Study of chain sequence in the controlled radical telomerization of vinyl acetate with Co(acac)2...

ORIGINAL PAPER

Study of chain sequence in the controlled radicaltelomerization of vinyl acetate with Co(acac)2 catalyst in bulk

Mohammad Ali Semsarzadeh & Sahar Amiri

Received: 25 January 2012 /Accepted: 10 May 2012 /Published online: 24 May 2012# Springer Science+Business Media B.V. 2012

Abstract Kinetics of radical telomerization of vinyl acetate(VAc) with CDCl3 as telogen in the presence of AIBNinitiator and ZnCl2 and Co(acac)2 as catalysis in bulk andsolution telomerization was studied through 1HNMR andGPC. The telomerization of vinyl acetate in chloroform hasindicated a random chain sequence with free radical initia-tors. Co(acac)2 catalyst, in this reaction controls the tacticityof the telomer chain. The narrower molecular weight distri-bution of telomer with its higher yield is predominantlysyndiotactic. 1HNMR study of the microstructure of thetelomers revealed that the sequential arrangement of CH2

and CH groups in telomer is controlled in bulk telomeriza-tion by Co(acac)2 catalyst.

Keywords Telomerization . ZnCl2 . Co(acac)2 .1HNMR .

Kinetic study of telomerization

Introduction

Telomerization is a reaction which forms oligomers withlow molecular weight with end group functionality. Telome-rization is defined as the process of reaction, under poly-merization conditions among molecule YZ, called telogen(chain transfer agent), and one or several molecules of apolymerizable compound M (Taxogen); the product X(Mn)Y is called telomere [1–5].

nM þ YZ ��!catalyst or initiatorY � ðMÞn � Z

Low molecular weight telomers are important because ofthe larger chain terminals and groups needed for reactions tobe used in ATRP of other surface reactions. Free radicaltelomerization of VAc with CHCl3 telogen has been doneearlier [6–9]. The potential use of telomer as a block struc-ture incorporated into the diblock or multi block copolymersof styrene, methyl methacrylate and methyl acrylate theimportance of which increased by discovery of ATRP. Also;incorporating of VAc telomers enhances macromolecularengineering application in this area [5, 10–12]. VAc telomerproduced as a macroinitior by free radical telomerizationthrought the ATRP reactions and synthesis of block copoly-mers of vinyl acetate-styrene which was reported earlier[13–20]. We have reported the molecular weight and itsdistribution of vinyl acetate macroinitiators on the ATRPkinetics reaction of acrylates [12–14]. The specific effect ofthe chains geometry, length, molecular weight and itsdistribution, conformation or the chemical composition ofvinyl acetate as well as, its reactivity on the ATRP polymer-ization has not been reported yet.

In this reaction, the complex catalyst Co(acac)2/DMFforms an active catalyst that activates VAc radicals in acontrolled radical telomerization and increases its conver-sion rate with a relatively low molecular weight distribu-tion. Contrary to what has been reported recently, thecomplex catalyst starts the reaction without any lengthyinduction time [19]. The reaction mechanism suggestedfor the fast and controlled reaction is a stabilized free-radical telomerization mechanism (SFRT) consisting ofthree steps:

1- Equilibrium complex formation of catalyst with DMFligand

2- Growth step from the active VAc radical3- Termination step.

M. A. Semsarzadeh (*) : S. AmiriPolymer Group, Chemical Engineering Department,Tarbiat Modares University,P.O. Box: 14155/143, Tehran, Irane-mail: [email protected]

J Polym Res (2012) 19:9891DOI 10.1007/s10965-012-9891-8

In Scheme 1. kp, kact, Kdeact and kt are kinetic constantsrelated to propagation, activation, deactivation and termina-tion rate constant and L is ligand (DMF) [19].

