Acceleration of the Single Electron Transfer–DegenerativeChain Transfer Mediated Living Radical Polymerization(SET–DTLRP) of Vinyl Chloride in Water at 25 °C

VIRGIL PERCEC, ANATOLIY V. POPOV, ERNESTO RAMIREZ-CASTILLO, OLIVER WEICHOLD

University of Pennsylvania, Roy & Diana Vagelos Laboratories, Department of Chemistry,Philadelphia, Pennsylvania 19104-6323

Received 9 August 2004; accepted 19 August 2004DOI: 10.1002/pola.20482Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The accelerated single electron transfer–degenerative chain transfer me-diated living radical polymerization (SET–DTLRP) of vinyl chloride (VC) in H2O/tetrahydrofuran (THF) at 25 °C is reported. This process is catalyzed by sodiumdithionite (Na2S2O4)-sodium bicarbonate (NaHCO3). Electron transfer cocatalysts(ETC) 1,1�-dialkyl-4,4�-bipyridinum dihalides or alkyl viologens were also employed inthis polymerization. The resulting poly(vinyl chloride) (PVC) has a number-averagemolecular weight (Mn) � 2,000–12,000, no detectable amounts of structural defects,and both active chloroiodomethyl and inactive chloromethyl chain ends. The molecularweight distribution of PVC obtained is Mw/Mn � 1.5. The surface active agents affordthe final polymers as a powder and provide an acceleration of the rate of polymeriza-tion. The role of ETC is to accelerate the single electron transfer (SET) step, whereasTHF enhances the degenerative chain transfer (DT) step. © 2004 Wiley Periodicals, Inc. JPolym Sci Part A: Polym Chem 42: 6364–6374, 2004Keywords: living polymerization; radical polymerization; vinyl chloride; poly(vinylchloride); sodium dithionite; iodoform; single electron transfer; degenerative chaintransfer; tetrahydrofuran; viologen; surfactant

INTRODUCTION

The recently discovered1 living radical polymer-ization (LRP) of vinyl chloride (VC) initiated withiodoform1–3 was found to proceed mainly via acombination of competitive single electron trans-fer (SET) and degenerative chain transfer (DT)mechanisms (Scheme 1) (SET–DTLRP).2,3 TheSET step of this polymerization is provided by oneelectron transfer agents. Such an agent can be azero valent transition metal [Cu(0)]1,2 or a non-transition metal compound such as sodium dithio-

nite (Na2S2O4), which in H2O dissociates slowlyto the electron-donor sulfur dioxide anion-radicalSO2

��.3 This electron-donor anion-radical plays therole of a one electron reducing agent. One electrontransfer to a halogen-containing group generatesan anion-radical that, in turn, decays to a halideanion and a radical. The radical adds monomermolecules and/or transfers an iodine atom fromthe initiator or the propagating polymer dormantspecies. Therefore, the chain transfer to iodocom-pounds represents the second major reaction step,namely DT. All the potential reactions occurringduring this polymerization were discussed in aprevious publication.3 When performed in waterSET–DTLRP of VC produces telechelic PVC withtwo chloroiodomethyl (�CHClI) active chain endsand a number-average molecular weight (Mn)

Correspondence to: V. Percec (E-mail: percec@sas.upenn.edu)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 6364–6374 (2004)© 2004 Wiley Periodicals, Inc.

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that can be controlled between 2,000 and 55,000.3

The PVC obtained at 25 to 35 °C by SET–DTLRPof VC (LRP–PVC) contains no detectable amountsof structural defects such as internal allyl chlo-rides and tert-alkyl chlorides. Because thesestructural defects are considered to provide theinitiation sites for the thermal degradation ofPVC,4 the LRP–PVC shows a higher thermal sta-bility than the commercial PVC of similar Mnobtained by the conventional free-radical poly-merization of VC. In addition, LRP–PVC has ahigher syndiotacticity than that of PVC synthe-sized at the same temperature by a free-radicalprocess.2,3 We have found that the process ofSET–DTLRP of VC consists of two stages: fastand slow.1–3 The breakpoint on the semilogarithmkinetic plots shows the transition from the faststage to the slow one. The fast stage consists of apolymerization process containing VC as an or-ganic liquid phase in equilibrium with VC in a gasphase and an aqueous phase. During this stagePVC exists as a fine dispersion. The apparent rateconstant of the fast stage is symbolized as kp1 onkinetic plots. The second slow stage is defined bykp2 and consists of the polymerization when VCforms the gas phase in equilibrium with its solu-tion in water and precipitated PVC. In most ex-periments the values of kp1 are 7–10 times largerthan the values of kp2.1–3

