Multiblock Copolymers of Styrene and Butyl Acrylate via Polytrithiocarbonate-Mediated RAFT ...

21 Pages·2011·1.83 MB·English

by  Bastian Ebeling

Polymers2011,3,719-739;doi:10.3390/polym3020719
OPENACCESS
polymers
ISSN2073-4360
www.mdpi.com/journal/polymers
Article
Multiblock Copolymers of Styrene and Butyl Acrylate via
Polytrithiocarbonate-Mediated RAFT Polymerization
BastianEbelingandPhilippVana(cid:63)
Institutfu¨rPhysikalischeChemie,Georg-August-Universita¨tGo¨ttingen,Tammannstraße6,D-37077
Go¨ttingen,Germany
(cid:63) Authortowhomcorrespondenceshouldbeaddressed;E-Mail: [email protected];
Tel.: [phone];Fax: [phone].
Received: 18February2011;inrevisedform: 18March2011/Accepted: 28March2011/
Published: 31March2011
Abstract: When linear polytrithiocarbonates as Reversible Addition-Fragmentation
chain Transfer (RAFT) agents are employed in a radical polymerization, the resulting
macromolecules consist of several homogeneous polymer blocks, interconnected by the
functional groups of the respective RAFT agent. Via a second polymerization with
another monomer, multiblock copolymers—polymers with alternating segments of both
monomers—can be prepared. This strategy was examined mechanistically in detail based
on subsequent RAFT polymerizations of styrene and butyl acrylate. Size-exclusion
chromatography (SEC) of these polymers showed that the examined method yields
low-disperse products. In some cases, resolved peaks for molecules with different numbers
of blocks (polymer chains separated by the trithiocarbonate groups) could be observed.
Cleavage of the polymers at the trithiocarbonate groups and SEC analysis of the products
showed that the blocks in the middle of the polymers are longer than those at the ends and
thatthenumberofblockscorrespondstothenumberoffunctionalgroupsintheinitialRAFT
agent. Furthermore, the produced multiblock copolymers were analyzed via differential
scanning calorimetry (DSC). This work underlines that the examined methodology is very
wellsuitedforthesynthesisofwell-definedmultiblockcopolymers.
Keywords: multiblock copolymers/segmented block copolymers; Reversible
Addition-Fragmentation chain Transfer (RAFT); polytrithiocarbonates; molecular weight
distribution/molarmassdistribution
Polymers2011,3 720
1. Introduction
Multiblockcopolymers(orsegmentedblockcopolymers)consistofalternatinghomogeneoussegments
oftwodifferentmonomerswhichareattachedcovalentlytoeachother. Ifthesemonomersarechemically
distinct,theydiffersignificantlyfromtherespectiverandomcopolymersorblendsofthehomopolymers.
Arguablythemostnotedpropertyofmultiblockcopolymersistheirabilitytoself-assembleintoordered
microstructureswhenthesegmentsaresufficientlylong[1,2]. Themicrophaseseparationofmultiblock
copolymers with long alternating hard and soft segments gives rise to their unique material properties
as thermoplastic elastomers [3,4]. Other industrial uses of the bulk polymers include the utilization
as compatibilizers [5,6] for blends of otherwise immiscible homopolymers and as pressure-sensitive
adhesives[7,8]. Certainmultiblockcopolymerscanformresponsivepolymerichydrogels. Thesearesmart
materialswhichhavepotentialapplicationsindrugdelivery,tissueengineering,andassensors[9–11].
Themotifofmultiblockcopolymerscanalsobefoundinnature,wherelongflexiblesegmentsalternate
with those which can form hydrogen bonds. The semi-crystalline structures that are formed in these
cases are responsible for the extraordinarily good material properties of several biopolymers [12]. It
seemspromisingtomimicthismotifandpreparemultiblockcopolymersthatcomprisehydrogenbonding
segments[13]. Ithasalreadybeenshownbyus andothersthatthesepolymershavedistinctmechanical
propertiescomparedtothecorrespondingpolymerswithouthydrogenbondingsegments[14,15].
