Synthesis of Waterborne Polyurethane by the Telechelic ,-Di(hydroxy)poly(n-butyl acrylate)

16 Pages·2017·2.36 MB·English

polymers
Article
Synthesis of Waterborne Polyurethane by the
Telechelic α,ω-Di(hydroxy)poly(n-butyl acrylate)
XinChen,ChiZhang,WeidongLi,LeiChenandWushengWang*
SchoolofChemistryandChemicalEngineering,AnhuiUniversity,111JiulongRoad,Hefei230601,China;
[email protected](X.C.);[email protected](C.Z.);[email protected](W.L.);
[email protected](L.C.)
* Correspondence:[email protected];Tel.:[phone]
Received:26January2018;Accepted:21February2018;Published:23February2018
Abstract: A key for the preparation of polyacrylate-based polyurethane is the synthesis of
hydroxyl-terminated polyacrylate. To our knowledge, exactly one hydroxyl group of every
polyacrylatechainhasnotbeenreported. Thehydroxyl-terminatedpoly(butylacrylate)(PBA)has
beensuccessfullysynthesizedbydegenerativeiodinetransferpolymerization(DITP)ofthen-butyl
acrylate(n-BA)using4,4(cid:48)-azobis(4-cyano-1-pentanol)(ACPO)anddiiodoxylene(DIX)asinitiator
andchaintransferagent,respectively,andsubsequentlysubstitutedreactionoftheiodine-terminated
PBA with β-mercaptoethanol in alkaline condition. The latter reaction was highly efficient, and
the terminal iodine at the end of polymer chains were almost quantitatively transformed to a
hydroxyl group. 2,2(cid:48)-Azobis(isobutyronitrile) (AIBN) and ACPO were used as initiators in the
DITPs of n-BA. The results demonstrated that they had a significant influence on the terminal
groups of the formed polymer chains. The structure, molecular weight, and molecular weight
distributionofthehydroxyl-terminatedPBAhavebeenstudiedby1H,13CNMR,andGPCresults.
Thecomponentsofhydroxyl-terminatedPBAweredeterminedbyMALDI-TOFMSspectra,andtheir
formationisdiscussed. ThebroadmolecularweightdistributionofthePBAandthedifferencein
thepolymerizationbehaviorsfromtypicallivingradicalpolymerizationareexplainedbasedonthe
resultsof1HNMRandMALDI-TOFMSspectra. Thehydroxyl-terminatedPBAhasbeensuccessfully
usedinthepreparationofPBA-basedpolyurethanedispersions(PUDs). TheaqueousPUDswere
stable,andbasedontheDSCresultsitcanbesaidthatthemiscibilityofhardsegmentswithPBA
chainswasimproved.
Keywords: poly(n-butyl acrylate); degenerative iodine transfer; free-radical polymerization;
waterbornepolyurethane
1. Introduction
Polyurethanedispersions(PUDs)aregenerallysynthesizedbythecondensationpolymerization
of oligomer polyols, diisocyanate, chain extender, and hydrophilic comonomers. The oligomer
polyolsusedinthepreparationofPUDsaremostlydi-functionaloligomerdiols,suchaspolyether
diols,polyesterdiols,andpolycarbonatediols[1]. Comparedtotheseoligomers,theadvantagesof
polyacrylatearegoodadhesiontothematrices,excellentlightstability,andchemicalresistance. PUDs
arewell-knownfortheirpeculiaradvantages,suchashighimpactstrengthatlowtemperatures,tear
resistance,andgoodelasticity[2–5]. Ifpolyurethaneandpolyacrylatearecombinedinonepolymerto
formpolyacrylatepolyurethane,thenewlyformedPUDsmayhavetheadvantagesofbothpolymers,
which may have potential applications [6–8]. At present, the commercially available polyacrylate
polyolshavebeenmanufacturedbythecopolymerizationofacrylatemonomersandhydroxy-alkyl
acrylatemonomers(e.g.,2-hydroxyethylacrylate),andthehydroxylgroupsarerandomlydistributed
inthepolymersidechains. Ingeneral,thehydroxylcontentis2–4%,andtheaveragefunctionalityis
Polymers2018,10,219;doi:10.3390/polym10020219 www.mdpi.com/journal/polymers
Polymers2018,10,219 2of16
greaterthanthree[9–11]. Becausethestructureandfunctionalityofthepolyacrylatepolyolchains
are inhomogeneous, and cross-linking reaction may occur when this kind of polyol reacts with
isocyanate,polyacrylatepolyolsareseldomusedtoprepareaqueousPUDs. Althoughpreparationof
thewaterbornepolyurethane–polyacrylatehybridemulsionshasbeenreported,lowcompatibility
between the polyurethane and the polyacrylate has limited its applications [12–16]. This problem
can be solved by using the polyacrylate as soft segments of aqueous PUDs. The key issue for
achievingthispurposeisthesynthesisoftheoligomerpolyacrylatediol. However,severalreview
papersmerelydiscussthesynthesisofthepolyacrylatediols[17,18];onlyoneU.S.patentwewere
abletofindintheliteratureinvolvedthesynthesisofhydroxyl-terminatedpolyacrylates[18]. Itis
difficulttobelievethatbothendsofeverypolymerchainarecappedwithahydroxylgroupbecause
2,2(cid:48)-azobis(isobutyronitrile)(AIBN)isusedinthepreparation,whichwillbediscussedfurther.
Livingradicalpolymerization(LRP)hasbeenshowntobeaviablemethodfortherationaldesign
ofpolymerswithpredictablemolecularstructureandnarrowmolecularweightdistributions[19,20],
andfundamentalkineticfeaturesoftheLRPhasbeendiscussed[19,20]. Amongseveralstrategiesof
LRPs,degenerativeiodinetransferpolymerization(DITP)hasbeenreceivingourspecialattention
becausetheiodinegroupattheendoftheformedpolymercanbealmostquantitativelytransformed
to a hydroxyl group via a simple procedure. However, other LRPs (e.g., atom transfer radical
polymerization and reversible addition-fragmentation chain transfer polymerization) can also be
used in the preparation of polyacrylate diols. DITP controls chain growth through a reversible
reactionofthegrowingchainradicalswithiodocompound,limitingirreversibleterminationofthe
growingchainradicals;themechanismofDITPandDITPinthepresenceofhalogenatedmonomers
have been discussed in detail [21]. A number of homopolymers and block copolymers including
polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(vinyl chloride) (PVC) have been
successfully synthesized through DITP [20,22–26]. In the case of solution polymerization, a large
varietyofsolvents(e.g.,toluene,anisole,etc.) canbeusedaspolymerizationmedia[20,22]. DITPhas
beensuccessfullyusedinemulsionpolymerizationforthepreparationofpolystyrene-b-poly(butyl
acrylate) [20,22,27]. Many studies have demonstrated that the DITP is a feasible method for
the synthesis of low-molecular-weight polyacrylate (M = 2000–6000 g mol−1), but the ratio of
n
iodine-end-cappedfunctionalgroupscannotreach100%(mostlyonly50–80%). Besidestheiodine,the
otherendgroupcomesfromtheinitiatorfragment[28,29]. Conversionoftheend-iodinegroupsinto
hydroxylgrouphasbeenstudied[18,30],buttheyieldedproductcannotbeusedinsynthesisofhigh
molecularweightpolyurethane.
