Synthesis of poly(vinyl acetate)-b-poly(dimethylsiloxane)-b-poly(vinyl acetate) triblock copolymers by iodine transfer polymerization

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European Polymer Journal 44 (2008) 318–328

EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj

Synthesis of poly(vinyl acetate)-b-poly(dimethylsiloxane)-bpoly(vinyl acetate) triblock copolymers by iodine transfer polymerization Jeff Tonnar, Emmanuel Pouget, Patrick Lacroix-Desmazes *, Bernard Boutevin Institut Charles Gerhardt – UMR 5253 CNRS/UM2/ENSCM/UM1 – Inge´nierie et Architectures Macromole´culaires, Ecole Nationale Supe´rieure de Chimie de Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France Received 3 October 2007; received in revised form 17 November 2007; accepted 22 November 2007 Available online 4 December 2007

Abstract Iodine transfer polymerization of vinyl acetate in bulk, initiated by a,a0 -azobisisobutyronitrile at 80 °C, has been successfully performed in the presence of an a,x-diiodo-poly(dimethylsiloxane) macrotransfer agent. The formation of a triblock copolymer PVAc-b-PDMS-b-PVAc has been proved by 1H NMR and size exclusion chromatography analyses. The analysis of the chain-ends has been performed using 1H NMR. It was found that a large amount of inverse chain-ends is present at the end of the polymerization. Moreover, the formation of several other side products by degradation of the functional chain-ends has been evidenced. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Block copolymer; Controlled radical polymerization; Iodine transfer polymerization; Poly(dimethylsiloxane); Vinyl acetate

1. Introduction The preparation of poly(vinyl acetate)–poly(dimethylsiloxane) (PVAc–PDMS) copolymers is of great interest, especially for the preparation of poly(vinyl alcohol)–PDMS amphiphilic copolymers [1–5]. Applications of PVAc-based copolymers

*

Corresponding author. Tel.: +33 4 67 14 72 05; fax: +33 4 67 14 72 20. E-mail addresses: jeff[email protected] (J. Tonnar), [email protected] (E. Pouget), patrick.lacroix-desmazes@ enscm.fr (P. Lacroix-Desmazes), [email protected] (B. Boutevin).

include coatings and adhesives [6,7]. The incorporation of PDMS blocks in the structure would enhance its flexibility and its resistance to moisture for instance. In spite of these potential applications, the preparation of such copolymers has been scarcely studied. One technique used to synthesize block copolymers of vinyl acetate and dimethylsiloxane consists in the use of a PDMS macroazoinitiator [8] or macroperoxyinitiator for the radical polymerization of vinyl acetate [9]. Tezuka et al. [1,10] synthesized triblock copolymers by a coupling reaction between a poly(vinyl acetate) containing chlorosilyl end groups and a PDMS containing NaOSi end groups. The other studies concern graft copolymers

0014-3057/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.11.026

J. Tonnar et al. / European Polymer Journal 44 (2008) 318–328

of poly(vinyl acetate) and poly(dimethylsiloxane) prepared by radical copolymerization of a PDMS macromonomer (dimethylvinylsilyl end-group) with vinyl acetate [2,3,5]. Lastly, Destarac et al. [11] claimed the controlled radical polymerization of vinyl acetate by MADIX from PDMS macrotransfer agents bearing xanthate moieties (MADIX: macromolecular design through the interchange of xanthate) to synthesize triblock and graft copolymers. Controlled radical polymerization (CRP) [12,13] includes a group of radical polymerization techniques which provide simple and robust routes to the synthesis of well-defined polymers and to the fabrication of novel functional materials. The general principle of the methods reported so far relies on a reversible activation– deactivation process between dormant chains (or capped chains) and active chains (or propagating radicals). To date, the most efficient CRP methods are nitroxide-mediated polymerization (NMP) [14], metal-catalyzed radical polymerization [13,15,16], reversible addition–fragmentation chain transfer polymerization (RAFT/MADIX) [17] and iodine transfer polymerization (ITP) [18]. Concerning the controlled radical polymerization of vinyl acetate, few techniques have been used: RAFT/MADIX with xanthates or dithiocarbamates [11,17,19–25], transition metal-catalyzed polymerization with complexes of iron or cobalt [26–29], organostibine mediated polymerization [30] and iodine transfer polymerization (ITP) [18,31–37]. Our work concerns the polymerization of vinyl acetate by ITP using an a,x-diiodo-PDMS macrotransfer agent to obtain a PVAc-b-PDMS-b-PVAc triblock copolymer. ITP has been successfully applied to the polymerization of styrene [38–41], acrylates [38,41–43], butadiene [44,45], vinyl acetate [31–37,46], fluorinated monomers [47–52] (tetrafluoroethylene, vinylidene fluoride, hexafluoropropene) and chlorinated monomers (vinyl chloride [53–55], vinylidene chloride [56]). In this work we studied the ITP of vinyl acetate to make PVAc-b-PDMS-b-PVAc triblock copolymers. To our knowledge, it is the first time that ITP is performed in the presence of an a,xdiiodo PDMS macrotransfer agent in the radical polymerization of vinyl acetate. ITP is an attractive alternative to MADIX since it does not involve the use of malodorous reactants and it has already led to commercial products [18].