Basedontheseapplications,severalmethodologicalcriteriacanbederivedwhichshouldbemetbya
suitable synthesis strategy: The strategy should be (i) easy to implement, i.e., should not involve high
costs orrequire a lot oftime and no specialequipment should be necessary. It shouldbe (ii) capableof
producingmoleculeswithlongsegmentswhich(iii)ideallyallhavethesamelength. Itshouldenablethe
possibility (iv) to produce polymers with a high number of alternating segments and, moreover, (v) to
control this number. Furthermore, (vi) thedistribution of the segments amongthe synthesized polymers
should be as narrow as possible. And after all, (vii) the method should be universally applicable, i.e.,
it should be deployable for a high number of different monomers and have a high tolerance towards
functionalgroups,solventsandimpurities. Uptorecenttimes,nosynthesisstrategyhasbeenfoundthat
meetsallofthesesevencriteriainareasonablyacceptablemanner.
Theconventionalmethodforthesynthesisofmultiblockcopolymersisthepreparationofendgroup
functionalized prepolymersthat arethen coupled togetherby means ofa coupling reaction. This method
isrelativelyeasytoimplement(althoughitstillrequiresatleastthreeindependentreactions)andisvery
versatile. Itwasalreadycarriedoutwithavarietyofdifferentmonomers. Aslongastheusedprepolymers
areequalintheirlengths,thesamewillholdtrueforthesegmentsintheproducedmultiblockcopolymers.
Ontheotherhand,thisstrategyentailssomefundamentallimitations: Becauseofthelowconcentrationof
reactivechainends,nomoleculeswithlongsegments(typically<5,000gmol−1)orwithhighnumbersof
alternating segments can be produced. Moreover, this number canhardly be controlled. Furthermore, the
segmentdistributionamongthemolecules(Schultz-Florydistributionwithidealkinetics)isratherbroad.
However,thecouplingapproachisclearlythemostwidelyusedstrategy,bothinindustryandacademia.
In principle, segmented copolymers can also be prepared via stepwise alternating addition of two
different monomers to a living polymerization, traditionally an anionic polymerization [16]. In this
way, multiblock copolymerswith aneasily controllablenumber ofalternating segmentswith arbitrarily
Polymers2011,3 721
specifiablelengthscanbeproducedwherebyideallyallmoleculespossessthesamenumberofalternating
segments. These advantages, however, are counterbalanced by an extremely high experimental effort
whenhighnumbersofsegmentsaretargetedbecauseforeachsegmentaseparatereactionisnecessary.
Untiltoday, toourknowledge,the highestnumberofsegmentsthatwas reachedbythismethodis 8[17].
Additionally,theapplicabilityofthisstrategyislimitedasonlycertainpairsofmonomerscanbeused.
With the advent of controlled radical polymerization (according to IUPAC: reversible-deactivation
radicalpolymerization,RDRP),theprincipleoflivingpolymerizationhasalsobeenintroducedtoradical
polymerization. Reversible Addition-Fragmentation chain Transfer (RAFT)polymerization [18,19] is
arguably themost versatileamong thesetechniques withrespectto temperature[20], solvent [21],and
monomer choice [22], and can be used to synthesize a wide variety of polymeric structures [23]. It
proceedsviaadegenerativechaintransfermechanism,whichinducesequilibriumbetweenpropagating
macroradicalsanddormantpolymericRAFTagentscarryingdithiomoieties[24]andhasalreadybeen
successfully employed to produce multiblock copolymers by stepwise addition of monomers to the
polymerization[25,26].
Trithiocarbonates (TTC) are unique RAFT agents in the sense that they are inherently bifunctional
so that a polymer chain is inserted on both sides of the functional group when it is connected to good
leaving groups. When a RAFT agent of trithiocarbonate type is employed in a radical polymerization,
only two polymerization steps are necessary to prepare polymers with three alternating segments [27]
and with every subsequent polymerization step the number of alternating segments of the polymers is
increased by two. The number of alternating segments which is produced within two polymerizations
canbefurtherincreasedbyutilizingaRAFTagentthatcontainsmultipletrithiocarbonategroupsalong
itsmainchain. ThisstrategywaspioneeredbyYouetal. [28]whousedittosynthesizepolymerswith
alternating segments of polystyrene and poly(methyl acrylate) in UV-initiated RAFT polymerizations.