As most other living radical polymerizations,SET–DTLRP of VC proceeds at 25 °C with a veryslow rate (kp1 � 0.015 h�1).3 Several options havebeen considered to increase the rate of the SET–DTLRP of VC. The first approach is to performthe polymerization at a higher temperature thatproduces a PVC with a low concentration of struc-

tural defects (35 °C). The second is to increase theSET agent, Na2S2O4. The third is to use suitabledispersants for the polymerization process. Theseoptions have been already investigated and havebeen reported in a previous publication.3

In this article we report the SET–DTLRP of VCaccelerated by addition of tetrahydrofuran (THF)as an additional nondegenerative chain-transferagent that has a chain transfer constant compa-rable with that of VC. In radical polymerization ofVC at 50 °C CsTHF � 2.4 � 10�3 although Cm� 1.23 � 10�3. No data are available for lowertemperatures.5 The acceleration of SET–DTLRPof VC with the aid of various electron transfercocatalysts (ETC) is also reported. The scopes andlimitations of the accelerated SET–DTLRP of VCwill be discussed.

RESULTS AND DISCUSSION

SET–DTLRP of VC Initiated with CHI3 andCatalyzed by Na2S2O4 in Water/THF in theAbsence and in the Presence of SurfaceActive Agents

The kinetic experiments reported in this publica-tion were performed in 50 mL glass high pressurereactors. Each data point on the kinetic plots rep-resents an individual polymerization experiment.

Figure 1(a) shows the kinetic plots forNa2S2O4/NaHCO3 catalyzed SET–DTLRP of VCat 25 °C in water/THF (2:1 v/v) carried out withthe initial molar ratio [VC]0 /[CHI3]0 � 200, (or at100% conversion the theoretical degree of poly-merization for the resulting PVC would be DPth� 200). A ratio of [Na2S2O4]0/[CHI3]0 � 2 wasused in the experiments reported in Figure 1 andin subsequent figures. The experiment fromFigure 1(a) produced an 82% conversion after68 h. The molecular weight distribution of theresulting LRP–PVC is Mw/Mn � 1.5. This value islower than the value obtained in the absence ofTHF.3 This kinetic exhibits two distinct slopes inthe semilogarithm conversion versus time plot.These linear dependences show a first-order ofreaction in the concentration of VC and a constantconcentration of radicals during the polymeriza-tion. The linear dependence between experimen-tal Mn and theoretical Mth supports a living poly-merization reaction. The fast stage of the poly-merization has kp1 � 0.039 h�1 and correspondsto two liquid phases (water and organic) in thereaction mixture. The second slope provides kp2

Scheme 1. SET–DTLRP of VC in water.

ACCELERATION OF THE SET–DTLRP OF VC 6365

� 0.015 h�1. This value is 2.6 times lower thanthat of kp1 [Fig. 1(a)]. This second slope belongs tothe slow stage suspension polymerization, when

the reaction mixture is composed of the liquidaqueous phase (water and dissolved THF, andinorganic salts) and the solid phase (solid PVCswollen by THF and VC). The breakpoint on thesemilogarithm plot from Figure 1(a) correspondsto the transition between these two stages. In thisexperiment this transition occurred after 28 h,when the monomer conversion was 63%. The ap-parent rate constants for both fast and slowstages are higher (2.6 and 7.5 times, respectively)than those for LRP of VC catalyzed by Na2S2O4/NaHCO3 in water at the same temperature with-out added THF and reported earlier (in that casekp1 � 0.015 h�1 and kp2 � 0.002 h�1).3 The LRP–PVC obtained from this polymerization consists ofsmall white solid spheres.

Different surface active agents were employedto increase the rate of polymerization [Fig. 1(b–d)]. When Brij 98 [polyoxyethylene(20) oleylether, C18H35(OCH2CH2)nOH, n�20] was used,the rate of the first stage increased to kp1� 0.050 h�1, whereas the rate of the second stagewas almost unchanged (kp2 � 0.013 h�1)[Fig. 1(b)]. After 48 h the conversion was 80%.The breakpoint of the two reaction rates was ob-served at 64% conversion after 24 h. The depen-dence of Mn on conversion and Mth was linear.The LRP–PVC had a Mw/Mn � 1.50.

