Chapter 39

Vinylidene Chloride Copolymerization with Methyl Acrylate by Degenerative Chain Transfer

P. Lacroix-Desmazes, R. Severac, and B. Boutevin

Laboratoire de Chimie Macromoléculaire, UMR-CNRS 5076, Ecole Nationale Supérieure de Chimie de Montpellier,

8 rue de l'Ecole Normale, 34296 Montpellier Cedex 5, France

Degenerative chain transfer copolymerization of vinylidene chloride (VC2) with methyl acrylate (MA) was investigated at 70°C in benzene. Different dithiocompounds ZC(S)SR were tested as chain transfer agents in the R A F T process (Reversible Addition-Fragmentation Chain Transfer) while 1­-phenylethyl iodide was tested as chain transfer agent in the ITP process (Iodine Transfer Polymerization). Dithioesters (Z= Ph) proved to be much more efficient to control VC2/MA copolymerization than both the xanthate (Z= OC2H5) and 1­-phenylethyl iodide. The higher apparent chain transfer constant was found for the dithioester with R= CH(CH 3 )C(O)OC 2 H 5 . Dithioesters had a pronounced effect on the kinetics, R= C(CH3)3 leading to the most important retardation effect. As illustrated by using the Predict® simulation package, the transfer to VC2 was thought to be responsible for the limitation of the attainable molecular weight in a living fashion. In spite of this side reaction, chain extension as well as a block copolymerization with styrene were successfully performed.

570 © 2003 American Chemical Society

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Introduction

The development of several living free radical polymerization processes (LFRP) in the two last decades opens the route to a wide range of well-defined polymers (predetermined molecular weight, narrow distribution and tailored architecture) ( i ) . In this field, the polymerization of styrenics, acrylics, methacrylics, and dienes in a living fashion has been extensively described in the literature. In contrast, monomers bearing halogen atoms on the reactive double bond have been only scarcely studied in living radical polymerization. In this work, we were interested in controlling the polymerization of vinylidene chloride (VC 2 ) .

Vinylidene chloride bears two halogen atoms at the alpha position. Few studies were reported in the literature on this class of halogenated monomers in living free radical polymerization. For instance, vinylidene fluoride was successfully copolymerized with hexafluoropropene by ITP (Iodine Transfer Polymerization), leading to commercial fluoroelastomers (2-4). Vinylidene chloride was used in ATRP (Atom Transfer Radical Polymerization) by Matyjaszewski et al.(5) as a comonomer in the polymerization of acrylonitrile. It was shown that the polymerization was limited to low conversion, but the amount of vinylidene chloride was only 5 mol%, so it is difficult to deduce some information on the A T R P of vinylidene chloride alone. In the nineties, some success was claimed by the Geon Company in the polymerization of vinyl chloride by ITP (d). More recently, vinyl chloride was also studied by metal catalyzed radical polymerization by Percée et α/.(7), best results being obtained with iodo compounds, making possible a combination with the ITP process. Finally, in a recent paper, we have reported the successful polymerization of butyl α/ρ/ια-fluoroacrylate by ATRP (8). From this Anal.ysis of the literature and from our own preliminary screening, we decided to focus on degenerative transfer processes, namely the R A F T process (Reversible Addition-Fragmentation Chain Transfer) (9) and the ITP process for the polymerization of vinylidene chloride (Scheme 1).

RAFT

ITP P„ + I—Pn - P„r - I + Pi?

( M ) 0 Scheme 1. Chain equilibration by R A F T and ITP processes (degenerative chain transfer)

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Because poly(vinylidene chloride) homopolymer has a low solubility in conventional solvents, we used methyl acrylate (MA) as a comonomer. The good solubility of the copolymer facilitates the characterizations by proton N M R and size exclusion chromatography. Reactivity ratios are close to one (r/=0.9, r2=0.95, with monomer 1=VC2) (70), indicating a statistical copolymer with almost no deviation in composition during the polymerization. It also means that the conversion is almost the same for both monomers. Therefore, in this work, we will refer to the monomer conversion without specifying the monomer. Furthermore, methyl acrylate is known to be compatible with both the R A F T (9) and the ITP (77) processes, so it should not have detrimental effect on the living copolymerization. In summary, this work aims at investigating the efficiency of R A F T and ITP processes for the copolymerization of vinylidene chloride with methyl acrylate. Special emphasis will be on the R A F T process because it is known to be a versatile and efficient process.

