Article pubs.acs.org/Macromolecules
Straightforward Synthesis of Symmetrical Multiblock Copolymers by Simultaneous Block Extension and Radical Coupling Reactions Antoine Debuigne,†,* Christophe Detrembleur,† Christine Jérôme,† and Thomas Junkers‡,* †
Center for Education and Research on Macromolecules (CERM), Department of Chemistry, University of Liege, Bldg B6a, Sart-Tilman, 4000 Liege, Belgium ‡ Polymer Reaction Design Group, Institute for Materials Research (imo-imomec), Universiteit Hasselt, Agoralaan Building D, B-3590 Diepenbeek, Belgium S Supporting Information *
ABSTRACT: In situ combination of a polymerization step with a coupling reaction is demonstrated to accelerate the synthesis protocols for symmetrical multiblock copolymers. Predici simulations and experiments prove on the example of cobaltmediated radical polymerization and coupling (CMRP/C) reactions that such synthesis strategy can be very effective and easy to conduct. Treatment of a cobalt-terminated poly(acrylonitrile) precursor with a mixture of acrylate and isoprene led to the rapid polymerization of the acrylate before isopreneassisted radical coupling of the macroradical chains forming a welldefined poly(acrylonitrile)-b-poly(acrylate)-b-poly(acrylonitrile) triblock. The degree of polymerization of the central block, resulting from the balance between propagation and coupling, could be tuned by adjusting the relative concentration and varying the structure of the acrylate and diene. The same convergent strategy also permits the synthesis of ABCBA-type pentablock copolymer starting from a cobalt-functional diblock. Simultaneous radical polymerization and coupling is thus a powerful macromolecular engineering approach for the straightforward design of symmetrical multiblock copolymers.
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INTRODUCTION Design and synthesis of complex block copolymer structures is nowadays accessible via a range of controlled polymerizations1−10 as well as click ligation reactions.11−14 Even though highly complex materials can be synthesized, constant efforts are still required to improve the existing protocols and to develop new methodologies that allow for faster, simpler, upscalable or simply more efficient synthesis procedures. Lately, radical coupling techniques such as atom transfer radical coupling (ATRC),15−20 nitrone-mediated radical coupling (NMRC),21−23 nitroxide radical coupling (NRC),24−29 and cobalt-mediated radical coupling (CMRC),30−33 have attracted significant attention for the generation of telechelic materials with symmetric (and asymmetric) block structures.34 Via such techniques, ABA triblock copolymers are directly accessible from activation of diblock copolymers that were preformed either via atom transfer radical polymerization (ATRP)1−3 or via cobalt-mediated radical polymerization (CMRP).8−10 Coupling efficiencies in these reactions are usually high and pristine materials can be obtained when dedicated coupling agents are employed that prevent disproportionation reactions. In case of CMRC, usually isoprene (IP) is used for that purpose.30,31 It is thereby noteworthy to add that IP also successfully coupled macroradicals generated upon photoirradiation of well-defined chains formed by tellurium-mediated radical polymerization (TERP).35 © 2013 American Chemical Society
Significant acceleration of the synthesis protocols could be achieved if the coupling reaction was combined with an in situ polymerization step. In such manner, a second polymer block could be polymerized during coupling, allowing to form ABA triblock structures from homopolymer precursors (thus removing one polymerization step prior to coupling). However, in such approach the problem arises that polymerization is in competition with the coupling reaction; block lengths can thus not be freely chosen and several side reactions may occur. In here, we demonstrate by simulation and experiment on the example of CMRC reactions that such approach can, however, be very effective and practically easy to conduct. As shown in Scheme 1, activation of a cobalt-terminated poly(acrylonitrile), pAN−Co(acac)2, can be triggered in presence of IP and butyl acrylate (BA). As a reminder, the room temperature addition of BA to polymers capped by Co(acac)2 resulted in fast an uncontrolled polymerization of BA36 because of the low bond dissociation energy of pBA−Co(acac) 2 and the high propagation rate coefficient, kp, of BA. Upon simultaneous addition of BA and IP to the pAN−Co(acac)2 precursor, we expect a rapid polymerization of the acrylate before chain growth is stopped by addition of an IP unit, which then in turn Received: September 16, 2013 Revised: October 17, 2013 Published: November 11, 2013 8922
