solvents monoxide copolymerization carried out in aproticorganic Influence of formic acid and water on the [Pd(OAc) (dppp)] catalyzedethene–carbon Applied Catalysis A: General

ContentslistsavailableatSciVerseScienceDirect

Applied

Catalysis

A:

General

j ourna l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p c a t a

Influence

of

formic

acid

and

water

on

the

[Pd(OAc)

2

(dppp)]

catalyzed

ethene–carbon

monoxide

copolymerization

carried

out

in

aprotic

organic

solvents

Andrea

Vavasori

,

Lucio

Ronchin,

Luigi

Toniolo

DepartmentofMolecularSciencesandNanosystems,Ca’FoscariUniversityofVenice,Dorsoduro2137,30123Venice,Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received16July2012 Receivedinrevisedform 21September2012 Accepted11October2012 Available online 23 October 2012 Keywords: Palladiumcatalyst Carbonmonoxide Ethene Solventeffect Polyketone

a

b

s

t

r

a

c

t

Thecopolymerizationofethenewithcarbonmonoxidecatalyzedby[Pd(OAc)2(dppp)]inanaprotic

sol-ventsuchas1,4-dioxaneornitromethaneisefficientlypromotedbothbyH2OandHCOOHandyieldsa

perfectlyalternatingpolyketone(PK).Theinfluenceoftheconcentrationofthepromoters,pressureand

temperatureonthecatalystproductivityandthelimitingviscositynumber(LVN)hasbeenstudied.The

productivityincreaseswiththeincreaseoftemperatureandpressure.TheLVNincreasesuponincreasing

thepressureandloweringthetemperature.At363Kand9.0MPa,inHCOOH/H2O/1-4,dioxane(2.7/1.35/1

molarratio),theproductivityis37.50kgPK(gPdh)−1(LVN2.77dLg−1).

LVNlowersuponincreasingtheconcentrationoftheacid,suggestingthatitisinvolvedinthe

protonol-ysischain-transferprocess.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

ThePd(II)-(chelating-diphosphine)catalyzedcopolymerization of ethene withCO toa perfectly alternated copolymer, named polyketone(PK),hasbeenwidelystudiedinthelast30years[1–11]. Thecatalytic activity is mostlyinfluenced by thenature ofthe chelatingligandandthecounteranion,althoughitdependsalsoon thenatureofthesolvent[1–10].Thecopolymerizationiscarriedout preferablyinmethanol,althoughinsomeinterestingpaperswater hasbeenproposedasanalternativesolvent[12–17].Wefoundthat theprecursors[PdX2(P-P)](X=AcO,Cl),inactiveinMeOH,turninto highlyactivecatalystswhenusedinH2O AcOHorH2O HCOOH [18–25].

Othersolventshavealsobeenutilized,suchasdichloromethane, THF,toluene,acetonitrile,1,4-dioxaneoracetone,howeverthe pro-ductivityis,ineachcase,wasverylow[26–28].

Hereafter, we report the results on the [Pd(OAc)2(dppp)] (dppp=1,3-bis(diphenylphosphino)propane) catalyzed CO–ethene copolymerization in aprotic solvents (1,4-dioxane andnitromethane)promotedbyHCOOHandH2O.

∗ Correspondingauthor.Tel.:+390412348577;fax:+390412348517. E-mailaddress:[email protected](A.Vavasori).

2. Experimental

2.1. Reagents

1,4-Dioxane, nitromethane (99%), 1,3-bis(diphenylphosphi-no)propane(dppp),CDCl3 and 1,1,1,3,3,3-hexafluoroisopropanol (99%)wereAldrichproducts;formicacid>99%,(AcrosOrganics).

Thecomplex[Pd(OAc)2(dppp)]waspreparedasreportedin lit-erature[29].

CarbonmonoxideandetheneweresuppliedbySIADCompany (‘researchgrade’,purity>99.9%).

2.2. Equipment

Gas-chromatographic analysis was performed on a Hewlett PackardModel5890,SeriesIIchromatographfittedwithaHP1, 30m×0.35␮m×0.53␮mcolumn(detector:FID;carriergas:N2, 0.2mL/min;oven:323K(2min)to473Kat15K/min).

FTIRspectrawererecordedonaNicoletMagna750instrument inKBrpowder.

AlltheNMR spectrawere recordedonaBrukerAvance 300 spectrometer.The1_HNMRand13_{CNMRspectraofthepolyketone} dissolvedin1,1,1,3,3,3-hexafluoroisopropanol/CDCl3 (10/1)were recordedusingtheInverse1_{H-GatedDecouplingTechnique.} 0926-860X/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.

