Polychloroalkanes as ATRP Initiators ... - ACS Publications

Chapter 17

Polychloroalkanes as ATRP Initiators: Fundamentals and Application to the Synthesis of Block Copolymers from the Combination of Conventional Radical Polymerization and ATRP 1-3

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Downloaded by NORTH CAROLINA STATE UNIV on September 21, 2012 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0768.ch017

M. Destarac , B. Boutevin , and Krzysztof Matyjaszewski 1

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213 Laboratoire de Chimie Macromoléculaire, Ecole Nationale Supérieure de Chimie de Montpellier, 8 rue de l'Ecole Normale, 34296 Montpellier Cedex 5, France 2

Polychloroalkanes CCl R (n = 2, 3 or 4) are tested as initiators in Cu-catalyzed atom transfer radical polymerization (ATRP) of methyl methacrylate ( M M A ) and methyl acrylate (MA), using the CuCl / 2,2'-bipyridine catalyst. 1,1,1-Trichloroalkanes, RCCl , are good initiators. For all the R groups tested, M increases with conversion and polydispersities are low (1.1 < M /M < 1.3). C C l induces a multifunctional initiation, with lower than predicted M values. This deviation, previously observed with Ru and Ni-based catalysts, has been explained by the doubling of chains resulting from the activation of a central -CCl - moiety, followed by a βscission. Trichloromethyl-terminated vinyl acetate and vinylidene fluoride telomers are synthesized and used to initiate A T R P of various monomers, leading to P V O A c and PVDF-based block copolymers. n

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Polychloroalkanes CCl R4_ (n=2, 3 or 4) have been used for a long time in redoxcatalyzed radical addition and telomerization (7). Their reactivity increases with increasing number of chlorine atoms per molecule. Because of their moderate reactivity, very few examples of initiation by mono- and dichlorocompounds can be found in the literature. On the other hand, trichloroalkanes and especially carbon tetrachloride combined with various transition metals were proved to be efficient initiating systems for the synthesis of low molecular weight telomers with high yields, most of time 1:1 adducts (7). In the case of 1,1,1-trichloroalkanes, RCC1 , it is known n

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Current address: Rhodia, Centre de Recherches d'Aubervilliers, 52 rue de la Haie Coq, 93308 Aubervilliers Cedex, France.

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© 2000 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


235 that if the presence of a trichloromethyl group is necessary to ensure a reasonable reactivity, the nature of the neighboring substituent R has an important impact on the overall reactivity of the system (i.e yield of 1:1 adduct). When applied to Cu-catalyzed A T R P (2,3) of styrene, the presence of the 2,2'-bipyridine ligand (bpy) adjusts the activity of the catalyst in such a way that 1,1,1-trichloroalkanes are fast and nearly quantitative initiators, even those weakly reactive in radical addition (3a,c,d). Moreover, a multifunctional initiation has been evidenced for some initiators containing an electron-withdrawing substituent in the α-position to the trichloromethyl group, e.g. CI or C 0 C H . The first section of this chapter deals with the case of M M A and acrylates. Trichloromethyl-terminated polymers were obtained by two different ways: first through the use of a CCl -terminated azo initiator (3c), second with the telomerization of some monomers (vinyl acetate - V O A c - (4) and vinylidene fluoride - V D F - (5)) with chloroform, leading to well-defined C C l ( M ) H telomers. A CCl -terminated poly(nbutyl acrylate) was synthesized using a CCl -terminated azo initiator (3c), and successfully used as a macroinitiator for A T R P of styrene to synthesize block copolymers. In the second part of this chapter, we describe the use of P V O A c and PVDF-based block copolymers by A T R P initiated by V O A c and V D F telomers, respectively. 2

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Initiation with Polychloroalkanes. Fundamentals.

MMA. A series of C C l R . -type initiators (Table I) was chosen to initiate A T R P of M M A in 50 vol.-% 1,2-dimethoxybenzene at 90°C. Figure 1 represents the evolution of M and M / M with conversion. It shows that all the initiators, in conjunction with the CuCl / bpy complex, promote a controlled polymerization. In all cases, the molecular weight increases during polymerization, and polydispersities are low ( M / M < 1.3 at high conversion). Polymerizations are fast (60-80% conversion after two hours according to initiator). C C 1 C H 0 H (1), C C 1 C H (2) and CC1 CH CF C1 (3) initiate polymerization with an efficiency factor/ (defined as Μ / M pc) equal to 0.70, 0.73 and 0.87, respectively. In order to explain this partial initiation, the initiator consumption was followed by G C . 56% of 2 is consumed after 12% monomer conversion in half an hour (M =4100 g / mol). After one hour, the analysis of a second sample shows 36% monomer conversion and no residual peak corresponding to 2 (M =7340 g / mol). This means that slow initiation is not responsible for the observed incomplete generation of chains from the initiator. A significant amount of radicals is presumably deactivated irreversibly in the early stages of polymerization via radicalradical coupling. Initiations by CC1 (4) and C C 1 C 0 C H (5) show an evolution of M with values matching theoretical predictions (M =([M]o/[RCCl ] )*(monomer conversion)* ( M W ) C B ) during the first half of the polymerization; then, the control over M is gradually lost with final values that are lower than expected. These results are consistent with works of Sawamoto (for 4 (6) and 5 (6c)) and Teyssie (4) (7) for n