The efficiency of chain transfer agents (CTA) depends on their structure (9). Especially, for R A F T agents Z-C(S)S-R, the nature of the activating group Ζ and the leaving group R strongly influences the reactivity of the transfer agent. Accordingly, we have tested three dithioesters (Z= Ph) with different leaving groups: benzyl derivative 1, ter/-butyl derivative 2, and l-(ethoxycarbonyl)-ethyl derivative 3 (Figure 1). We have also used a xanthate (Z= O C 2 H 5 ) 4 with a similar leaving group so that it can be compared to dithioester 3, giving an indication on the effect of the activating group Ζ which is either a phenyl or an ethoxy group. Lastly, we have used 1-phenylethyl iodide 5 as a chain transfer agent in ITP.

Herein, the kinetics of polymerization, the evolution of molecular weight and polydispersity with conversion, as well as the ability to prepare block copolymers will be discussed. Predici® will be used for numerical simulation in order to illustrate the possibilities and limitations of the living process for this system.

Figure 7. Structure of the reversible chain transfer agents 2-5 used in this work

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Experimental

Materials

Vinylidene chloride ( V C 2 , Aldrich, 99%), methyl acrylate (MA, Aldrich, 99%), and styrene (STY, Aldrich, 99%) were purified by vacuum distillation over anhydrous CaH 2 . 2,2' - azobisisobutyronitrile (AIBN, Fluka, 98%) was recrystallized from 95% ethanol. S-(thiobenzoyl)thioglycolic acid (Aldrich, 99%), benzyl mercaptan (Aldrich, 99%), 2-methyl-2-propanethiol (Aldrich, 99%), ethyl 2-mercaptopropionate (Lancaster, 98%), octamethylcyclotetrasiloxane (D4, Aldrich, 98%), and benzene (SDS, 99.9%) were used as received. Xanthate a-(O-ethylxanthyl) methyl propionate 4 and 1-phenylethyl iodide 5 were synthesized in our laboratory according to the procedure of Charmot et al(12) and Matyjaszewski et aL(ll), respectively.

General procedure for the synthesis of dithioesters 1-3

S-(thiobenzoyl)thioglycolic acid (5.3lg, 25 mmol) was dissolved in 30 mL of NaOH IN, in a 100 mL, three-necked, round-bottom flask equipped with a magnetic stirrer, under argon. Thiol (25 mmol) was added dropwise to the reaction mixture at room temperature. The reaction mixture was stirred for 4-8 hours and was extracted with benzene. The organic layer was washed once with aqueous NaOH I N solution and then three times with water, and dried over anhydrous sodium sulfate. Evaporation of the solvent under vacuum afforded the desired dithioester as a red liquid. Ή N M R δ (CDC13) : 1 (yield 71%) : 7.9(2H, d), 7.4(8H, m), 4.7(2H, s); 2 (yield 75%) : 7.9(2H, d), 7.4(3H, m), 1.7(9H, s); 3 (yield 65%) : 8.0(2H, d), 7.4(3H, m), 4.7(1H, q, /=7.46), 4.3(2H, q, /=7.24), 1.7(3H, d, 7=7.46), 1.3(3H, t, 7=7.24).