In this paper, macroinitiators of VAc by bulk and solutiontelomerization of VAc in the presence of various catalysisZnCl2 and Co(acac)2 are reported. The structure of telomersformed during the telomerization of vinyl acetate withCDCl3 as telogen or ZnCl2 and Co(acac)2 as catalysis isconsidered statically. This study provides the new syndio-tactic route to makes telomer with controlled chain struc-tures in synthesis of macroinitiators used in ATRP, and inthis way widens our scope in the macromolecular engineer-ing of block copolymers and then specific uses. 1HNMR hasbeen used to study tacticity of the telomers formed in thebulk or solution. Bernoullian chain statics has configuredcontrolled polymerization in telomerization with Co(acac)2catalyst leading to a telomer with new tacticity.

Experimental

Materials

Vinyl acetate (VAc, Merck-Schuchardt, > 99 %) was driedover calcium hydride, distilled under nitrogen (99.99 %)atmosphere. 2,2′- Azobisisobutyronitrile (AIBN, Fluka, ≥98 %) and chloroform-d (CDCl3, ARMAR Chemicals,Switzerland, 99.95 %), dimethylformamide (DMF, Merck,>99 %), cobalt acetyl acetonate (Co(acac)2, Merck, >99 %),Zinc chloride (ZnCl2, Merck, >99 %) methanol (Merck,>99 %) were used without further purification.

Telomerization

A three-neck round bottom flask equipped with a condenserand a magnetic stirrer was charged with samples with differentVAc/CDCl3/AIBN/Catalyst ratios concentrations were pre-pared (Table 1). Then the flask was immersed at 60 °C waterbath to initiate the reaction. After 5 h the reaction was stopped

by removing the flask from the bath and the unconvertedmonomer and chloroform were evaporated at room tempera-ture. After reaction for a few hours, the solutionwas diluted byacetone and eluted through alumina column in order to elim-inate the cobalt complex. The polymer was precipitated inTHF and refluxed for 2 h to decompose traces of unreactedinitiator. Finally THF was evaporated and the polymer wasdried under vacuum at 50 °C up to a constant weight [3, 4,20–21]. The dried polymer was used in determination ofmolecular structure with 1HNMR spectroscopy and GPC.

Characterization

1H NMR spectra were recorded with a Bruker DRX 500spectrometer (500 MHz) in deuterated chloroform. Molec-ular weight and its distribution (MWD) were recorded witha Waters 150C PS-calibrated GPC in THF at 35 °C and1 mL/min equipped with a refractive index detector and 104,103 and 500°A set of ultrastyrogel columns.

Results and discussion

Previously showed that DMF/Co(acac)2 controlled the poly-merization of vinyl acetate [19] and now controlled radicaltelomerization of vinyl acetate was studied in bulk and solutionin the presence of Co(acac)2. The chain sequence of the syn-thesized telomers was characterized by 1HNMR spectroscopytechnique. The kinetic parameters have been calculated from1HNMR spectra of each sample in various time intervals. Thereaction product is a mixture of vinyl acetate chain containingCCl3 end group (telomers) and vinyl acetate chains containinginitiator molecules at their ends. Typical 1HNMR recorded forreaction mixtures containing molar ratio of [VAC]0/[CDCl3]0/[AIBN]0/[Catalyst] in solution with 1 cc DMF or in bulk with0.2 cc DMF telomerization (Table 1) is shown in Fig. 1.

Signal assignment telomerization was reported in theliterature for VAc telomers synthesized by CHCl3, CCl4and CDCl3 and we compared it with the spectra recorded

Scheme 1 Mechanism of thestabilized radical controlledtelomerization of vinyl acetate

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for VAc telomers obtained by CDCl3 in the presence ofvarious catalysis. Assignments of the entire characteristicsignals with the chemical shifts of the corresponding pro-tons are given in Table 2.

Figure 1 shows that the progress of the telomerizationreaction increases the intensities of the signals related to theprotons of the monomer (VAc) incorporated into the telomerand polymer chains and the reaction progress be followed

Table 1 The bulk and solutionTelomerization of VAc withCDCl3 with AIBN initiator withCo(acac)2 and ZnCl2 as catalysisat 60 °C[time: 5 h]

Experiment No. VAc/CDCl3/AIBN/Catalyst DMF(cc) ½VAc�0molL�1 ZnCl2 Co(acac)2

1 1/2/0.01/0.01 0.2 2.42 ■2 1/2/0.01/0.01 1.0 1.36 ■3 1/2/0.01/0.01 0.2 2.42 ■4 1/2/0.01/0.01 1.0 1.36 ■5 1/2/0.01/0.0 0.2 2.42