Table1

Selected1_HNMRand13_{CNMRsignalsofPK.}

1_HNMR(ppm) 13_CNMR(ppm) C(O)CH2CH2 2.77 C(O)CH2CH2 35.73 C(O)CH2CH3 1.08 C(O)CH2CH2 212.65 C(O)CH2CH3 2.52 C(O)CH2CH3 6.91 C(O)CH2CH3 217.04 2.3. Copolymerization

Thecopolymerizationswerecarriedoutaspreviouslydescribed

[24,25].

In a typical experiment, 1000mg of [Pd(OAc)2(dppp)] (1.57_×10−3mmol) was added to 80mL of solvent contained inthebottleplacedintheautoclave.Theautoclavewasflushed witha1/1 mixtureof CO/C2H4 atroomtemperature with stir-ring. The autoclave was then pressurized with 0.5MPa of the gas mixture and then heated to 363K in ca. 10min without stirring. The pressure was then adjusted to the desired value (typically4.5MPatotalpressure)and,whilestirring,maintained constantthroughout theexperiment (1h, ratestirring 700rpm) bycontinuouslysupplyingthemonomersfromthereservoir.At theendoftheexperimenttheautoclavewasquicklycooledand carefullydepressurized.Thepolymerwascompletelyprecipitated byadditionof100mLofH2Oandtheslurryobtainedwasfiltered, washedwithwaterandacetoneanddriedundervacuumat343K. Thedriedpolymerwasweighedandtheproductivitywas cal-culatedaskgPK(gPdh)−1;thereproducibilitywaswithinca.5%. Lowmolecularweightproductseventuallyformedweredetected throughGCanalysisoftheliquidphase.

TheIRspectrashowtypicalstretchingsignalsofCOgroupsat 1695cm−1and−CH2−groupsat2915cm−1.

The13_{C NMR spectra,showsa singlecarbonylabsorptionat} 212.65ppm( C(O)CH2CH2 )andasingleresonanceforthe CH2 groupsat35.73ppm( C(O)CH2CH2 )intheratio1:2duetothe exclusiveperfectlyalternatedstructure[1].Themostrelevant sig-nalsarereportedinTable1.

2.4. Limitingviscositynumber(LVN)measurementsandaverage viscositymolecularweightcalculation

Theaverageviscositymolecularweightofpolymer hasbeen evaluatedas LimitViscosity Number (LVN)measurements. The LVNofa dilute PKsolutionwasdetermined byusingthe Hug-ginsrelationshipbetweentheviscositynumberandthepolymer concentrationbyextrapolationtozeroconcentration[30].ThePK solutionwaspreparedinm-cresolasasolventandtheviscositywas measuredbyusingaCannon–Fensketypecapillaryviscosimeter, thermostatedat298K.

Theaverageviscositymolecularweight(Mw)ofthepolyketone wasderivedfromtheLVNusingtheMark–Houwinkequation[31].

[]m-cresol,298K=1.01×10−4M¯w0.85

3. Resultsanddiscussion

3.1. PromotingeffectofHCOOH

ThepromotingeffectofHCOOHhasbeenstudiedintwo apro-ticorganicsolventshavingsignificantdifferentpolarity,suchas 1,4-dioxane(_ε=2.3)andnitromethane(_ε=39.4).Inbothsolvents thecatalyticactivitypasses througha maximumwhen HCOOH increases(Fig.1). 0 10 20 30 40 50 60 70 80 90 100 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 productivity, gPK gPd -1 h -1 HCOOH, molar % nitromethane dioxane

Fig.1. InfluenceofHCOOHconcentrationontheproductivityinnitromethane and 1,4-dioxane. Run conditions: [Pd(OAc)2(dppp)]=1.57×10−3mmol;

TsOH/Pd=100/1(molarratio);volumeofthereactionmedium(solvent+HCOOH) 80mL;363K;4.5MPa(CO/C2H4=1/1);1h;700rpm.