319

2. Experimental 2.1. Materials The synthesis and characterization of the a, x-diiodo poly(dimethylsiloxane) (Mn,SEC = 1340 g mol1, Mw/Mn = 1.6) is described elsewhere [57]. Vinyl acetate (VAc, Acros, 99%) was purified by vacuum distillation before use. a,a0 -Azobisisobutyronitrile (AIBN, Fluka, 98%) was purified by recrystallization in methanol. 2.2. General procedure for the polymerization Typically, AIBN (0.128 g, M = 164.2 g mol1, 0.78 mmol) and macrotransfer agent I-PDMS-I (1) (1 g, Mn,SEC = 1340 g mol1, 0.75 mmol) were dissolved in vinyl acetate (15 g, M = 86.09 g mol1, 174 mmol) and placed in a 25 mL glass reactor equipped with a condenser. After three freezethaw-pump cycles, the reaction was conducted at T = 80 °C during 2 h in the dark under argon atmosphere with magnetic stirring. Monomer conversion was determined by 1H NMR analysis on crude samples in CDCl3 (monomer CH3m0 CðOÞOCHn0 @ CHo0 Hp0 : Hm0 , 3H, 2.13 ppm; Hn0 , 1H, 7.26 ppm; Ho0 , 1H, 4.56 ppm; Hp0 , 1H, 4.87 ppm; Jtrans (n0 ,p0 ) = 13.98 Hz, Jcis (n0 ,o0 ) = 6.31 Hz, Jgem (o0 ,p0 )= 1.63 Hz. Polymer (–CH2 CHh0 ðOAcÞ–Þm : Hh0 , 1H, 4.7–5.2 ppm): monomer conversion¼ðI p0 þh0 I o0 Þ= I p0 þh0 100. Molecular weights were determined by size exclusion chromatography. 2.3. Characterizations Size exclusion chromatography (SEC) was performed on dried samples dissolved in toluene, with a SpectroSeries P100 pump equipped with a Shodex Rise-61 refractometer detector and two 300 mm columns thermostated at 30 °C (columns mixed-D PL-gel 5 lm from Polymer Laboratories: 2  102–4  105 g mol1 molecular weight range). Toluene was used as eluent at a flow rate of 0.8 mL min1. The chromatograms of the PDMS macrotransfer agent and of the PVAc-b-PDMS-bPVAc copolymers were recorded in negative mode due to the refractive index values of the polymers and eluent (nPDMS < ntoluene and nPVAc < ntoluene) (ntoluene = 1.496, nPDMS = 1.43 and nPVAc = 1.4665) [58,59]. Calibration was performed with polystyrene standards from Polymer Laboratories. 1H NMR

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analyses were performed in CDCl3 on a Bruker Avance 250 MHz spectrometer.

Despite the difficulty reported elsewhere to control the polymerization of vinyl acetate in solution [23], we were able to obtain high monomer conversion (75%) in bulk polymerization, although low monomer conversion was observed when the polymerization was performed at low initiator concentration (Table 1, run 1) or at low temperature (70 °C, Table 1, run 2). One explanation for these low conversions could be the side reaction of iodo chain-end decomposition (Scheme 2) [34]. One can assume that the conversion is low because in these experimental conditions the chain-end decomposition reaction is favored compared to polymerization. When working at high initiator concentration and high temperature, correct monomer conversion and an acceptable correlation between experimental and theoretical molecular weights with a rather low polydispersity index were obtained in less than 2 h (Table 1, runs 3 and 4). Small discrepancies between Mn,th and Mn,SEC are likely due to PS standards and/or the contribution of the initiators derived chains (see Supplementary data). The higher polydispersity index in run three can be assigned to a higher initiator concentration leading to a higher number of dead chains. In comparison with the study of Destarac et al. [11] by MADIX (Mn = 4200 g mol1, PDI = 1.35, monomer conversion = 62%), higher molecular weights and lower polydispersity indexes were reached.