6 1/4/0.01/0.0 1.0 1.36

Fig. 1 1HNMR of bulkTelomerization of VAc at 60 °Cfor 5 h without catalyst a bulk,in the presence of Co(acac)2catalyst c bulk d solutiontelomerization and in thepresence of ZnCl2 as catalystebulk f solution telomerization[Bulk: 0.2 cc DMF, solution:1.0 cc DMF]

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successfully [12]. The molar conversion of VAc (xVAc),the cumulative number-average degree of polymerization[(DPn)cum], the fraction of CCl3- and CDCl2-terminatedchains relative to the initiator-fragment-terminated chains[telomer percent (Telomer%)], and the ratio of deuteriumto chlorine abstraction from CDCl3 (RD/Cl; equivalently,the ratio of chains with deuterium x ends to those withchlorine x ends) with Eqs. (1)–(3), respectively [9] werecalculated based on the complete assumed signals:

xVAc ¼I4:7�5 þ I4:5 þ I3:9�4:1 þ I5:2�5:4 þ I6:3�6:4

I4:7�5:0 þ I3:9�4:1 þ I5:2�5:4 þ I6:3�6:4ð1Þ

ðDPnÞcum ¼ I4:7�5 þ I3:9�4:1 þ I5:2�5:4 þ I6:3�6:4

I3:9�4:1 þ I6:3�6:4ð2Þ

Telomer% ¼I2:8�3:2

2

I3:9�4:1 þ I6:3�6:4ð3Þ

Where Ii–j indicates the intensity of the proton(s) signalappearing at chemical shifts of i up to j . It was observed inthe previous study [2–4, 9] that (DPn)cum values calculatedby Eq. (2) were in good agreement with those measured bygel permeation chromatography. Table 3 shows character-ization tacticity of telomers from bulk and solution in thepresence of ZnCl2 and Co(acac)2 catalyst.

The signal about 6.35 ppm is assigned to proton ofmethane adjacent to terminal chlorine at the ω-end [4],

and the chain transformations in the solution telomerizationof VAc by CDCl3 at the presence of Co(acac)2 may extractdeuterium or chlorine radicals from CDCl3 and terminatethe chain. Peaks of methylene protons in the neighbours ofCCl3 (a) and initiator (a/) appear at 2.8–3.2 and 1.1–1.2 ppm, respectively. The methine protons of the firstVAc unit (b) in both molecules have a peak about 5.3–5.4 ppm. The peak at about 4.7–5.0 ppm is characteristicof the methine proton of the VAc repeating units in thebackbone of polymer and telomer molecules which ishigher in solution than bulk telomerization, telomers withCCl3 α-end groups increases in solution telomerization[2–14].

Methylene hydrogens at the end of both chains (f) areassigned to the peak at 4.0–4.1 ppm. The methyl group ofacetate is at 2 ppm (g). Protons of the methylene for therepeating units (c), the end of chains (e) the methyl group ofAIBN initiator(h) have a broad signals at 1.5 to about2.0 ppm in 1H NMR spectra [5, 8–10]. In telomerizationreaction with different initial ratios of CDCl3 to monomer(Ro0[S]o/[M]o) and initiator (AIBN) to monomer, thenumber average degrees of polymerization (Dpn) decreaseand the percent of telomers increases with higher ratio ofCDCl3 to monomer (Figs. 3 and 4).

