Living Radical Polymerization - ACS Publications

Chapter 26

Cobalt-Mediated Radical Polymerization of Vinyl Acetate: A New Tool for Macromolecular Engineering 1

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Downloaded by PURDUE UNIVERSITY on March 9, 2013 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch026

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Antoine Debuigne , Christophe Detrembleur , Rayna Bryaskova , Jean-Raphaël Caille , and Robert Jérôme 1,*

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Center for Education and Research on Macromolecules (CERM), University of Liège, Sart-Tilman, B6,4000 Liège, Belgium Solvay Research and Technology, Rue de Ransbeek 310, B-1120 Brussels, Belgium

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Cobalt(II)acetylacetonate is an effective mediator for the radical polymerization of vinyl acetate in bulk, suspension and miniemulsion, even when high molar mass is targeted. The mechanism, which basically relies on the reversible combination of the growing poly(vinyl acetate) (PVAc) chains with the cobalt complex, has been analyzed by UV-visible and NMR spectroscopies. The influence of both the temperature and the concentration of the initiator and the cobalt complex has also been investigated. With the macromolecular engineering of PVAc and poly(vinyl alcohol) (PVOH) in mind, a series of end-functional chains and PVAc (PVOH) containing block copolymers have been successfully synthesized.

372

© 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.


373 In spite of remarkable progress in living/controlled radical polymerization for the last decades, it is quite a problem to have the radical polymerization of vinyl acetate under control, particularly when high molecular weight and high monomer conversion are targeted (1-10). The origin of the problem has to be found in the high reactivity of the growing poly(vinyl acetate) radicals, which are not stabilized by the acetate substituent and engage themselves in transfer reactions to the monomer and to the polymer, and in irreversible termination reactions, as well. Recently, attention has been paid to cobalt complexes as potential mediators for the radical polymerization of VAc. In the presence of cobalt complexes, polymeric radicals can follow two different routes. Either dehydrocobaltation takes place and "catalytic chain transfer polymerization" is the actual mechanism (CCT) (11-13), or a cobalt-carbon bond is reversibly formed, in which the monomer can be inserted and a "cobalt mediated radical polymerization" (CMRP) is effective (14). The issue of the competition between these two processes depends on the structure of the cobalt complex and, above all, on the monomer. For example, methacrylic monomers with an abstractable hydrogen are prone to catalytic chain transfer (CCT) (13), whereas acrylic monomers are preferentially polymerized in a controlled manner, as originally reported by Wayland et al in the presence of cobalt porphyrin (14-16). Similarly to acrylates, vinyl acetate is a non-active CCT monomer, more prone to combine with the cobalt complex than to transfer. This propensity was a strong incentive to investigate the possible extension of the CMRP of acrylates to VAc. Recently, we reported on the unmatched control of the bulk radical polymerization of vinyl acetate in the presence of Co(acac) (17). Since then, Matyjasewski et al observed that the radical copolymerization of VAc and nbutylacrylate was also controlled by this cobalt complex (18). This paper aims at discussing recent results on the mechanism of the CMRP of vinyl acetate and on the extension of the bulk polymerization to polymerization in aqueous dispersed media, such as suspension and miniemulsion. The potential of this system in the macromolecular engineering of PVAc and easily derivatized poly(vinylalcohol) (PVOH) is also emphasized. 2

Experimental Section

Materials and characterization. A l l monomers were dried over calcium hydride, degassed by severalfreeze-thawingcycles before being distilled under reduced pressure and stored under argon. Cobalt(II) acetylacetonate (Co(acac) , >98% Merck), 2,2'-azo-bis(4-methoxy-2,4-dimethyl valeronitrile) (V-70) (Wako) were used as received. Size exclusion chromatography (SEC) was carried out in THF (flow rate : lmL min" ) at 40 °C with a Waters 600 liquid chromatrograph equipped with a 410 refractive index detector and styragel HR 2