HCOOHcaninfluencethecatalysisbecauseofseveraleffects: (i)itcanprovideahigherconcentrationofthePd(II) H+ initia-tor(seebelow)preventingdeprotonationthatultimatelyleadsto inactivePdmetal[5];(ii)itmayactivate/destabilizethe_␤-andthe ␥-chelateofthegrowingchainbyprotonatingtheoxygenatom coordinatedtothemetal,thusfavouringthechaingrowing pro-cess[32];(iii)itcausesanincreaseofthepolarityofthereaction medium(HCOOH,ε=59),whichcouldfavourtheformationofmore reactive“cationic”species.AthighHCOOHconcentration,however, theproductivitydecreases.Thismightbedue(i)totheincreaseof theconcentrationoftheconjugatebaseoftheacid,HCOO−,which competeswiththemonomersforthecoordinationonthemetal centreand(ii)totheloweringofthesolubilityofthemonomers (seeTable2),measuredaspreviouslydescribed[18,19,25].

InbothsolventsLVNdecreasesbyincreasingtheHCOOH con-centration(Fig.2),incontrastwithwhatobservedinH2O AcOH [21,23].Thetrendsuggestsadirectinvolvementoftheacidinthe

Table2

Henry’slawconstantsforCOandetheneindifferentreactionmedia.

Solvent HCO(MPa) Hethene(MPa)

HCOOH 1.5×103 _3.47×102

H2O 8.2×103 1.25×103

1,4-Dioxane 3.2×102 _3.4×101

Nitromethane 4.6×102 _3.86×101

HCOOH(mol%) 1,4-Dioxane Nitromethane

HCO(MPa) Hethene(MPa) HCO(MPa) Hethene(MPa)

5 346.5 38.2 488.7 43.9

10 374.7 42.1 519.8 48.6

20 438.2 54.6 585.9 59.1

40 599.8 86.4 745.7 93.3

80 1120.6 218.8 1205.0 224.4

H2Oa(mol%) 1,4-Dioxanea Nitromethanea

HCO(MPa) Hethene(MPa) HCO(MPa) Hethene(MPa)

10 478.5 55.0 651.2 61.2

20 662.2 78.5 869.0 86.3

40 1186.6 161.4 1270.1 173 60 2165.0 332.0 2284.3 347.9 80 4635.8 682.6 4894.2 695.8

0 10 20 30 40 50 60 70 80 90 100 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 LVN, d L /g HCOOH, molar % dioxane nitromethane

Fig.2. InfluenceofHCOOHontheLVNin1,4-dioxaneandnitromethane.Run con-ditions:thoseofFig.1.

chainterminationstep(seebelow).Inaddition,itcanbeduetothe loweringofthesolubilityofthemonomers.

Inordertofurtherimprovethecatalystperformance,wetested alsotheinfluenceofH2O,whichcanbeahydridesourceforthe catalyst[23–25].

3.2. PromotingeffectofH2Ointhesolvent

TheinfluenceofH2Oontheproductivityhasbeenevaluatedin thepresenceoftheacidinalowconcentration(5mol%),enough toavoidcatalystdeactivation(Pdmetalformation).Fig.3shows that the productivity passes through a maximum of 6.50 and 11.50kgPK(gPdh)−1whenH2Oca.40and60mol%innitromethane or1,4-dioxane,respectively.

ThepromotingeffectofH2Oaccordswiththeliterature[18–23], whereasthedecreaseofproductivityathighH2Oconcentration,is probablyimputabletotheloweringofmonomerssolubilityinH2O (Table2).

Fig. 4 shows the influence of H2O on theLVN, which is in therange1.18–1.20dLg−1,correspondingtoanaverageviscosity molecularweight(MW)of61–62kgmol−1.Itisinterestingtonote thatLVNispracticallyunaffectedbytheconcentrationofH2Oand bytheaproticorganicsolventused.

0 10 20 30 40 50 60 70 80 90 100 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 pr oductivity, gPK gPd -1 h -1 H₂O, molar % nitromethane dioxane

Fig.3. InfluenceofH2Oontheproductivityin1,4-dioxaneandnitromethane.Run

conditions:[Pd(OAc)2(dppp)]=1.57×10−3mmol;volumeofthereactionmedium

80mL;HCOOH5mol%;363K;4.5MPa(CO/C2H4=1/1);1h;700rpm.

0 10 20 30 40 50 60 70 80 90 100 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 LVN, dL/g H 2O, molar % dioxane nitromethane

Fig.4.InfluenceofH2OontheLVNin1,4-dioxaneandnitromethane.Run

condi-tions:thoseofFig.3.

3.3. OptimizationoftheHCOOH/H2O/1,4-dioxaneratio

Since in 1,4-dioxane a higher productivity is obtained, we optimizedtheHCOOH/H2Oratioundertheconditionsofthe max-imumofFig.3(56mLof1,4-dioxane).Theproductivityincreases fromca.11.50uptoca.18.35kgPK(gPdh)−1whenthemolarratio H2O/HCOOHisca.2/1,correspondingtothemolarfraction0.67mol H2O/mol(H2O+HCOOH)inFig.5.