Thepurposeofthestudyreportedhereinistosynthesizeawell-definedα,ω-di(hydroxy)poly(n-
butylacrylate)withrelativelylowmolecularweight,andthentouseitinthesynthesisofPUDswith
polyacrylateassoftsegment.
2. MaterialsandMethods
2.1. Materials
4,4(cid:48)-Azobis(4-cyano-1-pentanol)(ACPO)waspurchasedfromHenanLienchemInc. (Zhengzhou
China). 2,2(cid:48)-Azobis(isobutyronitrile)(AIBN)wasboughtfromEnergyChemical(Shanghai,China)
andpurifiedbyrecrystallizationinethanol. n-Butylacrylate(n-BA),α,α(cid:48)-dibromo-p-xylene,decane,
d-chloroform(CDCl ),andisophoronediisocyanate(IPDI)werepurchasedfromMacklin(Shanghai,
3
China). Toluene, 2-mercaptoethanol, 2,2-dimethylol propionic acid (DMPA), and acetone were
purchasedfromSinopharmChemicalReagentCo.,Ltd. (Shanghai,China). Calciumoxide,potassium
carbonate(K CO ),1,4-butanediol,andtriethylaminewereboughtfromAladdin(Shanghai,China).
2 3
Allotherchemicalswereusedasreceived.
Polymers2018,10,219 3of16
2.2. Methods
1H nuclear magnetic resonance (1H NMR) and 13C nuclear magnetic resonance (13C NMR)
(400MHz)wereperformedatroomtemperatureonAVANCEII400MHzspectrometersinCDCl
3
usingtetramethylsilaneasaninternalstandard.
Molecular weight and molecular weight distribution were measured on a PL-GPC120 gel
permeationchromatograph(GPC)equippedwithaPLGel5µmcolumn(300×7.5mmpurchasedfrom
PolymerLaboratories(SantaClara,CA,USA))andarefractiveindexdetector(G1362A1100/1200series
refractiveindexdetector(AgilentTechnologiesInc.,SantaClara,CA,USA)at40◦C.Tetrahydrofuran
(THF)wasusedassolventataflowrateof1mL/min. Themonodispersepolystyrenestandards(test
rangeofmolecularweightswasfrom1000to200,000g/mol,PolymerLaboratories)wereutilizedin
thecalibrationofM ,M ,andM /M . Sampleconcentrationwas1mg/mL,andsampleinjection
n w w n
volumewas300µL.
Mass-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF)
measurements were performed on a Bruker Autoflex III TOF/TOF mass spectrometer (Bruker
Daltonics Inc., Billerica, MA, USA). The instrument was operated in positive ion reflection mode
withanacceleratingpotentialof+20kV.trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-proprnylidene]
malononitrilewasusedasamatrixanddichloromethaneasasolvent. Sodiumiodidewasdissolvedin
methanolandusedastheionizingagent. Sampleswerepreparedbymixing2µLofpolymersolution
with2µLofmatrixsolution. Then,1.0µLofthesemixtureswasdepositedonatargetplateandthe
solventwasremovedinastreamofnitrogen.
The hydroxyl number of the α,ω-di(hydroxy)poly(n-butyl acrylate) was determined by the
phthalicanhydriderefluxmethodinaccordancewithASTMStandardD2849.
Differentialscanningcalorimetry(DSC)analysiswasconductedonaQ2000ofTAinstruments
(TA Instruments, New Castle, DE, USA) in the range of −80 ◦C to 150 ◦C. The experiments were
carriedataheatingrateof20◦C/minundernitrogenflowatarateof20mL/min. Thesampleswere
driedat65◦Cfor48hundervacuumpriortoanalysis.
Thestabilityofemulsionswascharacterizedbycentrifugationat3000rpm/minfor15minat
roomtemperature,andthenwereobservedtoevaluatewhetherprecipitationoccurred.
Particlesizeandparticlesizedistributionweremeasuredat25◦CwithZetasizerNanoseriesZS
90(MalvernInstrumentsLtd.,Malvern,UK).Thesamplesweredilutedto1.85%(massconcentration)
indistilledwater.
2.3. SynthesisofDiiodoxylene
Thesyntheticprocedurewassimilartothepreviousreport[23,31]. Sodiumiodide(0.08mol)and
α,α(cid:48)-dibromo-p-xylene(0.03mol)weredissolvedinacetoneundernitrogen. Thereactionwascarried
outatroomtemperature,andaftersubsidence,thestirringwascontinuedforanadditional30min.
Theproductwasprecipitatedbyaddingwater(250mL),andthesaltformedwasdissolvedinwater.
Then,theprecipitatewasremovedbyfiltration,washedwithwaterfivetimes,andthefinalproduct
wasobtainedbydryinginavacuumovenatroomtemperatureovernight.
2.4. DegenerativeIodineTransferPolymerizationofn-BA
n-BA(0.39mol),ACPO(0.02mol),diiodoxylene(0.04mol),andtoluene(120mL)wereplacedin
a500mLround-bottomflaskprotectedinthedark. Thereactionmixturewasheatedinanitrogen
atmosphereat60◦Cundermagneticstirring. After24h,thepolymerizationsolutionwascooledin
anicebath. Subsequently,potassiumcarbonate(K CO )wasaddedintotheflaskinordertochange
2 3
thepHofthereactionmixturetoalkalinity,andthenβ-mercaptoethanolwasaddedtothereaction
mixturefortransformationofend-iodinetoend-hydroxylgroup. Thereactionsolutionwascollected
byfiltrationthrough1.0µmmicroporousfilters,andtolueneinthecollectedsolutionwasremovedby
rotaryevaporation. Then,thetargetproductwaspurifiedbywashingwithethanolanddistilledwater.
Polymers2018,10,219 4of16
Aftertheethanolintheproductwasremovedinavacuumoven,asampledissolvedintoluenewas
heatedat110◦Cfortheremovalofwaterandthepurifiedproductwasobtained.