1


374 5

4

3

2

columns (four columns HP PL gel 5μιη 10 Â, 10 Â, 10 Â, 10 Â). Polystyrene standards were used for calibration. *H NMR spectra were recorded with a Bruker A M 400 Spectrometer (400 MHz) in CDC1 . UV-visible spectra were recorded with a Perkin-Elmer Ρ14 spectrophotometer equipped with a thermostat and a magnetic stirrer. An ultrasonic probe B. Braun labosonic 2000 (1=127 mm, 0 = 4mm, 200Hz) was used for the ultrasonication. 3

General procedure for the bulk polymerization of VAc. Bulk polymerization of vinyl acetate was initiated by V-70 at 30°C in the presence of Co(acac) under argon. The purple mixture was stirred and heated at 30°C. No polymerization occurred for few hours. During this induction period, the colour changed from purple to dark brown-green, followed by a substantial increase in the solution viscosity. The monomer conversion was determined gravimetrically after evaporation of the unreacted monomer.


2

Block copolymerization of VAc and styrene by C M R P . PVAc end-capped by a cobalt complex (M , EC =7000g/mol, M /M =1.18 , 0.29g, 0.41X10" mol ) was added into a glass tube, that was capped by a three-way stopcock and purged by three vacuum-argon cycles before addition of degassed styrene (1.5 ml, 131X10" mol). The mixture was heated at 30°C under stirring. After 24h, the reaction was stopped and the product was precipitated in heptane, filtered and dried in vacuo at 50°C. The unreacted PVAc macroinitiator was extracted from the PVAc-b-PS copolymer, with methanol in a Soxhlet extractor for 15h. 4

n

S

w

n

4

Synthesis of PVOH-b-PS copolymers by methanolysis of the ester groups of PVAc-b-PS. A solution of the PVAc-b-PS copolymer (200 mg) in THF (10ml) was added into a solution of potassium hydroxide (500 mg) in methanol (150ml, p.a). After 48h at room temperature under stirring, the PVOH-b-PS copolymer was collected by filtration and dried in vacuo at 50°C.

Results and Discussion Bulk Polymerization of VAc in the Presence of Co(acac)

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Whenever the bulk radical polymerization of vinyl acetate is initiated by V70 at 30°C in the presence of Co(acac) , a control is observed although after an induction period of several hours (12h, when [VAc]/[Co(acac) ][V70]=542/l/6.5) (17). Under these conditions, all the criteria of control are fulfilled, i.e., linear dependence of molar mass on conversion, narrow molar 2

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375 mass distribution (-1.2), linear time dependence of ln([M]o/[M]), and successful chain extension experiments.


Mechanism Because the cobalt(II) complex is a well-known trap for radicals, the radical polymerization of vinyl acetate is inhibited as long as the complex persists in the reaction medium. By analogy with the mechanism for the cobalt porphyrin mediated acrylate polymerization (14-16), a three-step mechanism based on the persistent radical effect has been proposed as shown in Scheme 1, i.e., (i) initiation of the vinyl acetate polymerization by the azoinitiator V-70, (ii) trapping of the carbon centered radicals by the persistent metallo-radical cobalt(II) complex, (iii) equilibrium between dormant and active species by cleavage of the Co-C bond, which is the basic mechanism of control.

VAc +

(0 R - f c H - C H J ^ C H ^ H -

OAc

R - N = N - R R = -C(CH )CN-CH -C(CH ) 3

2

3

2

OCH

OAc

3

(")

u

Co(acac)

2

VAc

R ^ C H

R

Ç H J ^ C H

OAc

2

D

— Ç H '

OAc

(iii) Kieact.

+ CoS

' 1 I ι

6.0

ι

1 1I 1I [ I M 1 I 1 ι , ι

5.5

5.0

4.5

4.0

ι

ι

ιι ι

3.5

I 1 I , 1 1 1 1 , 1

3.0

2.5

M 1

1 1 1 , 1 , 1 1 1

2.0

1.5

M M M 1 I ' I

1.0 ppm

J

Figure 2. HNMR (400MHz) spectrum ofpolyvinyl acetate) initiated in the bulk by V-70 in the presence of Co(acac) . (Μ = 18100g/mol, MJM^l.14). 2

η>NMR

Concentration of the Initiator and the Cobalt Complex The experimental dependence of the PVAc molar mass on the [VAc]o/[Co(acac) ]o ratio and the monomer conversion, is strong evidence that the cobalt complex is the control agent (17). As shown in Table I, the molar mass is predictable and increases with the [VAc]o/[Co(acac) ]o ratio, and the polydispersity is low, even at high [monomer]/[cobalt] ratios (Table I). 2