Polymerizations

R A F T solution polymerizations were carried out in a 300 cm 3 inox autoclave (Parr instrument). The reaction mixture of vinylidene chloride (64.80 g, 6.68X10"1 mol), methyl acrylate (14.39 g, 1.67X10"1 mol), transfer agent (3.96xl0' 3 mol), and benzene (77.60 g, 9 .95xl0 l mol) was introduced in the autoclave under nitrogen atmosphere. To launch the polymerization, a solution of A I B N (0.1445 g, 8.81X10"4 mol) in benzene (19.36 g, 2.48X10'1 mol) was added. Then, the reaction mixture was heated up to 70°C and the mechanical stirring speed was maintained at 200 rpm. The overall conversion was

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determined on aliquots, either by gravimetry (on samples quenched with hydroquinone and dried under vacuum at 40°C) or by *H N M R using octamethylcyclotetrasiloxane (D4) as internal standard. A l l R A F T copolymerizations of V C 2 / M A reported in this work were performed at 70°C with [VC 2]= 3.74 mol .L 1 , [MA]= 9.35X10"1 mol.L"1, [AIBN]= S.OOxlO"3 mol.L" l , [benzene]= 6.95 mol.L"1. Block copolymerization with styrene monomer was performed in a glass schlenk reactor under argon.

ITP polymerizations were carried out in benzene at 70°C with A I B N as initiator, in 10 mL Carius tubes sealed under vacuum after purging with argon. After appropriate time, tubes were removed from the oven (shaking frame), frozen, and opened. Conversion was determined by gravimetry.

Analysis

Size Exclusion Chromatography (SEC) was performed on crude samples with a Waters Associates pump equipped with a Shodex RIse-61 refractometer detector and two 300 mm columns mixed-D PL-gel 5 μηι from Polymer Laboratories (30°C). Tetrahydrofuran was used as eluent at a flow rate of 1.0 mL.min" 1. Calibration was performed with polystyrene standards from Polymer Laboratories. *H N M R spectra were recorded on a Bruker 200MHz instrument, chemical shifts are given in ppm using tetramethylsilane as reference and coupling constants are in Hz.

Numerical simulation

Numerical simulations of the copolymerization of V C 2 / M A were performed with the Predici® software package (75), version 5.35.1, used in moments mode. We used the copolymerization module of the software with the following rate constants (r=70°C): dissociation rate constant of A I B N £^/*w=3.166xl0" 5 s"1

(14% efficiency /ΑΙΒΝ=^^ (rough estimation); propagation rate constants *P.VC2=1785 L.mol^.s' 1 (15) and kPtMA=m00 L.mol^.s"1 (76); cross-propagation rate constants (70) kPtyc2/MA= 1983 L-mol^.s"1 and * P,MWC2=29160 L.mol' .s" ! ; termination rate constants & / f Vc2=4.16xl0 8 Lmol^.s" 1 (75) (dismutation mode) and £, t A M =6.44xl0 8 L.mol^s" 1 (77, 7S)(combination mode); cross-termination rate constant was estimated by the mean value of the individual rate constants ktyc2/m:=z(Kvc2^ktMA)l2^53x\Q% L.mol^s ' 1 (dismutation mode); transfer rate constant to V C 2 was calculated by an Arrhenius extrapolation from the work of Stockmayer (r=50-60°C) jfc,r,ra=n.35 L.mol" 1 .^ 1 (75).

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Results and Discussion

Synthesis of dithioesters

The synthesis of dithioesters is usually tricky (19). The usual way involves the addition of phenyl magnesium bromide on carbon disulfide, and a nucleophilic substitution reaction on an alkyl halide. We decided to by-pass this tedious step by using a very straightforward transesterification method, in biphasic conditions, adapted from the work of Leon et al. (20). The selected thiol reacts almost instantaneously with the sodium salt of the commercially available dithioester. The course of the reaction is visible thanks to the red color of the dithioester: the water phase quickly changes from red to uncolored and the red product separates from water and is recovered in high yields for all dithioesters 1-3. This is a very easy and quantitative synthetic route for primary, secondary and tertiary dithioesters in comparison with conventional methods. Of course, it is especially attractive when the appropriate thiol is readily available.