2.5. KineticsStudyofPoly(n-butylacrylate)byDegenerativeIodineTransferPolymerization
In order to study the kinetic features of this bulk polymerization system, we carried out the
polymerizationsat60◦Candtooksamplesperiodicallyatthedesignedpolymerizationtime(24h).
Monomerconversionwasdeterminedbygravimetricanalysis. Severaldropsof4-hydroxyanisole
(MEHQ)(0.5wt%)wereaddedto0.5mLofpolymersolutionevery30min. Then, weplacedthe
sampleinanicebath. Thesmallmoleculesinsolutionwerethenremovedbyvacuumdryingat65◦C.
Theweightbeforeandafterdryingwascompared,andmonomerconversionwasobtainedbythe
equation:
m
Conversion(%) = 3 ×100%
m − m
1 2
wherem istheweightofsolution,m istheweightofsolvent,andm istheweightofdriedpolymer.
1 2 3
Theobtainedpolymerwascharacterizedby1HNMRspectroscopy.
2.6. SynthesisofPolyacrylate-BasedPolyurethaneDispersions(PUDs)
Poly(n-butylacrylate)diol(6.67 × 10−3 mol)waschargedunderstirring, andtheisophorone
diisocyanate(IPDI)(0.03mol)wasthenslowlyaddedintothediol. Themixturewasthenheatedto
80–85◦Cuntilthetheoreticalisocyanategroup(NCO)contentoftheprepolymerwasreached,which
wasestimatedusingthedi-n-butylaminetitrationmethod. Then,thechainextenders1,4-butanediol
anddimethylolpropionicacid(DMPA)(9.40×10−3 mol)wereaddedintothemixtureatthesame
temperaturefor1.0hthroughadjustingtheviscositybyacetone. Distilledwater(106mL)wasthen
addedintotheneutralizedPUbyusingtriethylamine(9.4×10−3mol)asaneutralizingagentafter
addingcatalyst. Anaqueousdispersionof20wt%solidswasobtainedafterremovalofacetoneby
decompressingdistillation.
3. ResultsandDiscussion
3.1. DesignofDITPSystemforPreparationofHydroxyl-TerminatedPoly(butylacrylate)
Topreparelow-molecular-weightpoly(butylacrylate)(PBA)withbothendscappedwithhydroxyl
groupsviaDITP,thefirstissueisthedesignofanappropriatepolymerizationsystem. Accordingto
previousreports[18,32],hydroxyl-terminatedpolyacrylatecouldbepreparedbytheDITPofacrylates
usingdiiodoxylene(DIX)andAIBNasdi-functionalchaintransferagentandinitiator,respectively,
andthispatentclaimedthattheend-functionalizedpolymerswereusefulasreactiveintermediatesin
condensationpolymerization,chainpolymerization,andheterogeneouspolymerization. Therefore,
wefirstpreparedthePBAdiolsaccordingtothereportedmethod[18]. TheresultsarelistedinTable1,
andthemolecularweightwasdeterminedbyGPC,theOHvaluewasmeasuredbyphthalicanhydride
refluxmethod,andthefunctionalityofend-hydroxylgroupwascalculatedbasedon1HNMRspectra
asshowninFigure1. Surprisingly,weobservedthatthefunctionalityofend-hydroxylgroupinthe
resultant polymers was less than 60% (Table 1). Thus, it is necessary to clarify what happened in
thispolymerization.
Polymers2018,10,219 5of16
Polymers 2018, 10, x FOR PEER REVIEW 6 of 15
Figure 1a,b show the proton NMR spectra of the PBA obtained respectively from DITP for 1 h
Table1.Degenerativeiodinetransferpolymerization(DITP)conditionandresultsofn-butylacrylate
and c(no-mBAp)leutes icnogndvieiorsdiooxny olefn ne-B(DAI.X T)haen dpr2o,2to(cid:48)-na zsoigbinsa(ilsso abtu tδy =ro 4n.i0tr4i,l e2).3(6A, IaBnNd) 1a.s91tr panpsmfe raraeg ernestpaencdtively
ascribineidtia ttoo rt,hrees epsetcetriv melye.thylene group and the methine and methylene groups in the backbone of the
PBA chain; the aromatic protons and two methylene protons of the xylene units appear at δ = 7.05
and 2.51 ppm, respectively. Other proton signals oMf the PBA arOe Hmavraklueed in FigFuurnec t1io, naanldit ythoef vinyl
signaElxsp o.f* n-BA[n -mBAo]n:[oAmIBeNr a]:t[ C6T.3A8s, ]6.C14o,n av.n(d% 5).83 pp(gmmn c,oGolP−mC1p)lete(lmy gdiKsaOpHp/ega)r in eFnidg-uhryed 1robx; yall(l% o)f athese
results 1indicate tha1t 0P:1B:0A.5 was successfu8l9ly synthesiz2e3d1.0 The signal o15f methylene proto4n2s in the DIX
unit at 2δ = 4.45 ppm1 a0l:1m:1ost disappeare8d7, and a new2 s0i2g0nal at δ = 4.3200 ppm appeared i4n0 Figure 1a,b,
3 10:0.5:1 83 1910 25 54
which is attributed to the methine proton of the BA unit next to terminal iodine. This demonstrates
4 10:0.24:1 78 1680 26 49
that reversible transfer reaction of the growing chain radical with DIX occurred, forming p-
5 10:0.16:1 76 1540 28 48
iodomethylene benzyl radicals and then initiating n-BA to polymerize, producing PBA. Based on the
*Allofthereactiontimeswere7.5haccordingthereport[18].aCalculatedby([ICH2/2]/([ICH3/6]+[ICH2/2])×
integ1r0a0t,iwonhe rreatIiCoH 3oifs tthheei nsiteggnraall vaatl u4e.3o0f mpeptmhy lapnrodt otnhoef stihgeninailt iaatto 4r.f4ra5g pmpenmt,a ta1p.2p0r–o1.x3i0mpapmte,lyan 9d0I%CH 2oifs tthhee DIX
unitsi nwteegrrael lvoacluaeteodfm inet hthylee nmepidrodtolen sooff PβB-mAer ccahpatoinetsh.a nolat3.6–3.8ppm.

Figure 1. 1H NMR spectra of the polymers respectively obtained from (a) the DITP of n-BA using
Figure1. 1HNMRspectraofthepolymersrespectivelyobtainedfrom(a)theDITPofn-BAusing
AIBN as initiator and DIX as transfer agent for 1 h, and (b) for complete consumption of BA. (c) The
AIBNasinitiatorandDIXastransferagentfor1h,and(b)forcompleteconsumptionofBA.(c)The
polymer obtained after converting the iodide group at the ends of the polymer in (b) into the hydroxyl
polymerobtainedafterconvertingtheiodidegroupattheendsofthepolymerin(b)intothehydroxyl
group through reaction with β-mercaptoethanol.
groupthroughreactionwithβ-mercaptoethanol.