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η > ύ ι

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236 other transition metal catalysts. A s opposed to initiators 1, 2 and 3, a nearly quantitative initiation can be assumed considering the very good correlation between experimental and theoretical M below 4000 g / mol (Figure 1). In order to shed light on the peculiar effect of 4 and 5 on M , a deeper investigation of the initiation by CC1 has been conducted. n

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Table I. Initiator description Code 1 2 3 4 5 6 7 8

Structure CCI3CHPH CC1 C H CCI3CHPFP CC1 CC1 C0 CH CH CC1 CH HCC^CHpH CCUCHCiCHJClCCXCH, 3

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Figure 1. Evolution of number average molecular weight, M , and polydispersity, My/M with conversion in ATRP of MMA initiated by various RCCI3 initiators. RCCl /CuCl/bpy/MMA = 1 /0.5/1 /100 (50 vol.- % 1,2-dimethoxybenzene). T=90°C. Opened symbols relate to molecular weights and the filled ones to polydispersities. n

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237 Low molecular weight P M M A samples ( [ M ] / [Initiator] = 10) were synthesized with CC1 and a model compound, 2,4,4,4-tetrachloro~2-methyl butyrate CC1 CH C(CH3)C1C02CH3 (8) as initiators. These products were purified and analyzed by C N M R spectroscopy. 0

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Figure 2. CNMR analysis of an oligoMMA initiated with CCl (NMR solvent: acetone d- ). CCl /CuCUbpy /MMA =1/1/3/10, in bulk. Τ = 130°C. M =750, M /M =1.38. 4

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A C N M R spectrum of the oligoMMA initiated by CC1 is shown in Figure 2. In agreement with the spectrum of 8, the peak 1 at δ = 97 ppm has been attributed to a trichloromethylated chain end (CC1 -MMA-) and the series of peaks 3 between 67 and 69 ppm to a monochlorinated chain end (-MMA-C1). The broad signal 2 centered at 81.5 ppm represents a central dichlorinated carbon atom -MMA-ÇC1 -MMA- (a neighboring structure, - M M A - C C 1 - C H C C 1 - , has been characterized in a previous publication (8). The chemical shift corresponding to the central - C C 1 - moiety was reported to be located at 80.1 ppm). This result indicates that once oligomeric halides C C l ( M M A ) C l are generated from CC1 , the trichloromethyl terminal groups is rapidly activated by the metal catalyst. At this stage, polymerization proceeds with two propagating sites per chain. Results described in Figure 1 show that M increases linearly with conversion and roughly matches theoretical values up to about half of the polymerization. Beyond this point, another reaction contributes to generate new polymer chains. In order to investigate the possible initiation pathways once a difunctional initiation is established, two dichloroalkanes were tested as initiators: 2,2-dichloropropane CH CC1 CH (6) and 2,2-dichloroethanol H C C l C H O H (7). Indeed, these compounds do initiate polymerization, as shown in Table II. However, initiation is very slow, leading to much higher M than expected. Thus, once difunctional initiation is established and although slow compared to the activation of chain ends, an additional radical generation is likely to occur on the central - C C 1 - moiety, with a rate close to that of 6 or 7. If this type of central radical is able to initiate polymerization, a three-directional propagation is established, i.e a branching point is formed. The ratio of the intrinsic viscosity [η] of a branched molecule to that of a 4

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238 linear molecule with the same molecular weight for a uniform three-arm P M M A was reported as equal to 0.90 by Hatada et al. (9). Therefore, it seems unlikely that a change in hydrodynamic volumes between linear and branched P M M A could explain the M profile. A possibility is for the newly generated radical 9 (cf. Scheme 1), which has a structure analogous to those formed in copolymerization of methacrylate macromonomers (10), to generate a new P M M A growing chain and an a-chloro macromonomer 10 by β-scission (Scheme 1). The proposed mechanism is supported by the *H N M R analysis of an oligoMMA initiated by 8 (Figure 3). The series of peaks between 5 and 5.3 ppm has been assigned to the vinyl protons of 10 (CH2=CC1CH -), which is in excellent correlation with works of Rizzardo et al. on vinyl acetate analogs (signals at 5.15-5.3 ppm were attributed to vinyl protons CH2=CClCH -PVOAc) (11).


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Table II. A T R P of M M A initiated by 6 and 7. Initiator / C u C l / bpy / M M A = 1 / 0.5 / 1 / 30 (50 - vol. % 1,2-dimethoxybenzene). T=90°C. Initiator

t(hrs)

6 6 7 7

1 9 1 9 a)

Initiator Conv. (%) 8.2 18.1 5 14.4

MMA Conv. (%) 2.4 75.6 2.2 81.7

1 V I

MJM

M

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1.51 1.71 1.30 2.06

34320 100100 38600 120300

185 2380 180 2570

M =([M] /[Initiator] )* (monomer conversion)* (MW), nth

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Figure 3JH NMR spectrum of an oligoMMA initiated by CChCE C(CH )ClC0 CH 8. 8 / C u C l / b p y / M M A =1/1/3/10, in bulk. T= 130°C. M =780, MJM =1.44. NMR solvent: CDG . 2

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Scheme 1.


240 Acrylates. The use of RCC1 initiators in polymerization of M A has been investigated. Figure 4 shows the evolution of M and M / M with conversion for an initiation by 2 and 3. The very good correlation between experimental and theoretical M throughout polymerization shows that 2 and 3 are fast and nearly quantitative initiators for the polymerization of M A . Polydispersities are low at high conversions (M /M

Polychloroalkanes as ATRP Initiators ... - ACS Publications

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