Two suspension agents (SA) used in indus-try for the preparation of PVC were tested inthe polymerization experiments reported inFigure 1(c and d): hydroxypropyl methylcellulose(Methocel F50 or MF50) and poly(vinyl acetate)72.5% hydrolyzed (Alcotex 72.5 or Alc72.5). Thetotal amount of SA used was 910 ppm w/w rela-tive to VC. With the ratio [MF50]:[Alc72.5]� 820:90 ppm/ppm, the apparent rate constant ofpolymerization observed the fast stage of the re-action was kp1 � 0.085 h�1 [Fig. 1(c)]. This is atwofold increase from the one obtained withoutSA [Fig. 1(a)]. The rate constant of the slow stagestayed nearly constant (kp2 � 0.016 h�1). A highmonomer conversion, 90%, was achieved after70 h. The transition between fast and slow stagestook place at 80% VC conversion that was ob-tained after 24 h. The molecular weight distribu-tion of this PVC is Mw/Mn � 1.6 to 1.7, whereasthe values of Mn were very close to Mth [Fig. 1(c)].

A similar high polymerization rate for the firststage of the polymerization kp1 � 0.087 h�1 wasobserved when the ratio [MF50]:[Alc72.5] was455:455. The second stage was slightly faster (kp2� 0.022 h�1) than in the previous experiments.The change in polymerization rate occurred after

Figure 1. Na2S2O4/NaHCO3-catalyzed SET–DTLRPof VC initiated with iodoform in H2O/THF at 25 °C,[VC]/[H2O]/[THF] � 1/2/1 (v/v/v). (a) [VC]/[CHI3]/[Na2S2O4]/[NaHCO3] � 200/1/2/2.2 (mol/mol/mol/mol);(b) in the presence of surfactant Brij 98 [VC]/[CHI3]/[Na2S2O4]/[NaHCO3]/[Brij 98] � 200/1/2/2.2/0.25 (mol/mol/mol/mol/mol); (c) in the presence of surfactantsMethocel F50 (MF50) and Alcotex 72.5 (Alc72.5). [VC]/[CHI3]/[Na2S2O4]/[NaHCO3] � 200/1/4/2.2/(mol/mol/mol/mol); [MF50]/[Alc72.5] � 820/90 (ppm/ppm w/wrelative to VC); (d) in the presence of surfactants MF50and Alc72.5, [VC]/[CHI3]/[Na2S2O4]/[NaHCO3] � 200/1/4/2.2/(mol/mol/mol/mol); [Methocel F50]/[Alcotex72.5] � 455/455 (ppm/ppm w/w relative to VC).

6366 PERCEC ET AL.

24 h when the monomer conversion was 79%. Amaximum VC conversion of 93% was achievedafter 68 h. Mw/Mn was 1.5–1.7. A linear depen-dence of Mn versus Mth was observed [Fig. 1(d)],although Mn values were more scattered than forthe experiment performed with a higher contentof Methocel F50 that was presented inFigure 1(c).

The LRP–PVC obtained from all the experi-ments with added surfactants was a white pow-der.

Polymerization of VC Initiated with CHI3 andCatalyzed by Na2S2O4 in H2O/THF at 25 °C in thePresence of Electron Transfer Cocatalysts andSurface Active Agents

During the fast stage of this polymerization reac-tion both the initiator and the PVC dormant spe-cies are located, most probably, in the organicliquid phase. In such a situation, an organic elec-tron transfer (co)catalyst (ETC) could be used inthe SET–DTLRP of VC along with Na2S2O4 toenhance the rate of the reaction. Such ETC are1,1�-dialkyl-4,4�-bipyridinum dihalides or alkylviologens (AlkV2�).5–10 (Scheme 2, eq 1). Whenthe alkyl is methyl and the halide is chloride thecompound is known under the common namemethyl viologen (MV2�). When the alkyl is n-octyland the halide is bromide the common name isoctyl viologen (OV2�). Alkyl viologens can be re-duced via SET by Na2S2O4 in water to form rad-ical cations (AlkV�). AlkV2� are more soluble inwater than AlkV� that are more soluble in or-ganic solvents.5 Consequently, viologens are usedto transport electrons from an aqueous to an or-ganic phase.5–10 MV2� and OV2� were used as

cocatalysts together with Na2S2O4 to acceleratethe SET–DTLRP of VC in the absence and in thepresence of surfactants (Fig. 2).