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378 Although the polymerization of VAc was originally carried out with a [V70]/[Co(acac) ] ratio of 6.5, the polymerization control is maintained when this ratio is decreased by a factor of two (Table II). Indeed, the polydispersity remains low, and the experimental molar mass of PVAc increases again regularly with the monomer conversion in close agreement with the theoretical dependence based on the [VAc]o/[Co(acac) ]o ratio (Table II). For the polymerization to occur, the number of radicals formed in the medium should exceed the number of cobalt atoms. Matyjaszewski et al. reported in a recent publication that this system is effective even when this molar ratio is 1/1. (18) 2


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Table I. Bulk radical polymerization of vinyl acetate at 30°C for different VAc/Co(acac) molar ratios and constant V70/Co(acac) molar ratio (6.5) 2

Entry

2

[VAc]/ Time [Co(acac) ] 2

1 2 3 4

542 813 1626 2168

Conv (%)

24 22 14 14

64 70 56 60

Ai>, exp

(g/mol) (g/mol) 25 400 51400 70 500 99 000

M v /

30 100 49 000 78 400 112 000

"

1.21 1.23 1.22 1.33

x

^M^theor = ([VAc]o/[Co(acac) ]o) 86.09 χ conv. Data listed arefromref. 17. 2

Table II. Bulk radical polymerization of vinyl acetate at 30°C for different VAc/Co(acac) molar ratios and constant V70/Co(acac) molar ratio (3.25) 2

Entry

2

[VAc]/ Time Conv (%) [Co(acac) ] 2

a

M

1

136

2

271

3

542

nilheor

51 71 143 40 49 72 20 25 44

6 34 64 27 46 62 11 36 70

M theor. n>

(g/mol) (g/mol) 1200 5 800 11 100 8 500 13 000 16 700 7 300 19 300 30 000

700 4 000 7 500 6 300 10 700 14 500 5 100 16 800 32 700

MJM„ 1.20 1.17 1.20 1.14 1.15 1.24 1.23 1.20 1.40

. = ([VAc]o/[Co(acac) ] ) χ 86.09 χ conv. 2 0

According to the mechanism, the inhibition period in the VAc polymerization is the time required for the radicals released by V-70 to react


379 with the cobalt(II) complex and to form dormant species. Therefore, it is not surprising that the induction period is longer (19h instead of 12 h) when the [V70]o/[Co(acac) ]o ratio is decreased and less radicals are made available at constant Co(acac) concentration. However, the slope of the ln([M]o/[M]) versus time plot (Figure 3) and thus the polymerization rate appear to be independent of the V-70/Co(acac) molar ratio. 2

2

2


1.2

0

5

10 15 20 25 30 35 time (h)

Figure 3. Plot of ln([M](/[M]) versus time for the bulk radical polymerization of VAc, with different [V-70]/[Co(acac) ] molar ratios, at different temperatures. [VAc]/[Co(acac) ]= 542. (L) [V-70]/[Co(acac) ]= 3.25, at 30°C (·) [V-70]/[Co(acac) ] = 6.5, at 30°C (datafrom ref. 17) (m) [V70]/[Co(acac) ] = 3.25, at 50°C. 2

2

2

2

2

Influence of Temperature Finally, when the bulk radical polymerization of VAc is conducted in the presence of Co(acac) at 50°C instead of 30°C, the molar mass increases with the monomer conversion although a significant deviation from the ideal behavior is observed beyond 50% conversion and the polydispersity is much higher (Table III). This deleterious effect results from the shift of the equilibrium towards the active species, which makes irreversible termination more probable, but is also due to the transfer reactions to the monomer and the polymer whose contribution increases with the temperature. Moreover, the increase in temperature has a favourable kinetic effect, as assessed by a much shorter 2


380 inhibition period (2h instead of 19h) and a higher polymerization rate (Figure 3, comparison of • and A). So, temperature has a direct effect on the balance between polymerization rate and polymerization control.