3. Results and discussion 3.1. Synthesis of the macrotransfer agent The synthesis and characterization of the a, x-diiodo poly(dimethylsiloxane) (1) (Scheme 1) has been detailed in a previous publication [57]. The first step of the synthesis is the esterification of a hydroxypropyl telechelic PDMS (Mn,SEC = 970 g mol1) by 2-bromopropionic acid (90% yield). Then, bromine was substituted by iodine in refluxing acetone using sodium iodide (94% yield, Mn,SEC = 1340 g mol1, Mw/Mn = 1.6). This macrotransfer agent has been characterized by 1H NMR, SEC and elemental analysis [57]. 3.2. Polymerization of vinyl acetate Vinyl acetate was polymerized in bulk by ITP using (1) as macrotransfer agent. The theoretical molecular weight is given by Eq. (1) in which Mn(1) is the mean number average molecular weight of (1), mVAc is the mass of vinyl acetate, n(1) is the number of moles of (1) and avinyl acetate is the conversion of vinyl acetate. M n;theoretical ¼ M n;ð1Þ þ

mVAc  avinyl acetate nð1Þ

ð1Þ

O

CH3

CH3

O

CH 2 I

CH

C

O

CH2

CH 2 CH2

Si

O

Si

CH2

CH 2

O

C

CH

I

n

CH3

CH3

CH3

CH 3

(1) Scheme 1. a,x-Diiodo poly(dimethylsiloxane) (1).

Table 1 Polymerization of vinyl acetate by ITP in bulk Run

[AIBN]/[(1)]

T (°C)

Time

Conv. (%)a

Mn,th (g mol1)b

Mn,exp,NMR (g mol1)a

Mn,exp,

1 2 3 4

0.62 1.05 1.66 1.04

75 70 75 80

6h 21 h 1 h 40 1 h 40

0 38 73 74

–

– 9,020 15,900 15,900

– 7,100 12,120 13,000

a

9000 16,000 16,200

SEC

(g mol1)c

Mw/Mnc – 1.30 1.44 1.30

Determined by 1H NMR in CDCl3. Calculated by Mn,th = Mn,(1) + mVAc  (monomer conversion)/n(1) where Mn,(1) and n(1) are the molecular weight and moles of the macrotransfer agent a,x-diiodo poly(dimethylsiloxane) (1). c Determined by size exclusion chromatography with polystyrene calibration. b

J. Tonnar et al. / European Polymer Journal 44 (2008) 318–328

H CH2 C I O CH3 O

321

a) +

CH3COOH

CH2 CHO + HI +

H2O

b)

+ CH2 CH I O CH3 O

+

CH2 CHO

I

CH3 O

Scheme 2. Decomposition of iodo chain-ends in iodine transfer polymerization of vinyl acetate.

3.3. Evolution of the molecular weights with monomer conversion

18000 14000 12000 10000 8000 6000 4000 2000 0 0

20

40

60

80

Conversion (%)

Relative Intensity

To assess the controlled character of the polymerization, a kinetic experiment was conducted in bulk with [AIBN]/[(1)] = 1.04 at 80 °C (Fig. 1). At 0% conversion, the molecular weight is the molecular weight of the macrotransfer agent (Mn = 1340 g mol1, PDI = 1.6). The molecular weight increases with conversion and is very close to the theoretical evolution (Fig. 1a). At high conversion, a slight deviation between experimental and theoretical molecular weights is observed. Such a behavior has also been observed by Favier et al. [23] (MADIX polymerization) and it might be attributed to the irreversible transfer reactions to monomer and polymer involving the very reactive VAc radical [60]. Regarding the rather high [AIBN]/ [(1)] ratio used, the downward deviation is more likely attributed to the extent of the initiators derived chains (see Supplementary data). At high conversion, the slight tailing in the low molecular weight region is certainly due to a fraction of dead chains (Fig. 1b). However, the whole molecular weight distribution is shifted towards higher molecular weights as the conversion increases.