The whole route is illustrated in Scheme 1. In the first polymerization, homopolymer blocks grow
between the functional groups and on the ends of the polymer chains [29,30]. By a subsequent
radical polymerization with another monomer, multiblock copolymers can be produced. (The term
“block” is used ambiguously in the literature to describe both polymeric chains between particular
functional groupsand partsof homogeneousmonomeric compositionalong apolymer chain. To avoid
misunderstanding,thelatterwillbedenoted“segment”inthisarticle,while“block”willonlybereferred
to the parts of the polymers which are separated by the trithiocarbonate groups. With this definition,
the number of blocks in the polymers is not altered by the second polymerization step. Accordingly,
“multiblock(homo)polymer”describesa(homo)polymerwithseveraltrithiocarbonategroupsalongthe
mainchain. “Multiblockcopolymer”,however,isstillusedsynonymouslywith“segmentedcopolymer”,
as defined before.) When the RAFT agent possesses g functional groups, ideally polymers with 2g+1
alternating segments are prepared. In addition, RAFT polymerization with polytrithiocarbonates has
alreadybeenusedtopreparesegmentedcopolymerscomposedofpoly(N,N-dimethylacrylamide)(DMA)
and poly(N-isopropylacrylamide)(NIPAM)[31], of polystyrene andpoly(4-tert-butylstyrene)[32] and of
arandom DMA-NIPAMcopolymer andpoly(4-vinylpyridine)[33]. Ithas beenshown thatthisstrategy
canalsobecarriedoutpreparingthepolytrithiocarbonatesinsitubyemployingcyclictrithiocarbonates
inthefirstpolymerization[34,35]. ItgoeswithoutsayingthatinprinciplealsoRAFTagentswithother
chemicalgroups(e.g.,difunctionaldithiocarbamates[36])canbeutilized.
Polymers2011,3 722
Scheme 1. General strategy for the synthesis of multiblock copolymers using
polytrithiocarbonateRAFTagents.
This approach appears to be very promising for the synthesis of multiblock copolymers. The here
presented study was conducted to gain further insight into this method in order to be able to assess to
whatextentthe examinedstrategy meetsthe abovelisted methodologicalcriteria froman experimental
pointofview. Theidealsegmentdistributionamongthesynthesizedcopolymerswasalreadytreatedin
another article, where we showed that multiblockcopolymers prepared with the examined RAFT strategy
ideallyhaveahigherhomogeneitythanthosefromcouplingoffunctionalizedprepolymers[37]. Inthe
presentstudy,weexaminedthesynthesisofsegmentedcopolymersbasedonpolystyreneandpoly(n-butyl
acrylate) (pBA) with a polyfunctional RAFT agent of trithiocarbonate type. The prepared polymers
were probedby size-exclusionchromatography (SEC) anddifferential scanningcalorimetry (DSC). By
cleavageoftheproductsatthetrithiocarbonategroupsandanalysisoftheproductsadditionalmechanistic
informationcouldbeobtained.
2. Experimental
2.1. Materials
The anion exchange resin AMBERSEP(cid:13)R 900 OH (FLUKA, 20–50mesh) was dried for several
days under high vacuum. By titration of an aqueous suspension with hydrochloric acid, its
capacity was determined to be 3.5mmolg−1. To remove the inhibitor, styrene and butyl acrylate
(ALDRICH)werepassedthroughabasicalumina(ALDRICH,BrockmannI,∼150mesh,58A˚)column.
2,2(cid:48)-Azobis(isobutyronitrile)(AIBN, AKZONOBEL)wasrecrystallizedfrommethanolanddriedunder
high vacuum before use. Tetrahydrofuran (THF) used as the eluent in SEC (Carl Roth, stabilized with
2,6-di-tert-butyl-4-methylphenol)wasusedasreceived. Unlessotherwisespecified,allotherchemicals
werepurchasedfrom ALDRICH andusedwithoutfurtherpurification.
Polymers2011,3 723
2.2. Instrumentation
MolecularweightdistributionsweremeasuredbymeansofSECusingasystemcomposedofa JASCO
AS-2055PLUS autosampler, a WATERS 515 pump, three PSS SDV columns (8 × 300mm, nominal
particle size: 5µm, pore sizes: 105, 103 and 102A˚), a WATERS 2410 refractive index detector and a
VISCOTEK VE 3210 ultraviolet(UV)detector(settoawavelengthof310nm). THFat35◦Cwasusedas
theeluentataflowrateof1.0mLmin−1. TheSECset-upwascalibratedagainstPSSpolystyrenestandards
of very low dispersity. The molecular weight distributions were adjusted according to the principle
of universal calibration using Mark–Houwink parameters for linear pBA (K = 1.22×10−4dLg−1,
a=0.700)[38].