When the iodine-terminated PBA was treated with β-mercaptoethanol, the hydroxyl-terminated
BasedonthemechanismofDITPreportedinReferences[21,24],theDIPTmechanismofBAcan
PBA was obtained, and its proton NMR spectrum is shown in Figure 1c. We can see that the proton
bedepictedinScheme1. Thechaingrowthiscontrolledbyreversiblereactionofthegrowingchain
signals at δ = 4.45 and 4.30 ppm completely disappeared, two new proton signals appeared in Figure
radicalswithDIX,andafterthepolymerization,thexylenegroupsarelocatedinthemiddleofPBA
1c: one signal at δ = 3.73 ppm, which is ascribed to methylene protons next to the terminal hydroxyl
chain;however,thePBAwithoutxylenegroupcannotbeexcluded. Accordingtothismechanism,
group; another at δ = 2.67–2.96 ppm, which is attributed to methylene protons adjacent to an ether
thechainendgroups—whichareofespecialinterestinthisstudy—includeiodineandisobutyronitrile
sulfur atom. These results demonstrate that the substitute reaction of HOCH2CH2SK with PBA-I is
groups. Whentheβ-mercaptoethanolwasaddedintothereactionsystem,theiodineattheendof
almost quantitative. When we carefully analyze the 1H NMR spectrum in Figure 1c, we can observe
a proton signal at δ = 1.26 ppm, which is ascribed to methyl protons of the isobutyrontrile unit. Based
Polymers 2018, 10, x FOR PEER REVIEW 5 of 15
Based on the mechanism of DITP reported in References [21,24], the DIPT mechanism of BA can
be depicted in Scheme 1. The chain growth is controlled by reversible reaction of the growing chain
radicals with DIX, and after the polymerization, the xylene groups are located in the middle of PBA
chain; however, the PBA without xylene group cannot be excluded. According to this mechanism,
tPholey mecrhsa20in18 ,1e0n,d21 9groups—which are of especial interest in this study—include iodine 6aofn1d6
isobutyronitrile groups. When the β-mercaptoethanol was added into the reaction system, the iodine
at the end of the formed polymer chains could be converted to a hydroxyl group. In order to verify
theformedpolymerchainscouldbeconvertedtoahydroxylgroup. Inordertoverifytheseresults,
these results, 1H NMR was used to follow the polymerization, and typical 1H NMR spectra are shown
1HNMRwasusedtofollowthepolymerization,andtypical1HNMRspectraareshowninFigure1.
in Figure 1.
initation:
CN CN CN
H3C C N N C CH3 2H3C C +N2
CH3 CH3 CH3
chainpropagation:
CN CN
CH3 C +H2C CH H3C C CH2 CH
CH3 COOBu CH3 COOBu
CHNH H CN
H3C C CH2 C +nH2C CH H3C C CH2 CH
CH3 COOBu COOBu CH3 COOBu
chaintransfer:
CN CH2NH2
2 H3C C CH2 CH + I CH2 CH2 I 2H3C C CH2 CH I +
CH3 COOBu CH3 COOBu
chainpropagation:
H H
+2H2C CH C CH2 CH2 CH2 CH2 C
COOBu CH2OOBu CH2OOBu
H H
C CH2 CH2 CH2 CH2 C +nH2C CH
COOBu COOBu COOBu
degenerativechaintransfer:
H H CN CN H H
C CH2CH2 CH2 CH2 C +2H3C C CH2CH I 2H3C C CH2CH + I C CH2 CH2 CH2 CH2 C I
COOBu COOBu CH3 COOBu CH3 COOBu COOBu COOBu
CN CN
+ 2H3C C CH2CH I 2H3C C CH2CH + I I
CH3 COOBu CH3 COOBu
termination:
CN CN CN
H
2H3C C CH2 CH + H3C C CH2 C CH CH2 C CH3
CH3 COOBu CH3 COOBu COOBu CH3
CN CN CN
2H3C C CH2 CH H3C C CH2 CH CH CH2 C CH3
CH3 COOBu CH3 COOBuCOOBu CH3
OH
HS
I I HO OH
K2CO3
Scheme1.Mechanismofthedegenerativeiodinetransferpolymerizationofn-BA.
Scheme 1. Mechanism of the degenerative iodine transfer polymerization of n-BA.
Figure1a,bshowtheproton NMRspectraofthePBAobtainedrespectivelyfromDITPfor1h
andcompleteconversionofn-BA.Theprotonsignalsatδ=4.04,2.36,and1.91ppmarerespectively
ascribedtotheestermethylenegroupandthemethineandmethylenegroupsinthebackboneofthe
PBAchain;thearomaticprotonsandtwomethyleneprotonsofthexyleneunitsappearatδ=7.05
and2.51ppm,respectively. OtherprotonsignalsofthePBAaremarkedinFigure1,andthevinyl
signalsofn-BAmonomerat6.38,6.14,and5.83ppmcompletelydisappearinFigure1b;allofthese
resultsindicatethatPBAwassuccessfullysynthesized. ThesignalofmethyleneprotonsintheDIX
Polymers2018,10,219 7of16
unitatδ=4.45ppmalmostdisappeared,andanewsignalatδ=4.30ppmappearedinFigure1a,b,
whichisattributedtothemethineprotonoftheBAunitnexttoterminaliodine.Thisdemonstratesthat
reversibletransferreactionofthegrowingchainradicalwithDIXoccurred,formingp-iodomethylene
benzylradicalsandtheninitiatingn-BAtopolymerize,producingPBA.Basedontheintegrationratio
ofthesignalat4.30ppmandthesignalat4.45ppm,approximately90%oftheDIXunitswerelocated
inthemiddleofPBAchains.
Whentheiodine-terminatedPBAwastreatedwithβ-mercaptoethanol,thehydroxyl-terminated
PBAwasobtained,anditsprotonNMRspectrumisshowninFigure1c. Wecanseethattheproton
signalsatδ=4.45and4.30ppmcompletelydisappeared,twonewprotonsignalsappearedinFigure1c:
one signal at δ = 3.73 ppm, which is ascribed to methylene protons next to the terminal hydroxyl
group;anotheratδ=2.67–2.96ppm,whichisattributedtomethyleneprotonsadjacenttoanether
sulfuratom. TheseresultsdemonstratethatthesubstitutereactionofHOCH CH SKwithPBA-Iis
2 2
almostquantitative. Whenwecarefullyanalyzethe1HNMRspectruminFigure1c,wecanobservea
protonsignalatδ=1.26ppm,whichisascribedtomethylprotonsoftheisobutyrontrileunit. Basedon
thepolymerizationmechanismshowninScheme1,thisinitiatorfragmentshouldstandattheend
ofthePBAchain. Therefore,theobtainedPBAhastwoendgroups: hydroxylandisobutyrontrile.