Mn (g.mol-1)

16000

45 40 35 30 25 20 15 10 5 0 1.0E+02

PDI =1.20 PDI =1.26

1.0E+03

PDI =1.30

1.0E+04

1.0E+05 -1

Molecular weight (g.mol )

3.4. Structural characterization by 1H NMR analyses 1

Fig. 2 presents the H NMR spectrum of the crude product after the polymerization (Table 1, run 4). It has to be noticed that the protons He (q, 4.45 ppm) in the macrotransfer agent (1) have totally disappeared in the spectrum of the triblock copolymer PVAc-b-PDMS-b-PVAc. This was confirmed by 1H NMR analysis after complete removal of residual vinyl acetate under vacuum to avoid the

Fig. 1. Bulk polymerization of vinyl acetate by ITP at T = 80 °C: (a) Evolution of experimental molecular weight Mn (r) (Mn at 0% conversion is the molecular weight of the macrotransfer agent a,x-diiodo poly(dimethylsiloxane) (1), i.e., 1340 g mol1) and theoretical molecular weight (straight line, Mn,th = Mn,(1) + mVAc  (monomer conversion)/n(1) where Mn,(1) and n(1) are the molecular weight and moles of macrotransfer agent) (dashed line, theoretical molecular weight taking into account the initiators derived chains) versus monomer conversion. (b) Evolution of the molecular weight distribution: (d) conversion = 5%, (N) conversion = 16%, (r) conversion = 74%.

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J. Tonnar et al. / European Polymer Journal 44 (2008) 318–328

slight overlapping with the vinylic protons Ho0 of the monomer (spectrum shown in Supplementary data). Because of the thermal stability of the macro-

n'

transfer agent (1) [57], it is thus expected that it has reacted quantitatively with vinyl acetate. This result, in combination with the evolution of the molecular

p'+h' o'

a'

e'+j' f' d' r'

59.2

0.4

226.8 58.1

b'

q'

k' 4.7 2.6

c'+i'+l'+g'+m'

2.5

969.2

d

e

7.0

6.5

6.0

5.5

5.0

4.5

6.0 4.2

4.0

3.5

f' k' j' I

h' g'

CH3

C

O

CH 3

l'

C

2.5

2.0

1.5

1.0

a

4.0

68.0

0.5

0.0

a' d'

O

c' b'

CH2

CH CH2 CH CH2 CH me' O O

3.0

71.3

b

c

f

2.0 4.1

4.0

C

O

CH2 CH2

CH3

CH3

O

CH3

C

CH CH2 CH

CH2 Si

O

n

CH3

Si

CH 2 CH2

O

CH3

C

i'

H

n'

o'

C H p'

I

O

O

CH3

p

O

CH3

H C O C

O

CH 3

m'

f CH3

a d

O

CH2 I

CH

e

C

O

CH3

c

b

CH 2

CH 2

CH3

O

CH3

C

CH

CH2 Si CH3

O

n

Si

CH 2

CH 2

O

I

CH3

Fig. 2. 1H NMR spectrum (A) and formula (B) of the macrotransfer agent (1) and the final PVAc-b-PDMS-b-PVAc triblock copolymer in CDCl3: (run 4, Table 1) [AIBN]/[(1)] = 1.04, T = 80 °C.

J. Tonnar et al. / European Polymer Journal 44 (2008) 318–328

weights described above (Fig. 1), confirms the formation of the desired triblock copolymer. Moreover, the comparative study made by Guliashvili et al. [61] on the bond dissociation energies of the C–I chain-ends permits to predict that the cleavage of the chain-ends of the macrotransfer agent is easier (bond-dissociation energy = 44.6 kcal mol1 for the corresponding MA-I model compound, CH3CH(C(O)OCH3)–I) than the cleavage of the C–I chain-ends of the poly(vinyl acetate) blocks formed during the polymerization (bond-dissociation energy = 51.4 kcal mol1 for the corresponding VAc-I model compound, CH3CH(OAc)–I). Consequently, transfer reactions occur preferably with the chain-ends from the macrotransfer agent, leading to a complete consumption of the transfer agent in the early stages of the polymerization (around 5% of monomer conversion, indicating a high transfer constant of the macrotransfer agent (1) CT,(1) = ktr,(1)/kp > 20) [18] and the formation of the desired triblock copolymer. 1 H NMR analysis permits to calculate the monomer conversion by integrating the signal at 4.85 ppm (protons Hp0 and Hh0 ) (I p0 þh0 Þ and comparing it to the integration of the signal of the hydrogen Ho0 of the monomer (I o0 ) (Eq. (2)) (for simplicity, the contribution of the last monomer unit is neglected). Conversion ¼