Table III. Bulk radical polymerization of vinyl acetate initiated by V-70 in the presence of Co(acac) at 50°C. 2


Time (h) 4

Conv (%) 22

7 9

48 61

Μ p (g/mol)

(g/mol)

12 200 22 900 23 500

10 300 22 300 28 400

η>

eX

[VAc]/[Co(acac) ] = 542, [V70]/[Co(acac) ] = 3.25. ([VAc]o/[Co(acac)]o) * 86.09 χ conv. 2

2

MJM„ 1.23 1.37 1.63

a

M ^ .

=

2

Macromolecular Engineering Based on C M R P Controlled radical polymerization extensively contributes to impart welldefined molecular characteristics (length, chain-end, composition and architecture) to a large range of polymers. However, only few studies delt with the design of PVAc (19-25), which is an incentive to contemplate the potential of CMRP to prepare well-defined PVAc containing block copolymers and endfunctional PVAc chains, as well (Scheme 2). It was previously shown that the cobalt complex at the end of the PVAc chains is replaced by either a hydrogen atom or TEMPO upon addition of either propanediol or TEMPO to the VAc polymerization medium (PI and P2; Scheme 2) (26). Furthermore, the addition of these radical scavengers to the polymerization medium is a very efficient strategy for making PVAc free of metal contaminants. Having in mind the end-functionalization of PVAc, nonpolymerizable olefins, such as 3-butene-l-ol and l,2-epoxy-5-hexene, were also added to the polymerization medium, so end-capping the PVAc chains with an hydroxyl and an epoxy group, respectively (P3 and P4; Scheme 2) (26). Finally, well-defined PVAc and PVOH containing block copolymers have been prepared by a three-step approach (Scheme 2): (i) synthesis of PVAc chains end-capped by the cobalt complex, (ii) displacement of this complex by an abromoketone (or an α-bromoester) containing nitroxide (P5), (iii) polymerization of the second monomer by ATRP (27). PVAc-è-PS copolymers (P6) of a low polydispersity have been accordingly synthesized and converted into PVOH-6PS amphiphilic copolymers (P7) by methanolysis of the polyvinyl acetate) block. Polyvinyl acetate)-è-poly(ethylacrylate) (PVAc-6-PEA) and polyvinyl acetate)-è-poly(methylmethacrylate) (PVAc-6-PMMA) have also been prepared according to the same strategy.


381

OAc

R-^-ÇHJ-H PI

/|

R - | - C H C H J ^ C H Ç H - Coâcac^ r

P2

OAc

r

ï

OAc TEMPO

HX=CHR S / R = - C H - C H O H PS 2

2


CJHJSH

2

-CHJ-ŒLJ-CH-Q^

R-^CHj-Œ

P4

Coâcac^

b

OAc Styrene

OAc

Sty, CuBr, HMTETA, 110°C.

Ph P8,P9

KOH MeOH

ni

R^Œ CHJ^CH CH^Co( )(acac) R

r

OH

Ph

P10

2

OAc

OAc

P6

KOH MeOH R^ŒrŒÔ^^-ÇHj-Br OH

P7

Scheme 2. Synthesis of end-functional PVAc and PVAc and PVOH containing block copolymers. PI (7.2K), Ip = /. 15; P2 (12.0K), Ip = /. 15; P3 (10.5K), Ip = 1.30; P4 (10.5K), Ip = 1.20; P5 (6.9K), Ip = 1.15; P6 (PVAc(6.9K)-bPS(16600)), Ip = 1.30; P7(PVOH(3.5K)-b-PS(16.6)); P8 (PVAc(7.0K)-bPS(20K)), Ip = 1.68; P9 (PVAc(12.0K)-b-PS(10K)), Ip = 1.56; P10 (PVOH(3.6K)-b-PS(13K)). Synthesis ofthePl-P7was detailed in references 26 and 27.


382


Finally, the styrene polymerization has been directly initiated by a PVAcCo macroinitiator with formation of PVAc-b-PS diblocks. The efficiency of this two-step strategy has been assessed by the disappearance of the elution peak of the PVAc macroinitiator (Figure 4). Nevertheless, the styrene conversion is low (

Living Radical Polymerization - ACS Publications

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