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Scheme 1. Simplified Reaction Scheme for the Targeted Synthesis Strategy for ABA Block Copolymers
10 μm) and one PSS GRAM analytical column (30 Å, 8 × 300 mm, particle size 10 μm). The absolute molar masses of the pAN precursors were determined by SEC equipped with a multiangle laser light scattering (MALLS) detector in DMF/LiBr (0.025 M). The Wyatt MALLs detector (120 mW solid-state laser, k1/4 658 nm, DawnHeleos S/N342-H) measures the excess Rayleigh ratio Rh (related to the scattered intensity) at different angles for each slice of the chromatogram. The specific refractive index increment for pAN (dn/dc = 0.076 mL·g−1) was measured by using a Wyatt Optilab refractive index detector (k 1/4 658 nm). Data were processed with the Astra V software (Wyatt Technology). BA Polymerization Initiated from pAN−Co(acac)2. A solution of the alkyl-cobalt(III) initiator [(CH3O)(CH3)2C−CH2−C(CH3)(CN)−(CH2CH(OAc))∼4−Co(acac)2] in CH2Cl2 was introduced under argon in a Schlenk tube (0.73 mL of a 0.164 M stock solution, 0.12 mmol) and evaporated to dryness under reduced pressure at room temperature. The residue was dissolved under argon in DMSO (2.5 mL) and to the resulting solution was added AN (2.5 mL, 2.0 g, 38 mmol) at room temperature. The Schlenk tube was then immersed in an ice bath and stirred for 4 h at 0 °C. An aliquot was then withdrawn to evaluate the AN conversion by 1H NMR spectroscopy (22%) and to measure the molecular parameters of the pAN by SEC (Mn SEC DMF MALLS = 3900 g·mol−1, Đ = 1.09, dn/dc = 0.076 mL·g−1). After removal of the unreacted AN under reduced pressure at RT, the reaction medium was diluted with 2.5 mL of degassed DMSO before addition of BA (7.5 mL, 6.7 g, 52.3 mmol) under stirring at room temperature ([pAN−Co] = 0.01M, acrylate = 4.18 M). After few minutes, the reaction became exothermic and the viscosity of the medium increased rapidly. BA conversion, measured by 1H NMR in DMSO-d6, reached 77%. An aliquot was analyzed by SEC in DMF using PS as a calibration (Mn SEC DMF PS = 51000 g·mol−1, Đ = 1.5). The reaction medium was diluted in DMF before purification of the final polymer by several precipitations in MeOH/H2O (20:80) mixture. The copolymer was collected by filtration, dried under vacuum at 60 °C and then analyzed by 1H NMR in DMSO-d6 in order to establish the composition, i.e. pAN73-b-pBA362. (see exp 1 in Table 1). Synthesis of pAN−poly(acrylate)−pAN triblock copolymers. A solution of the alkyl-cobalt(III) initiator [(CH3O)(CH3)2C−CH2− C(CH3)(CN)-(CH2CH(OAc))∼4−Co(acac)2] in CH2Cl2 was introduced under argon in a Schlenk tube (0.73 mL of a 0.164 M stock solution, 0.12 mmol) and evaporated to dryness under reduced pressure at room temperature. The residue was dissolved under argon in DMSO (2.5 mL) and to the resulting solution was added with AN (2.5 mL, 2.0 g, 38 mmol) at r.t.. The Schlenk tube was then immersed in an ice bath and stirred for 7 h at 0 °C. An aliquot was then withdrawn in order to evaluate the AN conversion by 1H NMR spectroscopy (20%) and to measure the molecular parameters of the pAN by SEC (Mn SEC DMF MALLS = 6200 g·mol−1, Đ = 1.04, dn/dc = 0.076 mL.g−1). After removal of the unreacted AN under reduced pressure at RT, the reaction medium was diluted with 2.5 mL of degassed DMSO before addition at r.t. of a mixture of BA (7.5 mL, 6.7 g, 52.3 mmol) and IP (0.03 mL, 0.020 g, 0.3 mmol). A BA/IP stock solution was prepared on scale in order to improve the precision on
triggers combination of two macroradicals, effectively forming an ABA-type pAN−pBA−pAN triblock copolymer. In order to put the above-described reaction sequence in practice, we opted for a model-based experimental design strategy. On the basis of literature kinetic data, the process is first simulated using the program package Predici to test for feasibility of the reactions and to identify optimum reaction conditions. As will be demonstrated, the simulations confirm that the specific mechanism is in action if reaction conditions are chosen carefully. Successful chain insertion/coupling reactions are then performed on a variety of pAN precursors with butyl acrylate and a more specific urethane-containing acrylate, phenyl carbamate ethyl acrylate (PhCEA).37,38 Lastly, also the formation of a pentablock copolymer from a poly(vinyl acetate)-b-pAN precursor is described, highlighting the versatility and high efficiency of this process.