I p0 þh0  I o0  100 I p0 þh0

ð2Þ

Moreover, 1H NMR analysis allows to assess the number average molecular weight by integrating the reference signal of the two –CH2–Si methylene groups next to the Si atoms at 0.46 ppm (I b0 ) and comparing it to the CH group of the polymer backbone at 4.85 ppm (I p0 þh0  I o0 ) (Eq. (3)). M n;NMRðb0 Þ ¼ M n;ð1Þ þ

I p0 þh0  I 0o  M vinyl acetate I b0 =4

ð3Þ

in which Mvinyl acetate is the molecular weight of vinyl acetate (86.09 g mol1). The 1H NMR spectrum also permits the analysis of the chain-ends. The chemical shift of the normal iodinated chain-end –CH2 CHk0 ðOAcÞ–I is situated at 6.65 ppm [37] (Hk0 , (Fig. 3)) and should account for two protons (the factor two comes from the fact that the polymer is telechelic) if no degradation has occurred. Its integration (Ik0 ) compared to that of the reference signal (Ib0 ) allows the determination of the percentage of living chain-ends (%k0 ) (Eq. (4)). In our case, the percentage of living chain-ends

323 g'

H1 CH3 CH I

∗

O C

C

O

CH3

j'

h'

k'

CH2 CH CH2 CH n O O O

CH3 i'

H1

C

I

O

CH3 l'

k'

0.1

6.80

0.4

6.70

6.60

6.50

6.

Fig. 3. 1H NMR spectrum of the normal triblock copolymer chain-end (k0 ) and the proton resulting from the addition of HI on vinyl acetate (H1) in CDCl3.

is always around 20% at the end of the polymerization even in the case of medium monomer conversion. %k0 ¼ ½ðI k0 =2Þ=ðI b0 =4Þ  100

ð4Þ

The signal at 3.1 ppm in Fig. 2 can be assigned to the protons q0 situated near the iodine atom in the inverse addition of vinyl acetate at the chain-ends (Scheme 3) [37]. This kind of inverse chain-end was also observed in the case of iodine transfer polymerization of vinylidene fluoride [62–64] as well as in the case of conventional radical polymerization of vinyl acetate [65]. If all the chain-ends were inverse chain-ends, the integration (I q0 ) should account for 4 protons. Consequently, the percentage of inverse chain-ends (%q0 ) can be calculated using the following equation: %q0 ¼ ðI q0 =I b0 Þ  100

∗

ð5Þ

CH2 CH

CH CH 2 p q' O O

O

C

C

I

O

CH 3 CH 3 Scheme 3. Inverse head-to-head addition of vinyl acetate at the chain-ends.

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J. Tonnar et al. / European Polymer Journal 44 (2008) 318–328

Finally, the proton H4 could be assigned to the decomposition of the iodo chain-end after headto-head and tail-to-tail addition of the last three vinyl acetate monomer units (C, proton H4, 9.62 ppm, t, (Fig. 4)). In this case, this adduct accumulates in the medium because of its lower chain transfer constant due to the lower stability of the corresponding radical compared to that of the normal head-to-tail adduct. This lower reactivity is due to the presence of a –CH(OAc)–CH2– group in b position of the iodine (–CH(OAc)–CH2–CH2– CH(OAc)–I) compared to the presence of a –CH2– CH(OAc)– group in b position of the iodine in the normal head-to-tail adduct (–CH2–CH(OAc)– CH2–CH(OAc)–I) [18]. Because of its accumulation in the reaction medium, this iodo chain-end has a higher probability to be subject to degradation. This is why a high amount of aldehyde chain-ends resulting from the decomposition of this adduct is observed at the end of the polymerization. The possible formation of conjugated aldehydes [37,66] –CH@CH–CHO to explain the H3 and H4 signals cannot be completely ruled out. However, in our study, the corresponding protons situated on the