The hydroxyl functionality of the obtained PBA can be calculated based on integral values of the
signalsatδ=1.26and3.73ppm,andtheresultsarelistedinTable1. Theoretically,therearethree
typesofPBAbasedontheirdifferentendgroups,HO–PBA–OH,HO–PBA–I,andI–PBA–I(Irefersto
isobutyrontrilegroup). TheresultsinTable1showthatthehydroxylfunctionalityofallobtainedPBAs
isapproximately50%;themolarpercentageofHO–PBA–OH—whichisexpectedinthepreparationof
aqueousPUDs—waslessthan50%. Suchhydroxyl-functionalizedPBAwillsignificantlyinfluencethe
propertiesoftheformedaqueousPUDsbecausetheI–PBA–Iisinertinthereactionwithdiisocynates
andtheHO–PBA–Iterminatesthecondensationpolymerizationofdiolwithdiisocynatemonomers.
TosynthesizethePBAwith100%hydroxylfunctionality,thedesignofanewformulationisnecessary.
Basedontheabovediscussion,thelowhydroxylfunctionalityofthePBAisduetotheuseofAIBN
withouthydroxylgroupasinitiator;thus,theinitiatorACPOwithtwoterminalhydroxylgroupswas
selectedandusedintheDITPofn-BA.
3.2. DITPofn-BAUsingDIXandACPOasTransferAgentandInitiator
Inordertosynthesizelow-molecular-weightPBAwith100%hydroxylfunctionalityforapplication
inthepreparationofaqueousPUDs,lowfeedmolarratioofmonomertotransferagentisgenerally
required. Thus, the DITP of n-BA with feed molar ratio of [n-BA]/[ACPO]/[DIX] = 16:0.08:1 was
conductedat60◦Cfor24h. ThePBAwasobtainedbyprecipitation,filtration,anddrying,anditsGPC
curveand1HNMRspectrumweremeasured. Figure2showsitsGPCcurve,whichrevealsasingle
curvewithapronouncedtail. Theprobablereasonisirreversibleterminationofthegrowingchain
radicals,becauseattheinitialstageofpolymerization,fastdecompositionofinitiatorACPOproduced
toomanyprimaryradicals,andthenthegrowingchainradicalscouldnotbeefficientlycapturedby
transferagent(DIX),leadingtotheirirreversibletermination. ThetailoftheGPCcurveledtoabroad
molecularweightdistribution(M /M =1.67).
w n
After the iodine-terminated PBA was treated with β-mercaptoethanol in alkaline conditions,
thehydroxyl-terminatedPBAwasobtained,andits1HNMRspectrumwasmeasuredandisshownin
Figure3. Similartothe1HNMRspectrumofPBAobtainedbyinitiationofAIBN,thecharacteristic
ester methylene proton signal, the methane, and methylene proton signals in the PBA backbone
appearatδ=4.08,2.33,and1.96ppm,respectively,demonstratingthesuccessfulsynthesisofPBA.
TheiodomethyleneprotonsignalofDIXunitatδ=4.45ppmcompletelydisappeared, andanew
protonsignalclearlyappearedatδ=2.57ppm. Thisisbecausethereversiblereactionofthegrowing
PBAchainradicalswithDIXproducedp-iodomethylenebenzylradicalsandtheiodine-terminated
PBA,andtheformalradicalsinitiatedthepolymerizationofn-BAtoformPhCH –n-BAlinkage. So,
2
themethyleneprotonsignaloftheDIXwasshiftedfromδ=4.45to2.57ppm. Asinthediscussion
Polymers 2018, 10, x FOR PEER REVIEW 7 of 15
on the polymerization mechanism shown in Scheme 1, this initiator fragment should stand at the end
of the PBA chain. Therefore, the obtained PBA has two end groups: hydroxyl and isobutyrontrile.
The hydroxyl functionality of the obtained PBA can be calculated based on integral values of the
signals at δ = 1.26 and 3.73 ppm, and the results are listed in Table 1. Theoretically, there are three
types of PBA based on their different end groups, HO–PBA–OH, HO–PBA–I, and I–PBA–I (I refers
to isobutyrontrile group). The results in Table 1 show that the hydroxyl functionality of all obtained
PBAs is approximately 50%; the molar percentage of HO–PBA–OH—which is expected in the
preparation of aqueous PUDs—was less than 50%. Such hydroxyl-functionalized PBA will
significantly influence the properties of the formed aqueous PUDs because the I–PBA–I is inert in the
reaction with diisocynates and the HO–PBA–I terminates the condensation polymerization of diol
with diisocynate monomers. To synthesize the PBA with 100% hydroxyl functionality, the design of
a new formulation is necessary. Based on the above discussion, the low hydroxyl functionality of the
PBA is due to the use of AIBN without hydroxyl group as initiator; thus, the initiator ACPO with two
terPmolyinmaerls h2y01d8r,o10x,y2l1 9groups was selected and used in the DITP of n-BA. 8of16
3.2. DITP of n-BA Using DIX and ACPO as Transfer Agent and Initiator
ofthe1HNMRspectrainFigure1, theendgroupsofPBAcomefrominitiatorandtransferagent.