Nuclear magnetic resonance (NMR) spectra were recorded at room temperature on a VARIAN
UNITY 300 (1H, 13C) or a VARIAN INOVA 500 (1H) spectrometer, using deuterated chloroform as
solventandinternalstandard.
Differentialscanning calorimetry was carried outon a METTLER TOLEDO DSC 820 apparatus which
wasoperatedwiththeSTARe9.01software. Samplesof10–20mgwereheatedundernitrogenatmosphere
(flowrate14mLmin−1)from−52◦Cto170◦Candcooledbackto−52◦C.Toobtainthethermograms
the samples were heated again to 170◦C. The heating and cooling rates were 10◦Cmin−1. The glass
transitiontemperaturewastakenastheonsettemperatureofthestepsinthethermograms.
2.3. SynthesisofPolytrithiocarbonateA
In the synthesis of polytrithiocarbonate A with the lowest degree of polymerization (see Table 1),
cesium carbonate was employed as the base to produce trithiocarbonate ions [39]. Carbon disulfide
(409µL,6.79mmol)wasaddedtoasolutionofcesiumcarbonate(2.21g,6.79mmol)in5mLacetonitrile
at room temperature. The color of the suspension changed to red. After 10min of stirring, dimethyl
2,6-dibromoheptanedioate(739µL,3.40mmol)wasaddedandthecolorchangedtoyellowimmediately.
Afteranother21hofstirring,dimethyl2,6-dibromoheptanedioate(100µL,460 µmol)wasaddedagain.
Thesuspensionwas stirredfor45min andpouredintowater(100µL).Ayellowoilprecipitatedwhich
was isolated and washed with water (2×100mL) and methanol (2×100mL). The product (665mg,
67%)wasdriedundervacuumfor2d. SeesynthesisofB–EforNMRspectroscopicdata.
2.4. SynthesisofPolytrithiocarbonatesB–E
Inthesynthesis ofthepolytrithiocarbonatesB–Ewithhigherdegrees ofpolymerization(seeFigure 2
and Table 1), thetrithiocarbonate ions weregenerated atthe surface of an anion exchange resin[40,41].
Atypicalprocedure isreportedhere. Dry AMBERSEP(cid:13)R 900 OH (13g,46mmolOH−)was suspended
in carbon disulfide (65mL). The surface of the resin beads turned dark red immediately. Dimethyl
2,6-dibromoheptanedioate(5.00mL,23.0mmol)wasadded. The suspensionwasheatedtoboilingand
wasstirredfor65hunderreflux. Thecoloroftheresinbeadsandthesurroundingsolutionturneddark
yellow overtime. The reactionmixturewas cooledtoroomtemperature, pouredina frittedfunneland
rinsed subsequently first with carbon disulfide (300mL) and then with THF (300mL). Both obtained
fractionswereconcentratedundervacuum,washedwithwater(3×200mL)andmethanol(3×150mL)
and dried under high vacuum and were thus obtained as yellowish-orange oils, whereas the fraction
Polymers2011,3 724
obtained from rinsing with carbon disulfide (1.55g, 23%) was less viscous and paler in color than the
THFfraction(771mg,11%).
1H-NMR(300MHz,CDCl ): δ =1.40–1.70(m,CH –CH –CH ),1.85–2.14(m,CH –CH –CH ),
3 2 2 2 2 2 2
3.75 (s, COOCH ), 4.14–4.24 (m, CH–Br), 4.59–4.79 (m, CH–S) ppm. 13C-NMR (75.58 MHz,
3
CDCl ): δ = 22.1 (CH –CH –CH ), 30.5 (CH–CS ), 44.8 (CH–Br), 53.0 (COO–CH , CH–SCS ),
3 2 2 2 3 3 2
170.4(COO–CH ),219.2(S–CS–S)ppm.
3
2.5. Polymerizations1–9andI–VI
Polymerizations1–6ofstyreneand7–9ofBAwiththepreparedpolytrithiocarbonatesasRAFTagents
werecarriedoutinbulkat60◦CandambientpressureusingAIBNasinitiator. PolymerizationsI–VIof
butylacrylatewiththepreparedmultiblockstyrene-homopolymerswerecarriedoutinsolutionoftoluene
underthesameconditions. SeeTables2–4fortheindividualconditions. Theindicatedconcentrations
oftheRAFTagentsarereferredtothemolarmasseswhichweremeasuredbymeansofSECandtothe
numberofmolecules,ratherthanthenumberoftrithiocarbonategroups.