Figure 2(a) illustrates the kinetic plot of SET–DTLRP in the presence of added MV2�. When thisexperiment is compared with that reported inFigure 1(a) one can see that the first stage of thepolymerizations became faster (kp1 � 0.054 h�1),whereas the second step became slower (kp2� 0.009 h�1). A maximum conversion of 77% wasreached after 55 h. The breakpoint was observedafter 25 h with a conversion of VC of 68%. Thevalues of Mn are in good agreement with Mth.Mw/Mn � 1.5. The PVC obtained from these ex-periments represents small spheres that areslightly pink.

OV2� was employed together with the surfac-tant Brij 98 in the polymerization experimentsshown in Figure 2(b). In this case the first stagewas even faster (kp1 � 0.066 h�1) and second oneeven slower (kp2 � 0.007 h�1). The maximummonomer conversion was 80% after 68 h. VC con-version (70%) was obtained after 21 h. The result-ing PVC had Mw/Mn � 1.5. A linear dependence

Scheme 2. Electron transfer cocatalysis (ETC) medi-ated by alkyl viologens.

Figure 2. Na2S2O4/NaHCO3-catalyzed SET–DTLRPof VC initiated with iodoform in H2O/THF at 25 °Caccelerated with the electron transfer cocatalyst (ETC)alkyl viologen, [VC]/[H2O]/[THF] � 1/2/1 (v/v/v). (a)ETC is methyl viologen (MV2�) [VC]/[CHI3]/[Na2S2O4]/[NaHCO3]/[MV2�] � 200/1/2/2.2/0.0018 (mol/mol/mol/mol/mol); (b) in the presence of ETC octyl viologen(OV2�) and surfactant Brij 98 [VC]/[CHI3]/[Na2S2O4]/[NaHCO3]/[OV2�]/[Brij 98] � 200/1/2/2.2/0.0018/0.25(mol/mol/mol/mol/mol/mol).

ACCELERATION OF THE SET–DTLRP OF VC 6367

Mn versus Mth was observed. The resulting PVCwas also a pink powder.

Structural Analysis of LRP–PVC by NMRSpectroscopy

The 1H NMR spectra of three different PVC wererecorded in CD2Cl2 and are shown in Figure 3.The spectrum of the free-radical PVC synthesizedin bulk at 72 °C with azobisisobutyronitrile(AIBN) as an initiator is shown in Figure 3(a).The NMR spectrum of LRP–PVC obtained by ini-tiation with CHI3 and catalyzed by Na2S2O4/NaHCO3 SET–DTLRP of VC at 25 °C in water isshown in Figure 3(b), whereas that of an LRP–PVC obtained by initiation with CHI3 and cata-lyzed by Na2S2O4/NaHCO3 SET–DTLRP of VC at25 °C in H2O/THF is shown in Figure 3(c).

The spectrum of free-radical PVC [Fig. 3(a)]reveals the strong signals of main chain �CH2�

(1.9–2.6 ppm), �CHCl� (4.2–4.8 ppm). The CH3groups from the AIBN initiator rest are at 1.4–1.5 ppm. The signals of the structural defectsare: �HCACH� (5.7–5.9 ppm), transOCHACHCH2Cl (4.11 ppm, the broad signal of cis-CHACHCH2Cl is overlapped with �CHCl� at4.2 ppm) derived from head-to-head addition fol-lowed by 1,2-Cl shift(s) and Cl-abstraction, andthe internal allyl protons �CHACHCH2� (2.6–2.8 ppm). At 3.7–3.9 ppm one can see two multi-plets that correspond to �CH2Cl groups. Themultiplet at 3.7–3.8 ppm belongs mostly to the2-chloroethyl group CH2ClCH2� (a small part ofthis signal is, possibly, the signature of chlorom-ethyl branch-(CH2Cl)CHO).11,12 The 2-chloro-ethyl group can be a chain end derived from H-capture and a branch or part of a branch derivedfrom H-back-biting.4 The multiplet at 3.8–3.9 ppm belongs, most probably, to the 1,2-dichlo-roethyl group CH2ClCHCl�,11,12 which is from

Figure 3. 500 MHz 1H NMR spectra of PVC in CD2Cl2. (a) PVC obtained by free-radical polymerization of VC in bulk initiated with AIBN at 72 °C; (b) LRP–PVC fromNa2S2O4/NaHCO3-catalyzed SET–DTLRP of VC in H2O initiated with CHI3 at 25 °C;(c) LRP–PVC from Na2S2O4/NaHCO3-catalyzed SET–DTLRP of VC in H2O/THF initi-ated with CHI3 at 25 °C.