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EXPERIMENTAL SECTION
Materials. Vinyl acetate (VAc, >99%, Aldrich), acrylonitrile (AN, >99%, Aldrich), butyl acrylate (BA, >99% Aldrich) and DMSO were dried over calcium hydride, degassed by several freeze−thawing cycles before distillation under reduced pressure and stored under argon. Bis(acetylacetonato)cobalt(II) (Co(acac)2) (>98%, Acros) was stored under argon and used as received. 1,3-cyclohexadiene (1,3-CHD, 97% Aldrich) was degassed by bubbling argon for few minutes before use. 2,2′-Azobis(4-methoxy-2,4-dimethyl valeronitrile) (V-70) (Wako) was stored at −20 °C and used as received. The phenyl carbamate ethyl acrylate (PhCEA) was synthesized according to a previously reported procedure.38 The alkyl−cobalt(III) adduct initiator, i.e. [(CH3O)(CH3)2C−CH2−C(CH3)(CN)−(CH2CH(OAc))∼4−Co(acac)2], was prepared as described previously and stored as a CH2Cl2 solution at −20 °C under argon.39 All polymerizations were performed by classical Schlenk line techniques under argon. Methods. The 1H NMR spectra were recorded at 298 K on a Bruker Avance (500 MHz) instrument using dry DMSO-d6 as solvent with TMS as an internal standard; chemical shifts are reported in δ values (ppm) relative to internal TMS. The molar masses (Mn) and molar mass distributions (Đ) of the pVAc sample was determined by size-exclusion chromatography (SEC) carried out in THF (flow rate: 0.7 mL min−1) at 45 °C using a Malvern Viscotek TDA 305 liquid chromatograph equipped with a refractive index detector and styragel HR columns (four columns, HP PL gel 5 μm, 105 Å, 104 Å, 103 Å, 102 Å). The apparatus was calibrated with polystyrene samples. According to several reports, these elution conditions and calibration provide Mn values close to the exact molar mass of pVAc. The pAN precursors and the pAN-containing copolymers were analyzed by size-exclusion chromatography (SEC) in dimethylformamide (DMF) containing LiBr (0.025 M) relative to poly(styrene) (PS) standards at 55 °C (flow rate: 1 mL/min) with a Waters 600 liquid chromatograph equipped with a 410 refractive index detector as well as two PSS GRAM analytical columns (1000 Å, 8 × 300 mm, particle size 8923
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Table 1. Cobalt-Mediated Radical Coupling of pAN−Co in the Presence of Dienes and Acrylatesa pAN
copolymer
exp
Mnb (g/mol)
Đ
1 2 3 4 5 6
3900 6200 7400 7900 7300 7200
1.09 1.04 1.07 1.09 1.20 1.08
additives
[pAN]/[diene]/ [acrylate] (mol L‑1)
BA BA/IP BA/IP BA/IP BA/1,3-CHD PhCEA/IP
0.01/0/4.18 0.01/0.024/4.18 0.01/0.024/1.40 0.01/0.075/4.18 0.01/0.024/4.18 0.01/0.024/1.40
acrylate conv (%)
Mne (g/mol)
Đ
structuref
77c 7d 7d 3d 14d 19c
51000 33200 27300 33480 29000 34000
1.50 1.15 1.09 1.09 1.20 1.34
pAN73−pBA362 pAN116−pBA60−pAN116 pAN140−pBA21−pAN140 pAN149−pBA22−pAN149 pAN138−pBA124−pAN138 pAN135−PPhCEA40−pAN135
a
The addition of the acrylate/diene mixture to a the pAN−Co(acac)2 was performed at room temperature in DMSO. bDetermined by SEC− MALLS. cDetermined by 1H NMR of the crude mixture in DMSO-d6. dCalculated considering the incorporation of acrylate in the copolymer. e Measured by SEC in DMF using PS as calibration. fCalculated based on the exact Mn value of the pAN precursor measured by MALLS and the relative intensities of the comonomers signals in the 1H NMR copolymer spectrum. intensity of the NMR signals of VAc, AN, and BA units in the copolymer and the molar mass of the pVAc block determined by SEC, we established the composition of the copolymer, i.e., pVAc102− pAN107−PBA75−pAN107−pVAc102. Simulations. All simulations were carried out on an Acer TM 8472 equipped with an 2.3 GHz i5 Intel processor with the program package PREDICI version 7.1.0.