Furthermore, by keeping constant H2O/HCOOH=2/1 molar ratio, the productivity passes through a maximum of ca. 21.00kgPK(gPdh)−1 when HCOOH/H2O/1-4,dioxane is ca. 2.7/1.35/1(1,4-dioxaneisca.20mol%,Fig.6).Undersuchreaction conditionstheLVNis1.28dLg−1,correspondingto67.13kgmol−1. ProductivityandLVNaresignificantlyhigherthanthoseobtainable with“cationic”[Pd(TsO)(H2O)(dppp)](TsO)inMeOH[32].

It is interestingto notethat without1,4-dioxane(solvent is H2O/HCOOH,2/1molarratio), theproductivitydecreasestoca. 11.00kgPK(gPdh)−1, and that in pure 1,4-dioxane no catalytic activityisobserved. 0 10 20 30 40 50 60 70 80 90 100 0 1500 3000 4500 6000 7500 9000 10500 12000 13500 15000 16500 18000 19500 produc tivity, gP K gP d -1 h -1 HCOOH, molar %*

Fig.5.EffectoftherelativeconcentrationofHCOOH-H2Oontheproductivityin

1,4-dioxane.Runconditions:[Pd(OAc)2(dppp)]=1.57×10−3mmol;volumeofthe

reactionmedium80mL,1,4-dioxane56mL;363K;4.5MPa(CO/C2H4=1/1);1h;

0 10 20 30 40 50 60 70 80 90 100 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 productivity, gPK gPd -1 h -1 dioxane, molar %

Fig. 6. Effect of 1,4-dioxane mol% at constant H2O/HCOOH molar ratio on

theproductivity.Runconditions:[Pd(OAc)2(dppp)]=1.57×10−3mmol;volume

ofthereactionmedium80mL;H2O/HCOOH=2/1(molarratio);363K;4.5MPa

(CO/C2H4=1/1);1h;700rpm.

3.4. InfluenceoftemperatureonproductivityandLVN

TheinfluenceoftemperatureontheproductivityandLVNhas beentestedintherange333–363Kunder4.5MPa(CO/ethene1/1). Table3showsthat theproductivityincreasesbyincreasingthe temperature,whereastheLVNdecreases.Theapparentactivation energyof12.7kcalmol−1,evaluatedbytheArrheniusplot,accords withareactionunderkineticcontrol[33,34].

3.5. InfluenceofthepressureonproductivityandLVN

TheproductivityandtheLVNincreasebyincreasingthetotal pressure.Under9.0MPa,CO/ethene=1/1,37.50kgPK(gPdh)−1are obtainedhavingLVN2.77dLg−1(MW=166.48kgmol−1)(Fig.7). 3.6. Onthechain-transferprocess

Itshouldbestatedfirstthatthecopolymerpresentsexclusively keto-endgroups.Thisfeaturewasfoundalsoforothercatalytic sys-temsusedinaH2O AcOHorH2O HCOOHmixture[18–25].The natureoftheendinggroupsdependsonthechain-transfer pro-cess.WhenH2O-AcOHwasusedasasolvent,itwasproposedthat thisprocessinvolvesprotonolysiswithH2Owithformationofa Pd(II) OHspecies,thatgeneratesthePd–hydrideinitiatorafterCO insertionfollowedbyCO2evolution(Scheme1,stepsc,e,andf). AllthismayholdalsointhepresentcasewhenH2Oisaddedto 1,4-dioxaneornitromethanewithdissolved5%HCOOH(Scheme1, stepsa–d)and alsowhenonly HCOOHisadded,becausesome H2Omightbepresentinthesolvent.However,inthislattercase, consideringthattheLVNsignificantlylowersuponincreasingthe concentrationofthe acid(cf.Fig.2), thechain-transfer process mightoccurthroughprotonolysiswiththeacid,withformationof thePKandofaPd(II)-formatespecies[35]whichwouldgenerate thePd(II)–Hinitiatorafter␤-hydridetransferandCO2evolution (Scheme1,steps c and g).Thissuggestionis supportedby an Table3

InfluenceoftemperatureontheproductivityandLVN.

