandTechnology,Shanghai200237,Thepreferredcoadsorptionmodeofdiester-typeelectronChina;orcid.org/0000-0003-3103-4011;donorDiBSandalkoxysilane-typeelectrondonorCMDMSEmail:liuzhen@ecust.edu.cnandDCPDMSisthechelatemodeonthe4-coordinatedMg2BopingLiu−CollegeofMaterialsandEnergy,SouthChinasiteadjacenttotheTiactivesite,whereastheEBmoleculeisAgriculturalUniversity,Guangzhou510642,China;preferentialtocoordinateonthe5-coordinatedMg1siteEmail:boping@scau.edu.cnthroughthemonodentatemode.ThebareactivesiteAuthorsTiCl2iBu/MgCl2(110)isregioselectiveandnonstereoselectiveXingGuo−SchoolofChemicalEngineering,EastChinaintheabsenceofanelectrondonorduringpropyleneUniversityofScienceandTechnology,Shanghai200237,polymerization.Therearetworeasons:(i)thesterichindranceChinabetweenthemethylgroupofthepropylenemonomerandtheLiangCui−PolyolefinResearchDepartment,PetrochinaMgCl2surfacecontributestothehighapparentactivationPetrochemicalResearchInstitute,Beijing102206,Chinaenergyof2,1-insertion;thus,the1,2-insertionismoreYisenWang−PolyolefinResearchDepartment,Petrochinafavorable;(ii)theα-CH3ofthegrowingchainformedafterPetrochemicalResearchInstitute,Beijing102206,Chinathe2,1-insertionpreventsthefurtherπ-complexationandJianjunYi−PolyolefinResearchDepartment,Petrochinainsertionofpropylene.PetrochemicalResearchInstitute,Beijing102206,ChinaThepresenceofEB,DiBS,CMDMS,orDCPDMSaroundJingwenSun−SchoolofChemicalEngineering,EastChinatheTiactivesitepromotestheactivityandretainstheUniversityofScienceandTechnology,Shanghai200237,regioselectivity.ItisworthnotingthatthestereoselectivitycanChinabesignificantlyenhancedbythecoadsorptionofEBorDCPDMSneartheactivesite,whichcanbeexplainedfromCompletecontactinformationisavailableat:boththeelectronicandthestericaspects.Theelectron-https://pubs.acs.org/10.1021/acs.jpcc.0c11273donatingdonorsenrichingtheelectrondensityoftheTiatommakeitmorefacilefortheTiatomattackingtheCCdoubleNotesbond.ThesterichindrancecausedbythephenylgroupofEBTheauthorsdeclarenocompetingfinancialinterest.orthecyclopentylgroupofDCPDMSresultsinthecontinuingpropylenemonomerinsertionintothegrowingpolymerchain■ACKNOWLEDGMENTSinthesameconfiguration.TheintroductionofsuitableWethankthefinancialsupportoftheNationalNaturalScienceelectrondonorscouldenhancetheactivityandthestereo-FoundationofChina(21674036),ShanghaiPujiangProgramselectivityoftheactivesite.Finally,thecopresenceofAlEt2Cl(18PJ1402500),andtheOpenProjectofStateKeyLaboratory8540https://doi.org/10.1021/acs.jpcc.0c11273J.Phys.Chem.C2021,125,8533−8542

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