the IP content. The reaction medium ([pAN−Co] = 0.01M, [BA] = 4.18M, [IP] = 0.024M) was then stirred for 1h at r.t.. An aliquot was then withdrawn in order to characterize the resulting pAN-b-pBA-bpAN by SEC (Mn SEC DMF PS = 33200 g·mol−1, Đ = 1.15). The reaction medium was diluted in DMF before purification of the final polymer by several precipitations in MeOH/H2O (20:80) mixture. The copolymer was collected by filtration, dried under vacuum at 60 °C and then analyzed by 1H NMR in DMSO-d6. Considering the BA content in the copolymer, the BA conversion was estimated at 7% and the composition of the copolymer was determined, i.e., pAN116− pBA60−pAN116 (see exp 2 in Table 1). The same experiment was reproduced with various [BA]/[IP] molar ratios (see exp 3 and 4 in Table 1). Moreover, the pAN− Co(acac)2 precursors were also treated under the same conditions with the [BA]/[1,3-CHD] or [PhCEA]/[IP] mixtures (see exp 5 and 6 in Table 1 for detailed conditions). PhCEA being a solid, it was dissolved in degassed DMSO containing the appropriate amount of IP prior addition to the pAN−Co(acac)2. The concentrations of each reactant are mentioned in Table 1. Synthesis of pVAc−pAN−pBA−pAN−pVAc pentablock copolymers. A solution of the alkyl−cobalt(III) initiator [(CH3O)(CH3)2C−CH2−C(CH3)(CN)−(CH2CH(OAc))∼4−Co(acac)2] in CH2Cl2 was introduced under argon in a Schlenk tube (0.87 mL of a 0.14 M stock solution, 0.12 mmol) and evaporated to dryness under reduced pressure at room temperature. The residue was dissolved in VAc (2.5 mL, 27 mmol) at room temperature and the reaction was carried out in bulk at 40 °C. After 6h, aliquots were withdrawn from the medium. The VAc conversion measured by gravimetry reached 28% and the molecular parameters of the pVAc were determined by SEC (Mn SEC THF PS = 8800 g·mol−1, Đ = 1.06; Mn SEC DMF PS = 12200 g·mol−1, Đ = 1.09). The residual VAc was removed under reduced pressure. Then, the pVAc−Co(acac)2 residue was then dissolved under argon in DMSO (2.5 mL) and added with AN (2.5 mL, 2.0 g, 38 mmol) at room temperature. The Schlenk tube was then immersed in an ice bath and stirred for 16 h at 0 °C. Aliquots were then withdrawn in order to evaluate the AN conversion by 1H NMR spectroscopy (31%) and to determine the molecular parameters of the pVAc−pAN diblock by SEC (Mn SEC DMF PS = 30300 g·mol−1, Đ = 1.08). After removal of the unreacted AN under reduced pressure at room temperature, the reaction medium was diluted with 2.5 mL of degassed DMSO before addition at room temperature of mixture of BA (7.5 mL, 6.7 g, 52.3 mmol) and 1,3-CHD (0.028 mL, 0.0240 g, 0.3 mmol). A BA/1,3CHD stock solution was prepared on larger scale in order to improve the precision on the 1,3-CHD content. The reaction medium ([pAN− Co] = 0.01M, [BA] = 4.18M, [1,3-CHD] = 0.024M) was then stirred for 3h at room temperature. An aliquot was then withdrawn in order to characterize the resulting pVAc-b-pAN-b-pBA-b-pAN-b-pVAc by SEC (Mn SEC DMF PS = 71000 g·mol−1, Đ = 1.14). The reaction medium was diluted in DMF before purification of the final polymer by several precipitations in MeOH/H2O (20:80) mixture. The copolymer was collected by filtration, dried under vacuum at 60 °C and then analyzed by 1H NMR in DMSO-d6 (see Figure 7). On the basis of the relative
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RESULTS AND DISCUSSION Modeling of Reaction Sequence. The direct insertion of a polymer block via block copolymerization with simultaneous radical coupling displays a rather complex kinetic scenario. While such approach can be realized using various reactive systems, kinetic constraints must be assessed and reaction conditions precisely tuned in order to allow for success reaction outcomes. Thus, the experimental study on ABA block copolymer formations was preceded by a kinetic modeling study employing the program package PREDICI on the CMRC system. For modeling of the reactions, the somewhat simplified reaction scheme as given in Scheme 2 was applied. An equilibrium between activated pAN macroradicals and Co(acac)2-terminated chains is assumed given the ability of this Scheme 2. Reaction Scheme Used in Predici Modeling