Sequence distribution of telomers

Chain sequence of telomers studied and characterized inTable 3 with meso(m) and rasemic(r) assignment of the

Table 2 Assignments andcorresponding chemical shifts ofthe protons of unreacted VAcmonomer and AIBN initiator,telomers, and polymers of theVAc telomerization by CDCl3 inpresence of AIBN initiator,ZnCl2 and Co(acac)2 ascatalysis in bulk and solutiontelomerization [4]

Protons VAc monomer AIBN inititor CCl3 terminatedtelomers

CCl2 terminatedtelomers

polymers

c 1.7 – – – –

d 2.0 – – – –

e – 1.6 – 2.8–3.1 –

f – – 2.8–3.1 4.9–5.1 –

g – – 5.3–5.4 1.6–2 –

h – – 1.6–2 4.8–5.1 1.6–2

i – – 4.8–5.1 3.9–4.1 4.7–5.0

j – – 3.9–4.1 1.8–1.9 3.9–4.2

k – – 1.8–1.9 6.3–6.4 1.8–1.9

Table 3 Characterization oftelomer of VAc in bulk andsolution telomerization at 60 °Cin 5 h with 1HNMR and GPC

No. Experiments Dpn Telomer% Mn(HNMR) Mn(GPC) PDI(GPC)

1 Bulk- Radical Telomerization 55.5 29.5 3135.14 3300 1.83

2 Solution- Radical Telomerization 51.5 25.9 2666.25 2940 1.73

3 Bulk-DMF/ZnCl2 65.35 38.1 3772.27 4015.9 1.95

4 Bulk-DMF/Co(acac)2 76.85 34.09 3775 4139.2 1.65

5 Solution-DMF/ZnCl2 76.85 17.5 4703.56 3678 1.85

6 Solution-DMF/Co(acac)2 43.9 46.5 4076.94 4139 1.53

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peaks for 1HNMR; chain sequence were identified andstatically calculated. The Bernouillan statistical model wasused to confirm the probability of random sequence intelomer. In this equation the mm, mr, and rr triad sequencespossess isotactic, atactic, and syndiotactic parts of the chain,respectively [9].

Pm ¼ 2mmþ mr

2ð4Þ

pr ¼ 1� pm ¼ 2rr þ mr

2ð5Þ

The relationship between pentad and triad are obtainedby Eqs. (3)–(5) enables us to calculate the probability ofmeso (Pm).

Sequence areas were calculated from integrations inFig. 1 through Eqs. (1)–(5), and the Pm value was calculatedfor all systems (Tables 4–6) [15–20]. The results indicate

that the racemic addition is almost higher than the meso onein the presence of Co(acac)2/DMF in solution than bulk(Prr00.4473 and 0.4205 in the presence of Co(acac)2/DMFin the solution than in the bulk), so telomer microstructurestend to be more syndiotactic in solution than in the bulk. Thevalue of Pr is always more than the value of Pm becausestereo hindrance of pendant groups tends to stay away fromeach other and form racemic sequences and syndiotacticmicrostructures[12–15].

Figure 2 showed that in bulk telomerization, the signal atthe range of 6.3–6.4 ppm shifted to the range of 7.3–7.4 ppm, which is negligible for bulk telomerization withoutcatalyst. In the presence of ZnCl2 and Co(acac)2, it isobserved that bulk polymerization at the presence of ZnCl2is sharper than that of Co(acac)2. The experimental data ofFig. 2 for Bernoullian model of chain propagation and theresults are shown in Tables 4–6. The results indicate that theracemic addition is almost higher than the meso one for Co(acac)2/DMF relative to ZnCl2/DMF. Stereoregularity of

Table 4 Assignment for the -CH2 - hydrogen spectrum of VAc units by H NMR and Bernoullian statistical model

Probability Chemical shift CH2: 1.8–1.9 ppm

Bulk DMF/Co(acac)2 Bulk DMF/ZnCl2 Solution DMF/Co(acac)2 Solution DMF/ZnCl2

Observed Bernoulliancalculated




rr 0.4265 0.4356 0.387 0.3969 0.35 0.3481 0.2411 0.2401

mm 0.1259 0.1156 0.1383 0.1369 0.169 0.1681 0.27 0.2601

rrr 0.295 0.2874 0.21 0.205 0.206 0.2053 0.1165 0.1176

mmm 0.0398 0.0393 0.0601 0.0506 0.07 0.0689 0.1331 0.1326

mmr+rrm 0.4651 0.4485 0.449 0.4662 0.487 0.4837 0.5139 0.4997

rmmr+mmr 0.2201 0.2025 0.23 0.2155 0.252 0.2568 0.3239 0.3172

mmmm 0.0101 0.0133 0.0172 0.0187 0.028 0.0282 0.066 0.0676

rrrr 0.198 0.1897 0.1673 0.1575 0.1279 0.1211 0.0571 0.0576

Table 5 Assignment for the OCOCH3 hydrogen spectrum of VAc units by H NMR and Bernoullian statistical model