6368 PERCEC ET AL.

the chain end originated from chlorine chaintransfer to monomer, and from the dichloroethylbranch formed by head-to-head addition of VCfollowed by two subsequent 1,2-Cl-shifts and fur-ther addition of the VC monomer.4,13

The 1H NMR spectrum of LRP–PVC obtainedin H2O [Fig. 2(b)] shows the intense signals of themain PVC repeat units �CH2CHCl� (�CH2� at1.9–2.6 ppm and �CHCl� at 4.2–4.8 ppm). Ste-reoisomers r and m of chloroiodomethyl �CHClIchain ends show resonances at 5.9 and 6.1 ppmand adjacent methylene protons CHClICH2� at2.6–2.9 ppm. The weak signal of the diiodomethylCHI2� group (the iodoform remainder) that rep-resents the LRP–PVC still growing in one direc-tion is also detected at 5.2 ppm. The sharp butvery small signal of trans-CHACHCH2Cl at4.11 ppm is present as well (its concentration is�0.1 per 1000 VC r.u). The signals of otherCH2Cl� groups at 3.7–3.9 ppm, as well as thesignal of the vinyl protons at 5.7–5.9 ppm arealmost nondetectable.

The LRP–PVC obtained in water/THF[Fig. 2(c)] demonstrates proton signals of the PVCmain chain (�CH2� at 1.9–2.6 ppm and�CHCl� at 4.2–4.8 ppm). The resonances of rand m stereoisomers of chloroiodomethyl �CHClIchain ends are at 5.9 and 6.1 ppm and thoseof their neighboring methylene protons CHCl-ICH2� are at 2.6–2.9 ppm. There is also a smallsignal of trans-CHACHCH2Cl at 4.11 ppm (itsconcentration is �0.15 per 1000 VC r.u.) and aquite intense signal of CH2Cl� belonging to theCH2ClCH2� fragment at 3.7–3.8 ppm. The signalof the 1,2-dichloroethyl group is absent. The veryweak signals of vinyl protons at 5.7–5.9 ppmmight belong to the �CHACHCH2Cl moiety.3

This 1H NMR spectrum is identical to the 1HNMR spectrum of LRP–PVC obtained by theCu(0)/tris(2-aminoethyl)amine (TREN) poly(eth-yleneimine) (PEI)-catalyzed polymerization of VCinitiated with iodoform in H2O/THF at 25 °C.1,2

Two-dimensional proton–carbon correlationNMR heteronuclear multiple-quantum coherence(HMQC) of LRP–PVC obtained via initiation withCHI3, catalyzed by the Na2S2O4/NaHCO3 SET–DTLRP of VC at 25 °C in H2O/THF is shown inFigure 4. This spectrum exhibits the signals ofthe �CH2� group (1.9–2.6 ppm in 1H and 45–48 ppm in 13C NMR), �CHCl� (4.2–4.8 ppm in1H and 55–60 ppm in 13C NMR), r and m stereo-isomers of chloroiodomethyl �CHClI chain ends(6.1 ppm in 1H, 24 ppm in 13C and 5.9 ppm in 1H,27 ppm in 13C NMR), and the adjacent methylene

group CHClICH2� (2.6–2.9 ppm in 1H and 53–56 ppm in 13C NMR), and the CH2Cl� group ofCH2ClCH2� structural fragment (3.7–3.8 ppm in1H and 41–42 ppm in 13C NMR). The rest of theiodoform �CHI� has a proton signal that is over-lapped with the signal of �CHCl� main groupand is detected at 4.6 ppm in 1H and 29 ppm in13C NMR. No signals of the �CHClOCHCl�fragment were detected at 4.5–4.7 ppm in 1H and75–80 ppm. However, these signals were found inthe spectrum of LRP–PVC obtained from SET–DTLRP in water3 and suggest the possibility ofradical combination during the SET–DTLRPof VC.

Neither 1H nor HMQC NMR spectra of theLRP–PVC obtained from the SET–DTLRP of VCin H2O/THF, catalyzed by Na2S2O4/NaHCO3showed the resonances that might be assigned tosulfur- or sulfur dioxide-containing fragments3

and the fragments derived from THF.14

Mechanistic Considerations

Na2S2O4 is the key reagent for the SET–DTLRPof VC initiated with iodoform. Besides its mainSET catalytic role, it scavenges oxygen,15 freeiodine,16 and other iodine-based impuritiespresent in commercial iodoform and, therefore,suppresses possible side reactions derived fromthese compounds during the polymerization pro-cess. The role of Na2S2O4 as a SET catalyst is toproduce the single electron reducing agent sulfurdioxide radical-anion SO2

�� by the slow dissocia-tion of dithionite dianion (Scheme 3, eq 2). SO2

�� isthe single electron reductant that provides thesource of electrons for SET both during initiationfrom CHI3 and propagation from dormant�CHClI groups (Scheme 3, eq 2, Scheme 4, eqs 6and 7, Scheme 5).