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metal complex to mediate the polymerization of AN.40,41 Exact numbers for the specific combination and dissociation rates are not available, thus these values have been subject to variation in the modeling. In a second step, isoprene addition to the activated chains is accounted for (reactions II−IV). Isoprene addition to pAN macroradical, associated with a specific addiction rate kad,IP is therefore taken into account, followed by isoprene homopropagation (reaction III, rate coefficient kp,IP) and the classical CMRC reaction, that is termination of two isoprene-terminated macroradicals. For the termination reaction (associated with a general rate coefficient kt), no differentiation is made between macroradical species containing one or more isoprene units. The acrylate polymerization is taken into account by an initial BA addition step onto pAN macroradicals (with kad,BA) followed by homopropagation of BA (kp,BA) and addition of isoprene to a growing BA chain. Crosspropagation of BA with isoprene-terminated radical chains was not taken into account (see discussion below). Then, formation of the desired product occurs in reaction VIII. This specific termination reaction is to a certain degree in competition with other termination reactions as given in reactions IX−XI. Reactions X and XI yield in principle the same product as reaction VIII (differing only in chain length), however, conventional termination of BA chain termini may occur via disproportionation thus its occurrence compromises the purity of the product (formation of AB diblock copolymers rather than ABA triblocks). BA termination reactions are thus assumed to exclusively proceed via disproportionation to stay on the side of caution and to visualize the impact of this side reaction. Termination between macroradicals with AN or BA chain termini with IP-terminated chains have not been included in the model to simplify the reaction scheme. IP addition (as will be shown below) is a very fast process and hence radicals with IP units at the chain end will always appear in much abundances than the more transient counterparts. Such simplification is justified and indeed in previous modeling studies it was shown that conventional termination does not play a significant role in such systems.42 Regardless, even if such reaction occurs, it still favors formation of the desired products and hence the simplification made causes an underestimation of successful product generation. All termination rate coefficients are assumed to be equal for the different radical combination reactions. kt was set to 5 × 107 L·mol−1·s−1 which resembles kt of butyl acrylate43 and should be within an order of magnitude of any of the described reactions in dilute solution.44 kc was set to 105, kd to unity. Choice of these equilibrium parameters is somewhat arbitrary, but lay within the expected range for a CMRP of AN. It should be noted that the termination rate parameter and the equilibrium constant are mutually dependent. Higher levels of termination are counteracted by a faster release of pAN radicals. Regardless, for the competition between IP and BA addition to the radicals, the overall level of radical concentration (and thus precise equilibrium constant and termination rate coefficient) is without consequence. Homopropagation rate coefficients (at 20 °C) for butyl acrylate,45 isoprene46 and acrylonitrile47 were taken from literature. Homopropagation rate coefficients of AN are required to calculate crosspropagation rate coefficients from copolymerization parameters. Copolymerization between AN and BA is generally associated with r1 = r2 = 1, kad,BA was thus set to be equal to kp,BA. For copolymerization of isoprene with acrylates, literature report r1 = 0.75 and r2 = 0.12.46 This results in an kad2,IP of roughly 1.15 × 105 L·mol−1·s−1 at 20 °C. At the