Allpolymerizationmixtureswerethoroughlydegassedviathreefreeze–pump–thawcyclesandthen
evenlyportionedintothreetofivecappedglassvialsinanargon-filledglove-boxwhichweretheninserted
into a heating block. The reactions were stopped by cooling the vials in ice water after distinct time
periodsand residualmonomer and solvent(I–VI)were removed byevaporation. Monomerto polymer
conversionwasdeterminedbygravimetry.
2.6. CleavageofthePolymers
Aminolysiswithoutfurthertreatment
Themultiblockpolymer(10mg)wasdissolvedinTHF(3mL)andn-butylamine(1.0mL,10mmol)
wasadded. Themixturewasshakenfor1datroomtemperature. Afterdrying,colorlesspolymerswere
obtained.
AminolysiswithMichaeladditionsealing
Atypical procedureis described[42]. Themultiblock polymer(10mg)and averysmall amountof
tris(2-carboxyethyl) phosphine hydrochloride (TCEP·HCl) were dissolved in THF (6mL) and n-butyl
amine(0.20mL,2.0mmol)wasadded. Thesolutionwasshakenfor36handBA(0.2mL,1.4mmol)was
added. Afteranother24h,thecolorlesspolymerswereprecipitatedbyadditionofmethanolanddried.
3. Results
3.1. PreparationofthePolyfunctionalRAFTAgents
The general structure of the polyfunctional RAFT agents that were used in this study is depicted in
Figure 1. It is comparable to the one used by You et al. but contains one methylene group less per
unit. Thesepolytrithiocarbonatesweregenerallypreparedbythepolycondensationreactionofdimethyl
2,6-dibromoheptanedioateandtrithiocarbonateanions. Twodifferentstrategieswerefollowedtoproduce
Polymers2011,3 725
thetrithiocarbonate anionsviathe reaction ofcarbondisulfide withabase: The firstone(procedure 1)
involvestheproductionofthetrithiocarbonateanionswithcesiumcarbonateasthebase[39]. However,
it could only be successfully employed to prepare polyfunctional RAFT agents of very low degree of
polymerizationatroomtemperature. Atelevatedtemperaturesorlongerreactiontimes,aside-product
wasformed,presumablybecauseofthereactionofthedibromide’sestergroups[43]. Thesecondstrategy
(procedure 2) was based on the formation of the trithiocarbonate anions on the surface of an anion
exchangeresin. Youetal. publishedasynthesisforpolytrithiocarbonatesusingachlorideionexchange
resin [40] and it can be assumed that they used the products for their above mentioned study. Here, a
hydroxy-functionalized exchange resin was employed directly [41], whose capacity was measured by
titrationwithhydrochloricacidpriortouseinordertobeabletoaddthestoichiometricallyexactamount
of hydroxide ions to the system. The fact that the solubility of the produced trithiocarbonates in carbon
disulfide decreased with increasing chain length enabled the possibility of fractionating them directly
withinthework-upofthereactionbyfirstwashingtheresinwithcarbondisulfideandthenwithTHF.
Figure1. GeneralstructureoftheemployedRAFTagents.
ThepreparedtrithiocarbonateswereexaminedbymeansofSEC.Thetheoreticallyexpectedpositions
of theindividuallowmolar masspeaks inthe SECcurvesof thepolytrithiocarbonates, which correlateto
theindividualdegreesofpolymerization,canberationalizedonthebasisoftheirgeneralstructurewhich
isshowninFigure1:
M =gM +2M +(g−1)M
n,RAFT TTC EG MG
(1)
=346.01gmol−1+g·294.42gmol−1.
M is the average molar mass of the RAFT agents with an average of g trithiocarbonate groups,
n,RAFT
M ,M andM arethemolarmassesoftrithiocarbonategroups,endgroupsandmiddlegroups(see
TTC EG MG
Figure1).