Effect of the nature of the activating group Ζ on R A F T copolymerization

R A F T agents 3 and 4 were tested to study the effect of the nature of Ζ (Figure 2). As expected, without transfer agent (blank experiment), the molecular weight is almost constant and the polydispersity is close to 2. With dithioester 3 as transfer agent, we observed an increase of the molecular weight with conversion while the polydispersity decreased down to about 1.5. This accounts for a control of the copolymerization by this dithioester. In contrast, with xanthate 4 as chain transfer agent, the molecular weight is rather high from the beginning of the polymerization (although lower than for the blank experiment, indicating a limited ability for transfer) and increases only slightly with conversion, with a polydispersity index still higher than 1.8. So, dithioester 3 is much more efficient than xanthate 4 as a R A F T agent. This result agrees well with the general knowledge on R A F T (9, 21) : the ethoxy group is not a very good activator for the radical addition to the thiocarbonyl (lower apparent chain transfer constant for xanthates in comparison with dithioesters).

Effect of the nature of the leaving group R on R A F T copolymerization

Dithioesters 1-3 were tested to study the effect of the nature of the leaving group R (Figure 3) . With the benzyl derivative 1, the molecular weight increases with conversion and the polydispersity index remains close to 1.6. With the tert-butyl derivative 2, the overall behavior is essentially the same except that the

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polydispersity decreases slightly at the beginning of the polymerization. Dithioester 3 leads to a better control of the molecular weight and a smaller polydispersity index. The smaller molecular weight is obtained with dithioester 3, indicating that C T A 3 has the highest apparent transfer constant in this series. The higher slopes of M„ versus conversion for derivatives 1 and 2 may arise from the lower ability of the expelled radicals to reinitiate the polymerization. Indeed, the benzyl radical slowly adds to monomers such as V C 2 and M A (&add=430 M^.s* 1 at 23°C), and the ierf-butyl radical quickly adds to monomers such as M A ( W = l . l x l 0 6 M'l.sl at 27°C) but it suffers from possible side-product formation by disproportionation (isobutylene formation) (22).

Kinetics of RAFT copolymerization

The kinetics of R A F T copolymerization in the presence of C T A ' s 1-4 is shown in Figure 4 for a targeted molecular weight of 20 000 g.mol*1. A blank experiment without transfer agent is also given as a reference. Xanthate 4 has almost no effect on the kinetics, but we have also previously shown that it is a rather poor reversible transfer agent. Concerning dithioesters, the benzyl derivative 1 shows a retardation effect while the terr-butyl derivative 2 is even slower. Finally, among dithioesters 1-3, C T A 3 gives the fastest polymerization. Moreover, for higher targeted molecular weight (dithioester 2, targeted molecular weight 50 000 g.mol"1), the retardation effect is no longer visible. Thus, dithioesters cause an important retardation effect depending on their structure and concentration. This behavior has also been reported by others for R A F T polymerizations (9, 23-25). It is difficult to rationalize this retardation effect because the RAFT process involves many equilibria, especially when copolymerization is concerned.

ITP copolymerization

1-phenylethyl iodide 5 was tested in ITP copolymerization (Table I), A good correlation was found between experimental and theoretical molecular weight at high conversion, but the polydispersity index was higher than for R A F T copolymerization with dithioesters. This accounts for a lower apparent transfer constant for 5 in comparison with dithioesters 1-3, as encountered in the case of styrene polymerization (26).

Limitations of the living copolymerization

In this part, we will illustrate the limitations of the living process related to vinylidene chloride. A numerical simulation of the blank experiment, using the

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0% 10% 20% 30% Conversion

40% 50%

Figure 2. Evolution ofMn (black) and Ip (white symbols) versus conversion for the RAFT copolymerization of VCi/MA : without CTA (;o), in the presence of CTA 3 (mo) and CTA 4 (A,â)for theoretical Mn = 20 000 g.mol'1 ( ).