same time, addition of an acrylate unit to an isoprene terminus should be relatively slow, thus this reaction was not further considered. For the copolymerization system AN−IP, r1 = 0.03 and r2 = 0.45 was determined,26 resulting in an kad,IP of 1.1 × 105 L·mol−1·s−1. As a starting point for the simulation, a distribution of the pAN−Co(acac)2 species with an average degree of polymerization DPn of 55 with a dispersity of 1.1 was assumed at a concentration of 0.02 mol·L−1. First, only the influence of isoprene on the reaction was assessed, thus no presence of BA was assumed. With kp,IP = 2.8 L·mol−1·s−1 and a concentration of cIP 0.1 mol·L−1, simulations demonstrate that in the CMRC reaction exactly 2 isoprene units are built in the resulting coupling product, which is good agreement with previous investigations.31 However, since various homopropagation rates are reported in literature for IP, also the extreme case of kp,IP up to 300 L·mol−1·s−1 and concentrations up to 1 mol·L−1 were investigated.
Figure 1. Decrease in isoprene concentration and number of built in isoprene units into the final polymer product depending on kp,IP .
Figure 1 depicts the different outcomes from these simulations. While 2 IP units (thus one unit per pAN macroradical) are built in almost instantaneously due to the
Figure 2. Influence of BA concentration on the outcome of the coupling experiment in absence of isoprene. The horizontal line marks the starting average chain length of the pAN precursor. 8925
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300 L·mol−1·s−1, only 4 IP units are expected to be present in the final product. Thus, isoprene propagation does not need to be further considered, also for the reactions involving BA. In the following step, BA addition and propagation is tested for in absence of isoprene. In this manner, the competition between pAN radical termination and BA addition can be studied. Figure 2 shows the outcome of reactions at different BA concentrations. It must be noted again that acrylate termination is implemented in the model exclusively via disproportionation, thus the distributions only show diblocks rather than ABA triblock copolymers. As can be seen, under any BA concentration, growth of the chains can be observed (the starting DPn of the pAN block is marked by the horizontal line. However, with the shoulder emerging on the high molecular weight side of the distribution, it becomes apparent that at low acrylate concentrations pAN homotermination becomes a significant side reaction. A 1 molar concentration of the acrylate may be seen as the lower limit in order to make acrylate addition fast enough to avoid this side reaction. The shift of the associated chain length distribution corresponds to an average of 25 acrylate units being added to the pAN chain before BA termination had occurred. In the final experimentthat is in presence of acrylate and isoprenea minimum of about 1 mol·L−1 acrylate is thus required. In the next step, the full reaction scheme was then activated in the model and the isoprene content varied at a fixed acrylate concentration of 6 mol·L−1. The outcome of these simulations is visualized in Figure 3. Isoprene addition is in competition with acrylate addition to the pAN macroradicals. kad,IP is about 30 times higher than kp,BA, requiring thus a good balance between acrylate and isoprene concentration in order to allow for a successful experiment. In fact, as the figure shows, decreasing levels of the desired ABA triblock copolymer (pAN−pBA−IP2−pBA−pAN) is seen when the isoprene content is increased due to the formation of unwanted pAN−IP2−pAN homopolymer resulting from the direct isoprene-assisted coupling reaction of the pAN precursor. At the same time, however, a minimum of isoprene is also required to promote the coupling of BA-terminated chains. Thus, the amount of desired product formed passes a maximum at around cIP = 0.03 mol·L−1. Below that level, conventional termination becomes increasingly important, potentially disturbing the product integrity. It must thereby be stressed that the model assumes a worst case scenario. Since BA chains will also terminate significantly by combination, higher product purities than given in Figure 3 can be safely assumed, thus product purities of at least 90% should be expected which is a reasonable result when compared to conventional radical coupling procedures or ABA triblock synthesis procedures, respectively. The optimal isoprene concentration regime can be