Because no appropriate calibration data were available, the SEC curves were measured using the
calibration for linear polystyrene. The SEC curve of polyfunctional RAFT agent A (see Table 1),
which was synthesized via procedure 1, is shown in Figure 2(a). The numbers in the diagram indicate
the positions of the maxima in the number-weighted distribution. The UV detector signal—measured
at 310nm, where trithiocarbonates are known to absorb radiation [41]—nicely matches the refractive
index(RI)detectorsignal. Thisindicatesthatmoleculeswithhighermassespossessmoretrithiocarbonate
groups [44], which is also underlined by the expected structure of almost equidistant peaks. The first
Polymers2011,3 726
peak,though,canonlybeseenintheRIsignalwhereforeithastobeassignedtoareactantorabyproduct
withouttrithiocarbonategroups. FortheanalysisofallSECcurves,thepeakat387gmol−1 wasthefirst
one that was considered. Comparing the positions of the peaks with the expected ones, it can be seen
that the measured absolute positions and the distances between the peaks are considerably lower than
the theoretical values, most likely due to the non-appropriate calibration. Nevertheless, the measured
average molar masses of the prepared RAFT agents were used to calculate their average number of
trithiocarbonategroupsaccordingtoEquation(1). Ithastobekeptinmindthatthesevaluesareonlya
roughapproximationandprobablytoolow. ForpolytrithiocarbonateAthisyieldsanaveragenumberof
g =1.34trithiocarbonategroupsperchain. Figure2(b)showstheSECcurvesofthepolyfunctional
GPC,A
RAFT agents C and D (see Table 1) that were prepared within the same reaction performed according
toprocedure2. CwasobtainedbyrinsingwithCS andDbyrinsingwithTHF.Thesecurvesillustrate
2
that by procedure 2, RAFT agents with higher degree of polymerization could be produced than with
procedure1andthatthefractionationofthereactionproductswassuccessful. Again,thegoodagreement
oftheRIsignalandtheUVsignaldemonstratesthatmoleculeswithhighermolarmassalsopossessmore
trithiocarbonategroups,whichisinlinewiththestructuralexpectations. Generally,SECanalysisgave
valuesofg =4.09–5.33forthefractionobtainedbyrinsingwithCS andg =12.0–18.5forthe
GPC 2 GPC
fractionobtainedbyrinsingwithTHF.
Table 1. Overview of polyfunctional RAFT agents that were employed for further
polymerizationsinthisstudywithsynthesismethod,averagenumbersofTTCgroupsgand
numberaveragemolarmassesM ,determinedviaSECandNMR.Thelastcolumnindicates
n
thepolymerizationinwhichtheywereemployed(seeTables2and3)and,inparentheses,the
copolymerizationforwhichtherespectivepolymerizationproductswereused.
polyRAFT synthesis fraction M g ∗ M † g usedfor
n,SEC SEC n,NMR NMR
A procedure1 — 7.41×102 1.34 (6.40×102) 1‡ 1(→I)
B procedure2 CS 1.66×103 4.46 1.55×103 4.09 2(→II),7
2
C procedure2 CS 1.57×103 4.16 1.38×103 3.50 3,4(→III),8
2
D procedure2 THF 5.43×103 17.3 5.03×103 15.9 5(→IV),9
E procedure2 THF 3.87×103 12.0 2.76×103 8.19 6(→V,VI)
∗CalculatedfromM byEquation(1).
n,SEC
†Calculatedfromg byEquation(2).
NMR
‡Avalueunder1wasmeasured,presumablyduetoresiduedibromide.
It was already shown by You et al. that the average number of trithiocarbonate groups within the
prepared polyfunctional RAFT agents can also be determined by measuring their NMR spectra and
calculating the ratio ofthe intensityI at 4.7ppm (protoninα-position toa trithiocarbonategroup)
4.7ppm
andtheintensityI at4.2ppm(protoninα-positiontoabromineatom)[40]:
4.2ppm
I
4.7ppm
g = (2)
NMR I
4.2ppm
The thus determined value for polytrithiocarbonate A lies under the minimum value of 1, which
can possibly be explained by residual dibromide in the sample contributing only to I . This is
4.2ppm
Polymers2011,3 727
a considerable factor of inaccuracy of this method when polytrithiocarbonates with low degree of
polymerizationareexamined. Ontheotherhand, alsotheexaminationoftrithiocarbonateswitha very
high number of trithiocarbonate groups becomes very unprecise because in these cases, it has to be
dividedby verysmall valuesofI . Thedetermined averagenumbers oftrithiocarbonategroups for
4.2ppm
procedure 2 were g =2.89–4.09 for the CS fraction and g =8.19–15.9 for the THF fraction
NMR 2 NMR
(seeTable1).