A more precise analysis of the chain-end signals permits the quantitative analysis of the decomposition products (Fig. 4). At 9.69, 9.64 and 9.62 ppm, aldehyde-terminated chains (protons H2, H3, H4 in (Fig. 4)) can be detected. From our analysis, it appears that three different aldehyde chain-ends can be created by decomposition of the iodo endgroups resulting in the three different signals observed by 1H NMR. The first specie (A, (Fig. 4)) is the one described by Iovu et al. [34] which corresponds to the decomposition of the normal iodo chain-ends (proton H2, 9.69 ppm, t, (Fig. 4)). The second one (B, (Fig. 4)) could be assigned to the decomposition of the monoadduct (proton H3, 9.64 ppm, t, (Fig. 4)). This monoadduct decomposes faster than the other adducts (di-, tri-,. . .) because of its lower stability towards decomposition (Scheme 2). In fact, the carbonyl group in c stabilizes the carbocation, resulting from the decomposition of the iodo chain-end, by the formation of a five-member ring (Scheme 4). The decomposition being favored, it explains why we observe a high amount of aldehyde resulting from the thermal decomposition of the monoadduct (Fig. 4).

H3 H

B ∗

∗

H H2 CH2 CH CH2 n O C

H3 H4

O

C

CH CH2

CH2

O

O CH3

O

O

CH3

A

H H4 *

O

O C C O

H2 H5 H H 3C

CH2 CH CH CH2 CH2 n O O

CH3 CH3

C

C O

H5

0.1

9.80 9.80

9.70 9.70

0.4

9.60 9.60

9.50 9.50

9.4 0 9.40

Fig. 4. 1H NMR of the final triblock copolymer decomposition products observed at the end of the polymerization with a zoom on the zone of the spectrum where the decomposition products can be observed (CDCl3).

J. Tonnar et al. / European Polymer Journal 44 (2008) 318–328 O CH2

O

C

325

I CH

CH2

CH

O

CH2

I

O CH3

CH

C

O

C

O CH O

CH3

CH2

O

C H 3C

Monoadduct CH3

Scheme 4. Stabilization of the carbocation resulting from the decomposition of the iodo chain-ends by the carbonyl group in the monoadduct.

H1 H2C

CH3

CH

C

I H

HI

O

CH O

O

C

CH3

H5

H3C O

O

CH3

Scheme 5. Addition of HI on vinyl acetate and decomposition into acetaldehyde.

olefin group –CH@CH–CHO of such conjugated aldehydes (expected at 6.1 ppm) were not observed. In the case where all chain-ends are decomposed to aldehydes, the integration of the signals of the aldehyde terminated chain-ends (IH2 + IH3 + IH4) should account for 2 protons. Consequently, the percentage of dead aldehyde terminated chain-ends (%H2+H3+H4) can be calculated using the following equation: %H 2þH 3þH 4 ¼ ½ðI H 2þH 3þH 4 =2Þ=ðI b0 =4Þ  100

ð6Þ

The formation of aldehyde chain-ends is accompanied by the formation of hydriodic acid (Scheme 2). The signal H1 (Fig. 3) is the signal of the proton situated near the iodine atom in the addition product of hydriodic acid on vinyl acetate CH3CH(OAc)–I (VAc–I) [18,67] (Scheme 5). Acetaldehyde (providing from the decomposition of the VAc–I adduct, Scheme 5) is also detected in a small quantity (H5, 9.71 ppm, q, (Fig. 4)). The decomposition mechanism is the same as the one described in Scheme 2. In Fig. 2, the signal of the protons r0 at 3.7 ppm may be assigned to the protons –CH2 –CHr0 ðOHÞ– created by the acid-catalyzed hydrolysis of a small amount of vinyl acetate units in the triblock copolymer (extent of hydrolysis: I r0 =½I r0 þ ðI p0 þh0  I o0 Þ ¼ 1:5%) (Scheme 6). The presence of hydriodic acid (resulting from the hydrolysis reaction of iodo chain-ends, Scheme 2)

in the reaction medium may be responsible for this acidic hydrolysis which has also been reported with hydrochloric acid [68]. So, 1H NMR analysis of the chain-end degradation permits to determine the percentage of each type of chain-ends during the polymerization (Fig. 5). The percentage of living chain-ends –CH2CH(OAc)–I (%k0 , Eq. (4)) decreases very rapidly with monomer conversion (from 80% at 5% conversion down to 20% at 74% conversion). The percentage of aldehyde terminated dead chain-ends –CH2C(O)H (%H2+H3+H4, Eq. (6)) increases very quickly at the beginning of the polymerization but remains quasi-stable (maximum of 25%) during the polymerization. The percentage of inverse chain-ends –CH(OAc)CH2–I (%q0 , Eq. (5)) increases linearly with monomer conversion with a final percentage of 62% at 74% conversion.