Probability Chemical shift OCOCH3:2–2.1 ppm






rr 0.4573 0.4489 0.35 0.36 0.3225 0.3136 0.255 0.2601

mm 0.1192 0.1156 0.172 0.16 0.1932 0.1936 0.2495 0.2401

rrr 0.3011 0.3007 0.224 0.216 0.1824 0.1756 0.1289 0.1326

mmm 0.0364 0.0359 0.065 0.064 0.0829 0.0851 0.1108 0.1176

mrr+mmr 0.4571 0.4421 0.475 0.48 0.3951 0.4927 0.5131 0.4989

mmr+rmmr 0.1981 0.1947 0.2442 0.2487 0.188 0.2775 0.3151 0.3073

rrrr 0.2023 0.2015 0.1281 0.1296 0.1605 0.1545 0.063 0.0676

mmmm 0.0125 0.0118 0.026 0.0256 0.036 0.0374 0.125 0.12

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the chains tend to be syndiotactic microstructure withoutcatalyst [9].

Table 4 showed that for –CH2-, for bulk telomerization,structure tends to be –m-r-r-r-r-m-; Co(acac)2 produces syn-diotactic chains but ZnCl2 produces random chains. In so-lution telomerization; structure tend to be –[−r-m-m-]-[−r-r-r-]-[−m-m-r-]- where Co(acac)2 produces syndiotac-tic(−r-r-r-) chains but ZnCl2 produces random chains(−m-m-r-).

Table 5 showed that for OCOCH3, for bulk telomerization,structure tends to be -r-r-r-r-m-r-m- where Co(acac)2 producessyndiotactic chains but ZnCl2 produces random chains. Insolution telomerization; structure tends to be –r-r-r-r-m-m-where Co(acac)2 produces syndiotactic(−r-r-r-r-) chains butZnCl2 produces random chains (−m-m-) [9].

Table 6 showed that for CH, for bulk telomerization, struc-ture tends to be -r-r-r-r-m-m-m-m- where Co(acac)2 produces

Table 6 Assignment for the CH hydrogen spectrum of VAc units by H NMR and Bernoullian statistical model

Probability Chemical shift CH: 4.8–5 ppm






rr 0.51 0.49 0.4456 0.4356 0.275 0.2601 0.2379 0.2304

mm 0.1 0.09 0.1202 0.1156 0.2425 0.2401 0.2655 0.2704

rrr 0.353 0.343 0.291 0.2874 0.1289 0.1326 0.1026 0.1105

mmm 0.028 0.027 0.04 0.0393 0.1108 0.1176 0.1420 0.1406

mmr+rrm 0.433 0.42 0.4541 0.4485 0.5131 0.4989 0.4891 0.4991

mmr+rmmr 0.17 0.1701 0.2211 0.2028 0.3151 0.3073 0.3301 0.3218

mmmm 0.010 0.008 0.0150 0.0133 0.125 0.12 0.084 0.0731

rrrr 0.250 0.2401 0.184 0.1897 0.063 0.0676 0.0550 0.0530

Fig. 2 1HNMR of BulkTelomerization of VAc at 60 ° Cfor 5 h in the presence of aDMF/Znl2, b DMF/Co(acac)2as catalysis, and c withoutcatalyst

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syndiotactic chains but ZnCl2 produces random chains. Insolution telomerization structure tends to be -m–r-r-r-m-m-where Co(acac)2 produces syndiotactic(−r-r-r) chains butZnCl2 produces random chains (−m-m-).

Three cross peaks at 1.62/4.80 ppm, 1.77/4.80 ppm, and1.82/4.80 ppm can be ascribed to the vicinal coupling be-tween methin proton of VAc-VAc unit with the backbonemethylene proton of VmV (Hb), VrV (Hc) and VmV (Ha).