Na2S2O4 is stable only under basic conditions.Therefore, a basic buffer such as NaHCO3 is nec-essary. NaHCO3 also supports the polymerizationof VC that requires basic conditions.17 NaHCO3also consumes SO2 that results from SET oxida-tion of SO2

�� (Scheme 3, eq 5). Control experimentsdemonstrated that no polymer was obtained byattempts to polymerize VC with CHI3 as an initi-ator and catalyzed by Na2S2O4 in the absence ofNaHCO3 buffer. Because sulfur- or SO2-contain-ing fragments were not found in LRP–PVC ob-tained by the SET–DTLRP of VC in H2O/THF,the only other possible role of SO2 generated bySET from SO2

�� is in reversible addition to grow-ing PVC radicals. This would create �SO2

� stable

ACCELERATION OF THE SET–DTLRP OF VC 6369

transient species that could aid the SET–DTLRPprocess. This possibility and other probable re-versible reactions that may form a carbonOsulfurbond were discussed in detail in a previous pub-lication and were shown in the suggested mecha-nism for the SET–DTLRP of VC in water.3

A diversity of other organic solvents was testedto accelerate the Na2S2O4 catalyzed SET–DTLRPof VC in H2O.18 However, only THF and to alesser extent 1,4-dioxane have enhanced the rateof this polymerization. The LRP–PVC obtained inH2O/THF does not contain CH2ClCHCl� frag-

ments derived from chain transfer to monomer orfrom head-to-head addition followed by two 1,2-Clshifts and further addition of VC to the radi-cal.4,13 The LRP–PVC obtained in H2O/THF con-tains only CH2ClCH2� fragments that could beformed from the H-transfer and/or H-back-biting.The 1H NMR spectra of LRP–PVCs obtained inH2O/THF at 25 °C and catalyzed by Cu(0)/TREN(PEI)2 or by Na2S2O4/NaHCO3 are identical andreveal the same structure for the PVC. 13C NMRanalysis of Bu3SnH(D) reduced LRP–PVC ob-tained from the Cu(0) catalyzed SET–DTLRP of

Figure 4. Carbon–proton correlation HMQC NMR spectrum of LRP–PVC in CD2Cl2.The polymer was prepared by Na2S2O4/NaHCO3-catalyzed SET–DTLRP of VC inH2O/THF initiated with CHI3 at 25 °C.

6370 PERCEC ET AL.

VC at 25 °C revealed a linear structure and nodetectable amounts of branches.2 Therefore, at25 °C back-biting does not occur in the SET–DTLRP of VC and the 2-chloroethyl group repre-sents the chain end, which is not active and whoseconcentration increases with the increase ofmonomer conversion. This chain end is, mostprobably, generated by H-transfer from THF

(Scheme 6). The resulting tetrahydrofuryl radicalcan transfer an iodine atom from iodoform and/ordormant LRP–PVC species and the iodo-THFproduct formed participates in a degenerativechain-transfer process that increases the rate ofDT. There is no evidence so far that the tetrahy-drofuryl radical can start the polymerization ofVC, although it is well established that it can add

Scheme 3. Dissociation and initiation steps ofNa2S2O4/NaHCO3 catalyzed SET–DTLRP of VC initi-ated with CHI3.

Scheme 4. Activation of dormant LRP–PVC speciesvia SET.

Scheme 5. Propagation and DT steps of Na2S2O4/NaHCO3 Catalyzed SET–DTLRP of VC initiated withCHI3.

Scheme 6. THF mediated reactions during SET–DTLRP of VC.

ACCELERATION OF THE SET–DTLRP OF VC 6371

the fluoroolefins.14 The formation of iodotetrahy-drofuran was reported first for radical reactions ofRFI in THF,19 then it was detected in the SET–DTLRP of VC mediated by Cu(0)/TREN (PEI) cat-alyzed LRP of VC in H2O/THF.2 Therefore, therole of THF in the SET–DTLRP of VC is complex.THF is miscible with H2O, although it is also agood solvent for VC and PVC. THF plays the roleof chain-transfer agent by forming iodo-THF de-rivative(s). All these combined factors produce acomplex mechanism of SET–DTLRP acceleration.