0% 10% 20% 30% Conversion

40% 50%

Figure 3. Evolution of Mn (black symbols) and Ip (white symbols) versus conversion for the RAFT copolymerization ofVC/MA : in the presence of CTA 1 (Α,Δ), CTA 2 (Φ, 0)t CTA 3 (mo) for theoretical Mn = 20 000 g.mol'1 (

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Ο 100 200 300 400 500 time (min)

Figure 4. Evolution ofln([M]</[M]) versus time for the RAFT copolymerization of VCJMA : without CTA (x), in the presence of CTA 1 (Δ), CTA 2 (0), CTA 3 (a), CTA 4 (o) for theoretical M„ = 20 000 g.moXl, and CTA 2 (Φ) for theoreticalM„= 50000g. mol'.

Table I. Copolymerization of V C 2 / M A at 70°C by ITP

Run RI [AIBN]/[RI] Time Conversion M„.lh h A S" 0.27 15h 88% 7 102 8 679 2.06 Β A" 0.35 15h 82% 26 494 17 300 2.01

β [Benzene]= 4.87 M , [VC2]= 5.31 M , [MA]= 1.59 M , [AIBN]= 2.25xl0'2 M , [5]= 8.36xl0' 2M; * [Benzene]= 5.73 M , [VC2]= 4.34 M , [MA]= 1.60 M , [AIBN]= 9.10xl0"3 M , [A]= 2.58Χ102 M .

rate constants from the literature, is given in Figure 5. When transfer to monomer is neglected (ktr,M-0), the obtained molecular weight is much higher than the experimental values. In contrast, when transfer to monomer is taken into account (4r,A/=7i.3J Lmotl.s\ the molecular weight approaches the experimental results (ktrtM seems overestimated). Although the numerical simulation should be considered with caution due to conflicting propagation rate constant values reported in the literature for V C 2 (27), Figure 5 suggests that there is an important effect of transfer to vinylidene chloride on the molecular weight. This should have a detrimental impact on the living copolymerization.

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100000

0% 10% 20% 30% Conversion

40% 50%

Figure 5. Evolution of Mn (black) and lp (white symbols) versus conversion for the copolymerization of VC/MA at 70°C in the absence of CTA : experimental data ( Φ, <>), numerical simulation with ktrtM-0 (Mn : — A — , Ip : - - Δ - -), and numerical simulation with £ / Γ > Λ/=77.55 Lmol'^s'1 (Mn : -IP:—o- -).

A simple R A F T copolymerization model was constructed: a minimum of six transfer reactions was needed to complete the copolymerization module of Predici®. For simplicity, in order to be consistent with the copolymerization scheme, the apparent transfer rate constants were calculated from an overall apparent transfer constant Ctr,crA* Each transfer rate constant is related to the corresponding homo- or cross-propagation rate constant: k,rjj=CTRTCTA x kpjj (Scheme 2). Of course, such a simple kinetic scheme is not able to describe the retardation effect, but it is expected to give useful indications on the general trends of R A F T copolymerization for our system.

The effect of transfer to monomer on R A F T copolymerization was investigated for a targeted molecular weight of 20 000 g.mol"1 with dithioester 3. The simulation was performed with an arbitrary value C^CTA-^O chosen to approach the experimental results at low conversion for an easier comparison (Figure 6, left). The difference between the two simulations (with or without transfer to monomer) is relatively small, indicating that there is a rather low effect of transfer to vinylidene chloride on the molecular weight up to about 50% monomer conversion for a targeted molecular weight of 20 000 g.mol"1. Figure 6 (right) depicts the situation for various targeted molecular weights, taking into

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• γ • -• γ· - ktrt22=Qrkp,22

κ

κ •Υ*-- P2

#m P2,m ktrt22~CtrkPt22

Pl!m Scheme 2. Transfer reactions introduced in the R A F T copolymerization model for numerical simulation (P i t n and Di,„ stand for propagating chains and dormant chains respectively, with monomer i as the last unit).