ThIent eormrdienra ltoio dsiynnethoefsPizBeA –loIwch-mainolsewcualsarc-owneviegrhtte dPtBoAa hwyidthro x1y00l%gr ohuypdbroyxryela cftuionnctoiofnPaBliAty– Ifwori th
apHplOic–aCtioHn CinH thSeK p,rwephaircahtiiosnc oofn fiaqrmueeodusb yPUitDs 1sH, loNwM feRedsp mecotlraurm raitnio Foifg muroen3o.mTehre tos itgrnaanlsfoefr mageethnitn e
2 2
is pgreonteornalilny threeqtueirrmedin. aTlhnu-Bs,A thuen iDtaItTδP =o4f .3n0-BpAp mwcitohm fpeleedte mlyodlaisra prapteioa reodf ,[ann-BdAtw]/[oApCrPotOo]n/[sDigIXna] l=s at
16:δ0.=082:.16 7waansd c3o.n62d–u3c.9te3dp aptm 60a p°pCe aforerd 2i4n hF.i gTuhree 3P,BwAh iwchasa roebrteasinpeedct ibvye lypraetctripibiutatteidonto, ftiwltroamtioenth, yalnedne s,
dryreinspge, catnivde liyts aGdPjaCc ecnutrvtoe saunldfu 1rHa nNdMhRy dspoexcytlrugmro uwpesr.e Tmheeaspurroetdo.n Fsiiggunrael 2o fshthowe ste irtms GinPaCl h cyudrrvoex, yl
whgircohu rpe,vweahlisc ha scionmglees cfurorvme twheithA Ca PpOroninoiutinatcoerd, atalsiol. aTphpe eparrosbaatbδle= r3e.a6s2o–n3 .i9s3 irprpevme.rsBiebclea utesremthineaptiroonto n
of nthuem gbreorwoifntgh echtwaion mraedthicyallesn, ebeacnadutshee apt hthene yinleintieali nsttahgeeD oIfX pioslythmeesraimzaeti,othne, fcaosnt tdriebcuotmiopnorsaittiioono fotfh e
iniitniaittioart oArCAPCOP Opraondducteradn tsofoer magaennyt pDrIimXatoryt hreadteicramlsi,n aanldh ytdhreonx tyhleg grorouwpicnagn cbheaeinst rimadaitceadlsb caosuelddo nnotth e
bei neftfeigcireantitolyn craaptitouorefdth beys itgrannaslfseart aδg=en3t. 6(D2–I3X.)9,3 leaanddin7g.0 t4op tphmeir, airnrdeviser0s.i0b7l/e 1t—ermcloinsaettioont.h Tehfeee tdaiml oofl ar
ther aGtiPoCo fcu[ArvCeP lOed]/ t[oD aI Xb]ro=a0d. 0m8/o1le.cular weight distribution (Mw/Mn = 1.67).
Polymers 2018, 10, x FOR PEER REVIEW 8 of 15
CH2CH2SK, which is confirmed by its 1H NMR spectrum in Figure 3. The signal of methine proton in
the terminal n-BA unit at δ = 4.30 ppm completely disappeared, and two proton signals at δ = 2.67
and 3.62–3.93 ppm appeared in Figure 3, which are respectively attributed to two methylenes,
respectively adjacent to sulfur and hydoxyl groups. The proton signal of the terminal hydroxyl group,
which comes from the ACPO initiator, also appears at δ = 3.62–3.93 ppm. Because the proton number

of the two methylene and the phenylene in the DIX is the same, the contribution ratio of the initiator
ACFPigOFui greau n2red. G2 .terlGa penelsrpfmeerer amatiegoaentni cothn rDochmIXrao tmotoga rtaothpghera ypt (ehGrymP(CGin)P caCul r)hvceyusdr vorfeo stxhoyefl p thogelryopmuoeplyr mocbaetnra ionbbeetda iefnrseotdmimf trahoteme Ddt IhTbePaD soeIfT dnP -oonf the
intBegAnr wa-BtiiAtohnw f erietahdt ifmoee oodlfam rt hroaelat isroirsga ontifao [lsnso- BaftA[ nδ]-/ B[=AA 3C]./P6[2OA–]C3/DP.9IO3X] ]/a =nD 1dI6X /7]0.=.00481 /6p1/ p(0Mm.0n8,, G/aP1Cn (d=M 3i2s0 00. 0g7/=m/13o—2l0).0c lgo/sme otol) .the feed molar
n,GPC
ratio of [ACPO]/[DIX] = 0.08/1.
After the iodine-terminated PBA was treated with β-mercaptoethanol in alkaline conditions, the
hydroxyl-terminated PBA was obtained, and its 1H NMR spectrum was measured and is shown in
Figure 3. Similar to the 1H NMR spectrum of PBA obtained by initiation of AIBN, the characteristic
ester methylene proton signal, the methane, and methylene proton signals in the PBA backbone
appear at δ = 4.08, 2.33, and 1.96 ppm, respectively, demonstrating the successful synthesis of PBA.
The iodomethylene proton signal of DIX unit at δ = 4.45 ppm completely disappeared, and a new
proton signal clearly appeared at δ = 2.57 ppm. This is because the reversible reaction of the growing
PBA chain radicals with DIX produced p-iodomethylene benzyl radicals and the iodine-terminated
PBA, and the formal radicals initiated the polymerization of n-BA to form PhCH2–n-BA linkage. So,
the methylene proton signal of the DIX was shifted from δ = 4.45 to 2.57 ppm. As in the discussion of
the 1H NMR spectra in Figure 1, the end groups of PBA come from initiator and transfer agent. The
terminal iodine of PBA–I chains was converted to a hydroxyl group by reaction of PBA–I with HO–

FFiigguurree 33.. 11HH NNMMRR ssppeeccttrruumm ooff tthhee ppoollyy((bbuuttyyll aaccrryyllaattee)) ((PPBBAA)) ssyynntthheessiizzeedd bbyy DDIITTPP ooff tthhee nn--BBAA wwiitthh
ffeeeedd mmoollaarr rraattiiooo off[ n[n-B-BAA]/]/[[AACCPPOO]]//[[DDIIXX]] == 1166//00..0088//11 aatt 6600 °◦CC ffoorr 2244 hh ((MMn,GPC == 33220000 gg//mmooll)). .AACCPPOO::
n,GPC
44,,44′(cid:48)--aazzoobbiiss((44--ccyyaannoo--11--ppeennttaannooll))..
The 13C NMR spectrum in Figure 4 supports the structure of hydroxyl-terminated PBA, which
The 13C NMR spectrum in Figure 4 supports the structure of hydroxyl-terminated PBA,
was obtained based on analysis of its 1H NMR spectrum in Figure 3; the signal of the terminal
whichwasobtainedbasedonanalysisofits1HNMRspectruminFigure3;thesignaloftheterminal
hydroxylmethylene carbon appears at δ = 64.29 ppm, and the ascription of the other carbon signals
hydroxylmethylenecarbonappearsatδ=64.29ppm,andtheascriptionoftheothercarbonsignalsin
in the hydroxyl-terminated PBA are marked in Figure 4.
thehydroxyl-terminatedPBAaremarkedinFigure4.

Figure 4. 13C NMR spectrum of the PBA synthesized by DITP of the n-BA with feed molar ratio of [n-
BA]/[ACPO]/[DIX] = 16/0.08/1 at 60 °C for 24 h (Mn,GPC = 3200 g/mol).