The role of ETC is to accelerate the transfer ofan electron from the H2O phase to the organicphase and also to transfer the iodide-anion fromthe organic phase to the H2O phase (Scheme 7).This accelerates the SET step of SET–DTLRP.

EXPERIMENTAL

Materials

Vinyl chloride (VC, 99%) was purchased from Al-drich and distilled before use. Iodoform (99%),and sodium dithionite (85%) were purchased fromLancaster. Sodium thiosulfate (99.5%) and 1-bro-mooctane (99%) were purchased from Acros Or-ganics. Tetrahydrofuran (THF, 99%), methylenechloride (99.5%), methanol (99.8%), sodium bicar-bonate (99�%), dimethylformamide (DMF, 99%),and dimethyl sulfoxide (DMSO, 99%) were pur-chased from Fisher Scientific. Alcotex 72.5 waspurchased from Harlow Chemical Co., UK.Methocel F50 was purchased from the DowChemical Company. Octyl viologen was synthe-sized according the literature procedure.10 Allother chemicals were purchased from Aldrich andwere used as received.

Techniques1H (500 MHz), 13C (125 MHz), and HMQC NMRspectra were recorded on a Bruker DRX500 at305 K in CDCl3, CD2Cl2, and THF-d8 with tetra-methylsilane as an internal standard. Gel perme-ation chromatography (GPC) analysis was per-formed on a PerkinElmer Series 10 high-pressureliquid chromatograph equipped with an LC-100column oven (22 °C), a Nelson Analytical 900Series integrator data station, a PerkinElmer785A UV–vis Detector (254 nm), a Varian Star4090 RI detector, and 2 AM gel (10 �m, 500 Å and10 �m, 104 Å) columns. THF (Fisher; HPLC-grade) was used as an eluent at a flow rate of1 mL/min. Mns and Mws were determined againstpolystyrene standards and were corrected usingthe universal calibration20 with the followingMark–Houwink parameters for PVC: K � 1.50� 10�2 mL/g, a � 0.77.21

The samples used for spectral analysis wereprecipitated twice from THF or CH2Cl2 solutionsinto MeOH and dried in vacuo (1 Torr) at 23 °C.For GPC analysis the THF solution of polymerswere purified from surfactants (if used) by centrif-ugation and subsequently were passed through asmall basic alumina column.

Typical Procedure forNa2S2O4-NaHCO3–Catalyzed SET–DTLRP of VC inWater–THF at 25 °C

In a typical experiment, a 50 mL Ace Glass 8648#15 Ace-thred pressure tube equipped with bush-ing and plunger valve was charged with a previ-ously degassed mixture of water (6 mL) and THF(3 mL), then filled with argon, closed, and fro-zen with MeOH/dry ice. Then, the initiator(CHI3, 85.5 mg, 0.22 mmol), catalyst (Na2S2O4,75.6 mg, 0.43 mmol), buffer (NaHCO3, 40.1 mg,0.48 mmol), and precondensed VC (3 mL,0.043 mol) were added. The exact amount of VCwas determined gravimetrically after the reac-tion. The tube was closed and degassed throughthe plunger valve by applying reduced pressureand filling the tube with Ar 15 times at �40 °C.The valve was closed and the reaction mixturewas stirred in a water bath at 25 °C � 0.5 °Cbehind a protective shield. After 33 h, the tubewas slowly opened, the excess of VC was allowedto evaporate and the reaction mixture was pouredinto MeOH (150 mL). The polymer was recoveredby filtration and dried in a vacuum oven to aconstant weight to give 1.78 g (66.1%) of whitePVC, Mn � 8,200, Mw/Mn � 1.47.

Scheme 7. SET step mediated by ETC.

6372 PERCEC ET AL.

Typical Procedure forNa2S2O4-NaHCO3–Catalyzed SET–DTLRP of VC inWater–THF at 25 °C in the Presence of theSurfactant Brij 98

This experiment was performed as the previ-ous one, except that only 6.3 mg of Brij 98(0.055 mmol) was added to the reaction mixture.The reaction time was 44 h; 2.16 g (72.2%) PVCwas obtained. Mn � 9, 300, Mw/Mn � 1.49.