expected to be between 0.02 and 0.03 mol·L−1.Regarding block lengths, an addition of approximately 45 BA units is estimated before IP addition occurs (with cIP = 0.025 mol·L−1) for cBA = 6 mol·L−1 and 10 units for 1 mol·L−1 (resulting in 90 and 20 units in the final ABA polymer). As a final recommendation, one can conclude from the modeling study that the presence of small amounts of isoprene (or alternatively a spin trap, such as nitrones)48 is vital to the experiment, but its concentration need to be well adjusted to find the optimal kinetic window for formation of the desired ABA triblock copolymer structure. If too much isoprene chosen, then no acrylate block insertion may occur, if too little is employed, then inefficient coupling might occur due to
Figure 3. Product composition with respect to reactions IV and VII under variation of the isoprene content. (cacrylate = 6 mol·L−1.)
Figure 4. Overlay of size exclusion chromatograms the pAN−Co precursors (0.01 M) treated at room temperature with (A) BuA (4.18 M) or (B) a mixture of BuA (4.18 M) and isoprene (0.024 M).
very high addition rate compared to the pAN termination rate, no significant isoprene homopropagation occurs even when unrealistically high kp values are assumed as can be seen from the rather low change in isoprene concentration during the reaction even when cIP is set to 1 mol·L−1. Even with kp,IP of 8926
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Figure 5. 1H NMR analyses in DMSO-d6 of the pAN−pBA−pAN (upper spectrum) and pAN−pPhCEA−pAN (lower spectrum) prepared by simultaneous acrylate polymerization and isoprene-assisted radical coupling (see experiments 2 and 6 in Table 1, respectively).
Predici model is indeed an adequate representation of the true kinetic scenario, then symmetrical ABA triblock copolymers should form according to a multistep process including (i) homolytic rupture of the Co−C bond of the polymer precursor (eq I in Scheme 2), (ii) initiation and polymerization of BA (eqs V and VI), (iii) cross-addition of one isoprene unit (eq VII) and coupling of the newly formed allyl radical terminated chains (eq VIII). The feasibility of this type of “combo” process was assessed through a series of experiments reported Table 1 (exp 2−6). First, pAN−Co(acac)2 precursors with moderate molar masses (6000−8000 g·mol−1) and low molar mass distributions (Đ ∼ 1.1) were prepared following a wellestablished CMRP procedure. Typically, the controlled polymerization of AN is initiated in DMSO at 0 °C by an alkylcobalt(III) initiator containing an average number of four VAc units (R−(CH2CHOAc)∼4−Co(acac)2). These pAN samples were then treated with various amounts of acrylates and dienes. In our initial assay, we used the optimal concentration regime recommended by the theoretical calculations, i.e. 0.01M, 0.024 and 4.18 M for pAN−Co(acac)2, IP and BA, respectively (exp 2, Table 1). A much higher concentration of BA compared to IP is necessary to permit the BA propagation before coupling. In contrast to the same experiment performed without isoprene (exp 1, Table 1), no exothermic reaction followed the addition of the BA/IP mixture onto pAN−Co(acac)2. The reaction medium was stirred for 1 h and exhibited a rather low viscosity suggesting a low BA consumption, which was later evaluated to 7% by NMR. Nevertheless, the pAN SEC peak almost fully
disproportionation in the acrylate-type macroradicals. The block length of the acrylate block can be tuned by variation of the acrylate concentration. Isoprene homopolymerization plays under the elucidated reaction conditions only an insignificant role. Experimental Results. In previous studies, well-defined polymers formed by Co(acac)2-mediated radical polymerization were treated at room temperature with isoprene30−32 or butyl acrylate.36 The former additive led to the almost perfect doubling of the molar mass of the precursor due to the low propagation rate of dienes at room temperature and because the main termination mode of IP is coupling.30−32 In contrast, addition of BA to Co(acac)2-terminated polymers