T(K) Productivity(kgPK(gPdh)−1) LVN(dLg−1) 333 12.14 1.95 343 15.11 1.82 353 18.10 1.41 363 21.00 1.28 0 1 2 3 4 5 6 7 8 9 10 0 2500 5000 7500 10000 12500 15000 17500 20000 22500 25000 27500 30000 32500 35000 37500 40000 pr oduc tivity, gPK gPd -1 h -1 P_tot, MPa

Fig. 7.Influence of total pressure on the productivity. Run conditions: [Pd(OAc)2(dppp)]=1.57×10−3mmol;volumeofthereactionmedium80

mL;1,4-dioxane/H2O/HCOOH=2.7/1.35/1(molarratio);363K;CO/C2H4=1/1;1h;700rpm.

experimentcarriedoutundertheconditionsofthemaximum

pro-ductivityin1,4-dioxaneinthepresenceofHCOOH(cf.Fig.1),but

alsointhepresenceof5%ofaceticanhydride,whichhasbeenadded inordertoeliminateH2Ofromthereactionmedium.After1h reac-tion1.1gofPKwererecovered,productivity10.6kgPK(gPdh)−1, LVN=0.93dLg−1.ThefactthatinH2O AcOHorH2O HCOOHthe LVNincreasesupon increasingtheacidconcentration[21–23]is alsoinfavourofthesuggestionthatundertheconditionsofFig.2 thechain-transferprocessoccurthroughprotonolysiswithformic acid.ThecomparisonofFig.1withFig.2suggestsfurther com-ments.ItshowsthattheLVNdecreaseswiththeincreaseofHCOOH concentrationalsointherangeofHCOOHconcentrationwherethe productivityincreases.Asfarasthemolecularweightisconcerned, itdependsontherateofthechaingrowingprocesswithrespectto theprocessofthechaintermination.Asmentionedabove,theacid

Pd-H

Pd-CH

CH

Pd-CH

CH

-[C(O)CH

CH

]

-H

CH

CH

n CH

CH

n CO

Pd-OH

PdC(O)OH

CO

CO

CH

CH

-[C(O)CH

CH

]

-H

PK

H

O

Pd-OC(O)H

HCOOH

PK

HCOOH

H

O

where:PK is

a

b

c

c'

d

e

CO

g

f

Pd-OAc

H

O, CO

CO

HOAc

HCOOH

CO

HOAc

1

₂

Scheme1.Proposedpathwaysfortheactivationofthecatalyticprecursorandfor thechain-transferprocess.

maydestabilizethe␤-and␥-chelaterings,thusfavouringthechain growingprocess[36].However,thedecreaseofthesolubilityofthe monomersuponincreasingtheacidconcentrationhasanopposite effect.Wehavejustshownthattheacidisinvolvedinthe termi-nationstep.ThismightbeanotherreasonwhyLVNlowers.With methanol,ithasdemonstratedthattheterminationprocessoccurs viaanenolateformationstepfroma ␤-chelate,which isslower thanthesubsequentmethanolprotonolysis[37].Ifthisholdsalso withthereactionmediaofFig.2,thentheprotonolysiswillbe effec-tivetoaminorextentuponincreasingHCOOHconcentration.This meansthattheloweringoftheLVNismainlyduetothelowering oftheconcentrationofthemonomers.

Steps1and2 ofScheme1showhowtheprecursor maybe activatedtothehydridethatstartsthefirstcatalyticcycle.

4. Conclusion

HCOOH efficiently promotes the catalytic activity of [Pd(OAc)2(dppp)]in1,4-dioxane(11.20kgPK(gPdh)−1,CO/ethene 1/1, 4.5MPa, 363K) and nitromethane (8.5kgPK(gPdh)−1, CO/ethene1/1,4.5MPa,363K).LVNlinearlydecreasesby increas-ingthe HCOOHconcentration inboth solvents,which suggests a direct involvementof theacidin thechain termination step, althoughanadditionaleffectcanbedue totheloweringofthe solubilityofthemonomers.

Furthermore,ithasbeenfundthatalsoH2O,underacid con-ditions,efficientlypromotesthereactioninthesamesolvents.A combinationofbothpromotersleadstothebestresults.Thehighest productivityhasbeenobtainedinHCOOH/H2O/1-4,dioxaneinthe molarratio2.7/1.35/1(ca.37.50kgPK(gPdh)−1,LVNof2.77dLg−1, at363Kunder9.0MPaofCO/ethene=1/1).

Acknowledgements

Ca’FoscariUniversityofVenice(Ateneofunds2008–2011)and theMIUR(PRIN 2008)aregratefullyacknowledgedforfinancial support.AspecialthanktoDr.EmilianoCelegatoandDr.Marco Scattofortheexperimentalassistance.

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