∗

CH2 CH O O

p

∗

CH2 CH n

OH

C CH 3

Scheme 6. Product of the acid-catalyzed hydrolysis of some vinyl acetate units in the triblock copolymer (with p  n).

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J. Tonnar et al. / European Polymer Journal 44 (2008) 318–328

The aldehyde terminated chain-ends are dead in the sense that they will no longer participate to the controlled radical polymerization process.

Moreover, the –CH(OAc)CH2–I chain-ends from the inverse addition have a much lower chain transfer constant than the normal –CH2CH(OAc)–I

100

Percentage of chain-ends

90 80 70 60 50 40 30 20 10 0 0

10

20

30

40

50

60

70

80

Conversion (%) Fig. 5. Polymerization of vinyl acetate by ITP at T = 80 °C; evolution of the percentage of each type of chain-ends with monomer conversion: (d) percentage of transfer agent chain-ends –CH2 OCðOÞCHðCH3 Þ–Ið%e ¼ ðI e =2Þ=ððI b þ I b0 Þ=4Þ  100Þ, (r) percentage of living normal chain-ends –CH2CH(OAc)–I (%k0 , Eq. (4)), (j) percentage of aldehyde terminated dead chain-ends –CH2C(O)H (%H2+H3+H4, Eq. (6)), (N) percentage of inverse chain-ends –CH(OAc)CH2–I (%%q0 , Eq. (5)).

O

O

O

O

propagation (normal addition) CH3

O

O CH3

Highly reactivable chain-end

O

O

O

CH3

O

CH 3

CH3

CH3

O O

CH3

O

O CH3

CH• O

I

H C

Iodine transfer

CH•

CH3

O O

O

Moderately reactivable chain-end

O O

CH3

Iodine transfer CH• O

propagation (reverse addition)

O

H C O

CH 3

O

O

O

O

O O

O CH3 O

I

O

CH3

CH3

CH3

CH3

Tail-to-tail addition

O

O

CH 3

CH3

O

CH2 • O O

O

Iodine transfer

O CH 3

I

O

C H2

CH3

O

O O

O

CH 3

Tail-to-head addition

Poorly reactivable chain-end

CH3 CH3

O

CH3

CH 3

O O

O O

CH3

O O

O

Iodine transfer

I

CH2• O

O O

CH3

C H2 O

O CH3

O O

CH3

Scheme 7. Different reactions that occur in iodine transfer polymerization of vinyl acetate.

O CH3

Poorly reactivable chain-end

J. Tonnar et al. / European Polymer Journal 44 (2008) 318–328

chain-ends. For this reason, the transfer reactions preferably occur with the normal chain-ends leading to the growth of these chains, whereas the inverse terminated chain-ends are poorly reactivated (Scheme 7). Because of the chain-transfer constant lower than one for this primary iodinated chainend –CH(OAc)CH2–I [18,46], the accumulation of the inverse chain-ends should result in a loss of control of the polymerization. However, in our case, this effect is minored because of the presence of two chain-ends in the triblock copolymer: a chain can continue to grow even if one chain-end is no longer active. Nevertheless, when reaching high monomer conversion the number of dead and inverse chain-ends becomes high (more than 50%), leading to a broadening of the molecular weight distribution.

4. Conclusions The synthesis of PVAc-b-PDMS-b-PVAc triblock copolymers has been successfully performed for the first time in bulk by iodine transfer polymerization. The polymerization was initiated by a, a0 -azobisisobutyronitrile at 80 °C. Because of the competitive degradation of the iodinated chainends, a high conversion was only obtained when the polymerization was conducted at high temperature (>75 °C) and high initiator concentration ([AIBN]/[(1)] > 1). A kinetic experiment showed the controlled character of the polymerization and 1 H NMR proved the complete consumption of the macrotransfer agent. The results are fully consistent with the formation of a triblock copolymer structure. 1 H NMR analysis allowed the determination of the chain-end degradation products resulting from the decomposition of iodo end-groups. It also showed the presence of a large amount of inverse chain-ends –CH(OAc)CH2–I. These inverse chainends accumulated in the reaction medium due to their lower reactivity, leading to a slight broadening of the molecular weight distribution of the copolymers at high monomer conversion.

Acknowledgment The authors are grateful to Rhodia for their gift of a,x-dihydroxypropyl poly(dimethylsiloxane) Rhodorsil 1647V60.

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