TomakesurethatcompleterelaxationofallnucleiwasachievedwithintheNMRmeasurements,one
polytrithiocarbonatesamplewasre-measuredwithanadditionalrelaxationdelayof15secondsbetween
thepulses. Thismeasurementgaveanidenticalspectrumwhichdemonstratesthatthespectracanindeed
beinterpretedquantitatively.
Figure2. (a)SECcurve(solidline: RIsignal,dashedline: UVsignal)ofthepolyfunctional
RAFT agent A; (b) SEC curves (solid lines: RI signal, dashed lines: UV signal) of the
polyfunctionalRAFTagentsC(red,obtainedbyrinsingwithCS )andD(black,obtainedby
2
rinsingwithTHF).
3.2. Polymerizations
In order to produce segmented copolymers of styrene and BA, styrene is preferentially used for the
first polymerization step, as macromolecular RAFT agents with the better leaving groups are formed.
Nevertheless, to gain more insight into the system, three polytrithiocarbonates were also employed in
polymerizations with BA. All homopolymerizations were performed in bulk and initiated thermally
Polymers2011,3 728
with AIBN as initiator. The resulting polymers were examined by SEC (see Tables 2 and 3) using the
calibrationdataforthepurehomopolymerswhichseemreasonablywellapplicablesincetheRAFTagents
constituteonlyaminorpartofthefinalpolymer’smainchain.
Table 2. BulkpolymerizationsofstyrenewithdifferentpolyfunctionalRAFTagentsat60◦C
and ambient pressure using AIBN as initiator. The table lists the individual experimental
conditions, the gravimetrically determined monomer conversions of the samples, and the
correspondingM andpolydispersityindices(PDI)obtainedbytheRIandtheUVdetector.
n
RIdetector UVdetector
(cid:2) g (cid:3) (cid:2) g (cid:3)
timeinh conversion M PDI M PDI
n mol n mol
1a 3 0.082 1.34×104 1.39 1.09×104 1.56
1b 6.25 0.151 2.38×104 1.39 2.24×104 1.42
1c 9 0.204 3.20×104 1.42 3.05×104 1.43
1d 24 0.471 5.60×104 1.45 5.19×104 1.56
(RAFTagentA,c =10.5mmolL−1,c =5.4mmolL−1)
RAFT AIBN
2a 3 0.068 7.98×103 2.63 5.13×103 3.74
2b 6 0.130 1.86×104 2.46 1.61×104 2.84
2c 9 0.193 3.03×104 2.20 2.93×104 2.29
2d 24.18 0.451 7.10×104 2.01 6.63×104 2.07
(RAFTagentB,c =5.0mmolL−1,c =5.0mmolL−1)
RAFT AIBN
3a 5.32 0.123 1.83×104 2.17 1.44×104 2.68
3b 14.22 0.285 4.36×104 1.95 4.03×104 2.07
3c 20.52 0.369 5.40×104 2.00 4.77×104 2.20
(RAFTagentC,c =10.0mmolL−1,c =5.0mmolL−1)
RAFT AIBN
4a 4 0.068 6.87×103 2.66 4.99×103 3.36
4b 11 0.183 2.91×104 2.01 2.68×104 2.14
4c 15 0.241 3.98×104 1.93 3.78×104 2.00
4d 19 0.302 4.89×104 1.90 4.70×104 1.95
4e 26 0.397 6.07×104 1.94 5.78×104 1.99
(RAFTagentC,c =10.0mmolL−1,c =4.0mmolL−1)
RAFT AIBN
5a 3 0.154 1.38×104 4.34 1.22×104 4.84
5b 6 0.223 2.31×104 3.85 2.10×104 4.15
5c 12.5 0.335 4.51×104 2.98 4.21×104 3.11
5d 22.33 0.489 6.40×104 2.86 5.96×104 3.00
5e 25.75 0.590 7.55×104 2.77 6.19×104 3.30
(RAFTagentD,c =4.0mmolL−1,c =4.4mmolL−1)
RAFT AIBN
6a 4 0.078 3.44×103 4.13 3.61×103 3.87
6b 11 0.195 1.16×104 3.95 1.24×104 3.66
6c 15 0.255 2.34×104 2.63 2.27×104 2.69
6d 19 0.321 3.15×104 2.47 3.04×104 2.52
6e 26 0.398 4.13×104 2.43 4.11×104 2.43
(RAFTagentE,c =5.0mmolL−1,c =4.0mmolL−1)
RAFT AIBN