account transfer to monomer. For low targeted molecular weight (M„= 5000 g.mol"1), the control is good all along the polymerization. For intermediate targeted molecular weight (Α/Λ=20 000 g.mol"1), the increase of molecular weight is no longer linear above about 50% monomer conversion. Finally, for a targeted molecular weight of 50 000 g.mol"1, the molecular weight clearly reaches a plateau at high conversion. The final molecular weight is about four times lower than the targeted molecular weight and the polydispersity is high. In the same time, the percentage of dead chains increases up to high values. So, Figure 6 (right) shows that there is a strong limitation of the attainable molecular weight in the living radical copolymerization of V C 2 with M A . This was confirmed experimentally with dithioester 2: the experimental molecular weight at about 50% conversion was only 15 000 g.mol"1 (instead of the theoretical value of 25 000 g.mol"1 for a truly living polymerization in the absence of transfer to monomer) and the polydispersity remained high (7P=1.7-1.8).

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nd 5

0 00

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1. Per

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of d

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is i

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d in

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are

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582

Possibilities of the living copolymerization

In spite of the limitations discussed above, we were interested in the synthesis of block copolymers. In a first approach, we checked the possibility of chain extension. The copolymer prepared by ΓΓΡ was introduced as a macro transfer agent in copolymerization of V C 2 with M A (Table I). The SEC Anal.ysis clearly indicates an increase of the molecular weight, confirming the living nature of the ITP process for this system.

In a second approach, we first synthesized a short block by R A F T copolymerization of V C 2 / M A with dithioester 3 (targeted molecular weight 15 000 g.mol"1). The amount of dead chains could be estimated by numerical simulation of R A F T copolymerization. The polymerization was stopped at 55% monomer conversion, corresponding to 10% or 30% of dead chains without and with transfer to monomer, respectively. This first block 6 (M„=6800 g.mol"1, 7p=1.34) was then used as a macro transfer agent in R A F T polymerization of styrene at 70°C in bulk. According to the molecular weight of the first block, the targeted molecular weight of the second block was 23 000 g.mol' 1. Figure 7 shows that the polymerization follows pseudo-stationary conditions (left) whith a linear increase of the molecular weight and a small increase of the polydispersity index (right). Furthermore, as shown on Figure 8, the peak of the final diblock copolymer clearly shifts toward higher molecular weight and remains monomodal, whatever the detector (UV or RI). The tail in the low molecular weight region possibly accounts for the dead chains which were expected from the simulation. So, the successful reinitiation of the second block confirms the living nature of R A F T copolymerization of V C 2 / M A in the first step.

Conclusions

We have established that degenerative chain transfer processes such as R A F T and ITP were able to control copolymerization of V C 2 with M A . Among iodo compounds, xanthates, and dithioesters, the last ones proved to be the more efficient reversible chain transfer agents (higher apparent chain transfer constant) although they may cause a decrease of the polymerization rate (retardation effect). Dithioester 3 (PhC(S)SCH(CH 3)C(0)OC 2H 5) gave the best results whereas dithioester 2 (PhC(S)SC(CH 3) 3) lead to the most important retardation effect. Transfer to vinylidene chloride leads to a limitation of the attainable molecular weight, as illustrated by numerical simulations and corresponding experiments at high targeted molecular weight. In spite of this limitation, the living nature of the copolymerization by ITP and R A F T is confirmed by the possible chain extension and formation of block copolymers.

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200

400

tirre

(rri

n)

0.2

0.4

Co

nve

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n

Fig

ure

7.

RA

FT

bloc

k po

lym

eriz

atio

n

of

styr

ene

in

bulk

at

70°C

wit

h

6/A

IBN

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e (1

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in m

ol).

Lef

t : e

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M])

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e :

expe

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Rig

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us

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Rétention Time (min)

Figure 8. SEC chromatograms of the polymer precursor poly(VC2-co-MA) 6 and the diblock copolymer poly(VC2'CO'MA)-b-PS : refractive index detector (bold line), UV detector at 254nm (dashed line).

Acknowledgments. We thank Vincent Bodart and Jean-Raphaël Caille for valuable discussions and SOLVAY (Brussels) for financial support of this work. We thank B. Colomer and Y. Bastaraud for their contribution to this work.

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