Polymers 2018, 10, x FOR PEER REVIEW 8 of 15
CH2CH2SK, which is confirmed by its 1H NMR spectrum in Figure 3. The signal of methine proton in
the terminal n-BA unit at δ = 4.30 ppm completely disappeared, and two proton signals at δ = 2.67
and 3.62–3.93 ppm appeared in Figure 3, which are respectively attributed to two methylenes,
respectively adjacent to sulfur and hydoxyl groups. The proton signal of the terminal hydroxyl group,
which comes from the ACPO initiator, also appears at δ = 3.62–3.93 ppm. Because the proton number
of the two methylene and the phenylene in the DIX is the same, the contribution ratio of the initiator
ACPO and transfer agent DIX to the terminal hydroxyl group can be estimated based on the
integration ratio of the signals at δ = 3.62–3.93 and 7.04 ppm, and is 0.07/1—close to the feed molar
ratio of [ACPO]/[DIX] = 0.08/1.

Figure 3. 1H NMR spectrum of the poly(butyl acrylate) (PBA) synthesized by DITP of the n-BA with
feed molar ratio of [n-BA]/[ACPO]/[DIX] = 16/0.08/1 at 60 °C for 24 h (Mn,GPC = 3200 g/mol). ACPO:
4,4′-azobis(4-cyano-1-pentanol).
The 13C NMR spectrum in Figure 4 supports the structure of hydroxyl-terminated PBA, which
was obtained based on analysis of its 1H NMR spectrum in Figure 3; the signal of the terminal
hydroxylmethylene carbon appears at δ = 64.29 ppm, and the ascription of the other carbon signals
Polymers2018,10,219 9of16
in the hydroxyl-terminated PBA are marked in Figure 4.

FigFuigreu r4e. 143.C1 N3CMNRM spRecstpreucmtr uomf thoef PthBeAP sByAntshyensitzheedsi bzeyd DbITyPD oITf tPhoe fnt-hBeAn w-BitAh wfeietdh mfeoeldarm roatliaor oraf t[ino-of
BA[n]/-[BAAC]P/O[A]/C[DPOIX]]/ =[D 1I6X/0].=081/16 /a0t .6008 /°1C afto6r 024◦ Ch f(oMrn2,G4PCh =( M3200 g/m=o3l2)0. 0g/mol).
Polymers 2018, 10, x FOR PEER REVIEW n,GPC 9 of 15
TToo ffuurrtthheerr vveerriiffyy tthhee ssttrruuccttuurree aanndd tthhee tteerrmmiinnaall ggrroouuppss ooff PPBBAA,, iittss MMAALLDDII--TTOOFF mmaassss ssppeeccttrraa

wweerree mmeeaassuurreedd.. MMAALLDDII--TTOOFF mmaassss ssppeeccttrroommeettrryy iiss aa ppoowweerrffuull tteecchhnniiqquuee ffoorr ggaaiinniinngg mmeecchhaanniissttiicc
iinnssiigghhttss inintotop oploylmymereizraiztaiotniorne arcetaiocntisonbas sebdasoend thoen ptrheese npcreesoernacbes eonr ceabosfemnceec hoafn imsmec-shpaenciisfimc-rsepaecctiiofinc
preraocdtuiocnts p[3ro3]d.uFctigs u[r3e3]5. aFsighuorwes 5aa tyshpoicwasl Ma AtyLpDicIa-Tl OMFAmLDasIs-TsOpeFc tmruamss osfptehcetruhmyd roofx tyhl-et ehrymdirnoaxtyedl-
PteBrAmiwnaittehdM PBn,AGP wCi=th4 M28n0,GPgC /=m 42o8l,0 agn/mdoitl,s aenndl aitrsg eendlasrpgeecdtr supmectrraunmgi nragngfrionmg fr3o0m25 3t0o253 3to0 033g0/0m g/oml oisl
sish oshwonwinn iFni gFuigreur5eb .5bW. eWcea ncasnee seseev seenvesner sieesrioesf tohfe thPeB APBcAh acihnasitnhsa tthraetp reeaptewati twhitthhe tmhea mssaosfs aonf na-nB nA-
uBnAi tu(n1i2t8 (1g2/8m go/ml,osle, eseFei gFuigreur5eb 5),ba),n adndth tehset srutrcutcutruarlaflo fromrmuulalsasf oforre eaacchhs seerrieiesso off ppoollyymmeerrss——ddeeppiicctteedd
bbaasseedd oonn ththeeiri rmmasass—s—araer elisltiesdte idn TinabTlea b2l.e A2m.onAgm thoen gsevtheen sseervieesn, sseerriieess ,As iesr aie rselAatiivselay raebluantidvaenlyt
aPbBuAn dsaenrtiePs.B ABasseerdie so.n Btahsee dmoansst heofm saesrsieosf Ase,r ieas sAtr,uactsutrraulc tfuorraml ufolarm cualna cbaen dbeepdicetpeidc teadnda nids
iNsaN+Ha+OHCOHC2HCH2C2CHH2C2(HC2N(C)(NC)H(C3)HC3–)(Cn-–B(nA-)BnA–S)Cn–HSC2CHH22COHH2.O WH.hWenh ethnet hdeedgergeere eofo fppoolylymmeerrizizaattioionn ((DDPP))
wwaass 2233,, iittss nnuummbbeerr--aavveerraaggee mmoolleeccuullaarrw weeigighhtt( (MMnn)) ccoouulldd bbee ccaallccuullaatteedd aass 33115599.2.2g g//mmooll,, wwhhiicchh iiss vveerryy
cclloossee ttoo tthhee mmeeaassuurreedd vvaalluuee ((33115588..44 gg//mmooll))..

Figure 5. (a) Mass-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-
Figure5.(a)Mass-assistedlaserdesorptionionizationtime-of-flightmassspectrometry(MALDI-TOF)
sTpOeFct)r uspmecatnrudm(b )aenndl a(rbg)e denslpaergcterdu mspoefcttrhuemh yodfr othxey lh-tyedrmroixnyalt-etderPmBiAnaotebdta iPnBeAd boybtDaIiTnePdo fbny- BDAITwP itohf
fne-eBdAm owlaitrhr afteioedo fm[no-BlaAr] /r[aAtiCoP Oof] /[Dn-IBXA=]1/[6A/C0.P5O/1]/iDnItXo lu=e n1e6a/0t.650/1◦ Cinf orto2l4uhen(eM at 60= °4C2 80fogr/ m24o l)h.
n,GPC
(Mn,GPC = 4280 g/mol).