Typical Procedure forNa2S2O4-NaHCO3–Catalyzed LRP of VC in Water–THF at 25 °C in the Presence of Electron TransferCatalyst, Methyl Viologen

The experiment was performed as the typical pro-cedure presented for Na2S2O4-NaHCO3–cata-lyzed LRP of VC in water–THF at 25 °C, exceptthat only the electron transfer cocatalyst methylviologen (MV2�, 0.1 mg, 0.39 �mol)) was added tothe reaction mixture. The reaction time was 24 h;2.28 g (72.2%) PVC was obtained. Mn � 10, 200,Mw/Mn � 1.52.

Typical Procedure forNa2S2O4-NaHCO3–Catalyzed SET–DTLRP of VC inWater–THF at 25 °C in the Presence of ElectronTransfer Catalyst, Octyl Viologen and theSurfactant Brij 98

The experiment was performed as the typical pro-cedure presented for Na2S2O4-NaHCO3–cata-lyzed LRP of VC in water–THF at 25 °C, exceptthat only the electron transfer cocatalyst octylviologen (OV2�, 0.2 mg, 0.39 �mol) and 6.3 mg ofBrij 98 (0.055 mmol) were added to the reactionmixture. The reaction time was 66 h; 2.50 g(83.2%) of PVC was obtained. Mn � 10, 450,Mw/Mn � 1.59.

Typical Procedure forNa2S2O4-NaHCO3–Catalyzed SET–DTLRP of VC inWater–THF at 25 °C in the Presence of SurfactantsMethocel F50 and Alcotex 72.5 (820/90 ppm/ppm,w/w)

In a typical experiment, a 50 mL Ace Glass 8648#15 Ace-thred pressure tube equipped with bush-ing and plunger valve was charged with a previ-ously degassed 9 mL mixture of water and THF(volume ratio water : THF � 2:1), 132.3 mg of1.86% water solution of Methocel F50, and 6.4 mgof 4.24% water solution of Alcotex 72.5 ([Methocel

F50]:[Alcotex 72.5] � 820 ppm:90 ppm w/w rela-tive to VC). The tube was filled with argon, closed,and frozen with MeOH/dry ice. Then, the initiator(CHI3, 85.5 mg, 0.22 mmol), catalyst (Na2S2O4,152.2 mg, 0.87 mmol), buffer (NaHCO3, 40.1 mg,0.48 mmol), and precondensed VC (3 mL,0.043 mol) were added. The exact amount of VCwas determined gravimetrically after the reac-tion. The tube was closed and degassed throughthe plunger valve by applying reduced pressureand filling the tube with Ar 15 times at �40 °C.The valve was closed and the reaction mixturewas stirred in a water bath at 25 °C � 0.5 °C,behind a protective shield. After 40 h, the tubewas slowly opened, the excess of VC was allowedto evaporate, and the mixture was poured intoMeOH (150 mL). The polymer was separated byfiltration and dried in a vacuum oven to a con-stant weight to give 2.33 g (85.9%) of PVC. Mn� 8,900, Mw/Mn � 1.69.

Typical Procedure forNa2S2O4-NaHCO3–Catalyzed LRP of VC in Water–THF at 25 °C in the presence of SurfactantsMethocel F50 and Alcotex 72.5 (455/455 ppm/ppm, w/w)

The experiment was performed as the previousone, except that only 73.4 mg of the 1.86% watersolution of Methocel F50 and 32.2 mg of the 4.24%water solution of Alcotex 72.5 ([Methocel F50]:[Alcotex 72.5] � 455 ppm:455 ppm w/w relative toVC) were used. After a reaction time of 43 h,2.32 g (85.4%) of PVC was obtained. Mn � 10,290,Mw/Mn � 1.55.

CONCLUSIONS

An accelerated non-transition metal catalyzedSET–DTLRP of VC is reported. The sodium di-thionite-catalyzed reaction proceeds in water/THF at 25 °C, producing a PVC of Mn from 2,000to 12,000 with narrow Mw/Mn � 1.5. This PVC isfree of structural defects and contains both activeand inactive chain ends. The fast stage of thepolymerization was completed within 24 h whenVC conversion was 64–80%. Experiments withsurfactants resulted in white powders. The struc-ture of the PVC and the potential polymerizationmechanism were discussed. This acceleratedmethod is of interest for the synthesis of low-molecular-weight PVC that does not contain func-tional chain ends and contains a lower concentra-

ACCELERATION OF THE SET–DTLRP OF VC 6373

tion of structural defects and narrower Mw/Mnthan the polymer produced by the conventionalfree-radical process.

Financial support of the Edison Polymer InnovationCorporation, PVC Technology Consortium (EPIC) andthe National Science Foundation is gratefully acknowl-edged.

REFERENCES AND NOTES

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