Furthermore, a single peak centered at 1.60/1.80 (I) wasobserved in 2D NOESY NMR spectrum (Fig. 3) due togeminal coupling between the methylene protons (Ha andHb). A cross single peak at 4.78/4.67 ppm corresponds tothe long range vicinal coupling between the methin protonsof VV units in 2D spectrum of NOESY NMR (Fig. 3).

Effect of solvent to monomer initial ratios (Ro)

The effect of the solvent to monomer initial ratios (Ro) onDpn and the percent of telomer formation are shown in Figs.4 and 5 according to results of Table 4. The average degreeof polymerization decreases by increasing the initial ratio ofthe solvent (chain transfer agent) to monomer (Ro) as shown

Fig. 3 1H NOESY spectrum ofPVAc telomers in CDCl3 in thepresence of DMF/Co(acac)

Fig. 4 Effect of initial ratiosof CDCl3 to monomer intelomerization of VAc at60 °C for 5 h

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in Fig. 4, which means that the chain transfer reactions arecarried out more than propagation reactions. Increasing RIoalso decreases Dpn which is in agreement with the freeradical mechanism [2, 3]. At constant RIo, Fig. 5 shows thatthe percent of telomer Dpn increased by increasing [S]0/[M]0.This means that telomerization is preceded further than poly-merization by increasing the initiator concentration at high Ro

ratio. The concentration of trichloromethyl radicals increasesmore than vinyl acetate radicals created by the initiator.

The excellent stabilization of the trichloromethyl radicalsformed by resonance which involves the chloro free pairs ofelectrons are of great importance [3]. The kinetics of vinylacetate telomerization, conversion with time, is shown inFig. 6.

Conclusion

Telomerization of vinyl acetate at the presence of chloro-form as chain transfer agent and AIBN as initiator werecarried out. Molecular structure of the synthesized VActelomer with CCl3 end group were characterized well andthe effect of chloroform and AIBN on molecular structurewere determined.1HNMR kinetic experiments were used toinvestigate the radical telomerization of VAc with CDCl3 asa telogen in the presence of AIBN initiator, Co(acac)2 and

ZnCl2 as catalysts. Effects of R0 ([CDCl3]0/[VAc]0) andcatalyst were studied. Analysis of the 1HNMR spectraallowed us to calculate XVAc, DPn, telomer percentage.Moreover, it was observed that there are different mecha-nisms for the transfer reaction to telogen (CDCl3), i.e.,abstraction of either of deuterium or chlorine radicals fromCDCl3. As a result, two telomers with different α-ends, i.e.,trichloromethyl- or dichloromethyl-terminated VAc telomerswere obtained. Both telomers are expected to be efficientmacroinitiators in the ATRP. The results showed that withthe same conversion, XVAc increases with the increase ofmonomer concentration in the presence of ZnCl2 (or equallywith decreasing of telogen concentration). It was observedthat, under the same condition, by increasing the CDCl3concentration, DPn decreases. Also, DPn at the same conver-sion increases with the increase of monomer concentration.The effect of various catalysis on the telomer percentage wasinvestigated and it was observed that at the same conversion, itincreases by increasing the R0 and in the presence of ZnCl2.High telomer percentage is preferred when the synthesizedtelomers are used as a macroinitiator in the ATRP which insolution and bulk telomerization at the presence of ZnCl2conversion reached to 91 and 85 % respectively. The valueof Pr is always more than the value of Pm; because stereohindrance of pendant groups tends to stay away from eachother and form racemic sequences and syndiotactic

Fig. 5 Effect of initial ratiosof CDCl3 to monomer on %telomer in telomerization ofVAc at 60 °C for 5 h

Fig. 6 Conversion vs. reactiontime in telomerization of VAc inCDCl3 at 60 °C for 5 h

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microstructures. Telomers from bulk telomerization in thepresence of ZnCl2 tends to random microstructures, but inthe presence of Co(acac)2 tends to syndiotactic microstruc-tures. The germinal couplings of methylene protons and vic-inal coupling between methin protons of VV units were alsocharacterized by 2D NOESY NMR.

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Study of chain sequence in the controlled radical telomerization of vinyl acetate with Co(acac)2...

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