BasedonthecarefulanalysisofthestructuralformulasofsevenseriesofPBAsinTable2,wecan
conjectureinterestingreactionsoccurringinthepolymerizationandclarifyhowtheterminalgroups
Based on the careful analysis of the structural formulas of seven series of PBAs in Table 2, we
can conjecture interesting reactions occurring in the polymerization and clarify how the terminal
groups formed. As we mentioned previously, the end groups of PBA come only from the initiator,
ACPO, and transfer agent, DIX. This can be further confirmed by the results of MALDI-TOF analysis.
Both ends of the PBA capped with ACPO fragments include series D, E, and G; the transfer agent
fragment, I—which was substituted by 2-mercaptoethanol after polymerization—occupies both ends
of series B and F. Both end units of series A and C were ACPO and DIX fragments, respectively.
Because the ACPO fragment has one terminal hydroxyl group, and after iodine at the ends of PBA
was quantitatively substituted by KSCH2CH2OH (Figure 3), the PBA with hydroxyl groups at both
ends was successfully synthesized, which is consistent with the results based on the analysis of
1H NMR spectrum.
Table 2. Structural formulas of the PBA produced in DITP of n-BA, depicted based on MALDI-TOF
Mass Spectrum.
No. Structural formulaa (m/z)observation(g/mol) (m/z)calc. (g/mol)
A Na+HO–CPO–(n-BA)n–ME–OH 3158.4 (n = 23) 3159.2 (n = 23)
B Na+HO–ME–(n-BA)n–xylene–(n-BA)m–ME–OH 3232.5 (n2 = 23) 3228.2 (n2 = 23)
Na+HO–CPO–(n-BA)n–xylene–(n-BA)m–xylene–
C 3214.3 (n4 = 21) 3215.3 (n3 = 21)
(n-BA)p–xylene–(n-BA)q–ME–OH
Na+HO–CPO–(n-BA)n–xylene–(n-BA)m–xylene–
D 3271.6 (n3 = 23) 3274.4 (n3 = 23)
(n-BA)p–CPO–OH
Polymers2018,10,219 10of16
formed. Aswementionedpreviously,theendgroupsofPBAcomeonlyfromtheinitiator,ACPO,
andtransferagent,DIX.ThiscanbefurtherconfirmedbytheresultsofMALDI-TOFanalysis. Both
endsofthePBAcappedwithACPOfragmentsincludeseriesD,E,andG;thetransferagentfragment,
I—whichwassubstitutedby2-mercaptoethanolafterpolymerization—occupiesbothendsofseriesB
andF.BothendunitsofseriesAandCwereACPOandDIXfragments,respectively.BecausetheACPO
fragmenthasoneterminalhydroxylgroup,andafteriodineattheendsofPBAwasquantitatively
substitutedbyKSCH CH OH(Figure3),thePBAwithhydroxylgroupsatbothendswassuccessfully
2 2
synthesized,whichisconsistentwiththeresultsbasedontheanalysisof1HNMRspectrum.
Table2.StructuralformulasofthePBAproducedinDITPofn-BA,depictedbasedonMALDI-TOF
MassSpectrum.
No. Structuralformulaa (m/z)observation(g/mol) (m/z)calc.(g/mol)
A Na+HO–CPO–(n-BA)n–ME–OH 3158.4(n=23) 3159.2(n=23)
B Na+HO–ME–(n-BA)n–xylene–(n-BA)m–ME–OH 3232.5(n2=23) 3228.2(n2=23)
C Na+HO–CPO–(n-BA)n–xylene–(n-BA)m–xylene– 3214.3(n =21) 3215.3(n =21)
(n-BA)p–xylene–(n-BA)q–ME–OH 4 3
D Na+HO–CPO–(n-BA)n–xylene–(n-BA)m–xylene– 3271.6(n =23) 3274.4(n =23)
(n-BA)p–CPO–OH 3 3
E Na+HO–CPO–(n-BA)n–CPO–OH 3196.2(n=23) 3194.2(n=23)
F Na+HO–ME–(n-BA)n–xylene–(n-BA)m–xylene– 3181.1(n =21) 3180.3(n =21)
(n-BA)p–xylene–(n-BA)q–ME–OH 4 4
G Na+HO–CPO–(n-BA)n–xylene–(n-BA)m–xylene– 3122.1(n =20) 3124.2(n =20)
(n-BA)p–xylene–(n-BA)q–CPO–OH 4 4
a CPO: 4-cyano-1-pentanol (HOCH2CH2CH2(CN)(CH3)C); ME: 2-mercaptoethanol (SCH2CH2OH); xylene:
CH2C6H5CH2;n2=n+m;n3=n+m+p;n4=n+m+p+q.
Thereactionsoccurringinthepolymerizationcouldbegainedbasedonthepresenceorabsence
ofmechanism-specificreactionproducts[33].BasedontheDITPmechanismlistedinScheme1,wecan
depicthowsevenseriesofthePBAinTable2areformed;thereareprobablyseveralroutestoreach
thesePBAspecies,andhereinwedescribeonlyoneofthem. Theintermediatesfortheformationof
finalPBAspeciesaredepictedinScheme2. Theprimaryradicalsformedviadecompositionofthe
ACPOinitiatesthepolymerizationofn-BAtoyieldPBAchainradicalI,andthenitreversiblyreacts
withDIXtoformPBA–Iandp-iodomethylenebenzylradical. Thelatterinitiatesn-BAtopolymerize,
yieldingthePBAchainradicalII.WhentheiodomethyleneofradicalIIreactswiththegrowingchain
radicalorprimaryradicalandtheninitiatesthepolymerizationofn-BA,thegrowingchainradical
III is formed. If the growing chain radical I continuously grows through chain propagation and
degenerativechaintransfer,seriesAisformed. IrreversibleterminationofradicalIyieldsseriesE.
SeriesBisformedthroughcontinuousgrowthofradicalII,andthenradicalIIIiscappedwithiodine
viaitsreactionwithgrowingchainradicals. CouplingreactionofradicalIwithradicalIIIproducethe
PBAspeciesIV;afterreactionwiththegrowingchainradicals,theresultantradicalIViscoupledto
formseriesD.TerminationreactionsofradicalIIIwithradicalIIorradicalIV,respectively,yielded
PBAspeciesVorVI.RadicalVisterminatedwithradicalIIIorwithradicalIV,respectively,toafford
series F or series C, and series G is produced through coupling reaction of radical IV with radical
VII.Therefore,thehydroxyl-terminatedPBAsobtainedbyDITPofn-BAareamixturecomposedofa
seriesofPBAwithdifferentnumbersofDIXunitsandterminalspecies. Indeed,thereactionsinthe
DITPofn-BAaremorecomplicatedthanthatdescribedabove;allthesereactionswillinfluencethe
polymerizationkineticsandtheformationofpolymerchains,whichwillbediscussedlater.