resulted in chain extension reactions which are fast and uncontrolled.36 This point is illustrated by experiment 1 in Table 1. A welldefined pAN−Co(acac)2 precursor was reacted with BA at 25 °C and produced a pAN-b-pBA diblock with a relatively broad distribution (Đ ∼ 1.5). The reaction was exothermic and completed within few minutes confirming the uncontrolled character of the chain extension. Nevertheless, no residual peak corresponding to unreacted pAN macroinitiator was detected on SEC chromatograms (Figure 4A), in agreement with the theoretical predictions presented in Figure 2. Moreover, the shoulder observed on the high molar mass side of the pAN-bpBA main SEC peak most probably results from the radical coupling of few copolymer chains. Yet, the key question is whether simultaneous addition of BA and IP to pAN−Co(acac)2 could lead to block extension and radical coupling reaction in a single step as predicted. If the 8927
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Macromolecules
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mass increase associated with a low BA conversion. Nevertheless, the question how much of the BA was indeed incorporated into the polymer product remains at this point still debatable. Nevertheless, NMR analyses of the final product confirmed the presence of BA units in the copolymer (Figure 5, upper spectrum) as indicated by signal c at 4.1 ppm corresponding to the −CH2−O(CO)− protons of pBA. On the basis of the molar mass of the pAN precursor (6200 g· mol−1) and relative intensities of the NMR signals relative to the AN and BA units, we evaluated the degree of polymerization of the pBA central block to 60 (corrected to 7700 g· mol−1). In other words, the isoprene-assisted coupling reaction occurred after addition of 30 BA units. This value is consistent with our predictions (45 and 10 for 6.0 mol·L−1 and 1.0 mol· L−1, respectively) given the concentration of BA that we used (4.18 mol·L−1). In a previous mechanistic study on CMRC,31 we evidenced by NMR the insertion of 2 isoprene units in low molar masses coupling products. Here, the number of inserted IP unit in the middle of the triblock could not be determined because the olefinic protons of the IP units could not be detected on the NMR spectra due to of the rather high molar mass of the triblock copolymers. Next, we highlighted the dependence the pBA block length on the BA/IP molar ratio, which regulates the balance between propagation and coupling (see experiments 2−4). Decreasing the acrylate concentration by a factor 3 with the same IP concentration (0.024M) decreased the pBA block length in the same proportions (compare experiments 2 and 3 in Table 1). SEC curves indicate that the coupling was also almost quantitative in exp 3 (Figure S1A). The DPn of the pBA block also dropped from 60 to 22 between experiments 2 and 4 when multiplying the concentration of the IP coupling agent by 3 (Table 1). Again, the pAN−pBA−pAN triblock appeared free of unreacted chains on the SEC chromatograms (Figure S1B). The DPn of the poly(acrylate) block of our triblocks should not only depend on the concentration of the additives but should also vary with the structures of the acrylate and diene. Indeed, substitution of IP for a diene with higher degree of substitution should delay the cross-addition reaction and permit the insertion of more acrylate before coupling. As a confirmation, the molar mass of the pBA central block doubled when we used 1,3-cyclohexadiene (1,3-CHD) (compare experiments 2 and 5 in Table 1). As shown by the SEC chromatograms (Figure S1C), the coupling efficiency remained good with 1,3-CHD. Then, we replaced BA by 2(phenylcarbamoyloxy)ethyl acrylate (PhCEA) (exp 6 in Table 1), a urethane-containing acrylate with exceptionally high propagation rate constants due to the preorganization of the monomer via hydrogen bonding and dipole effects.37,38 In this case, the NMR analysis (Figure 5, lower spectrum) indicate that the DPn of the middle block of the pAN−pPhCEA−pAN triblock was twice higher compared to the one of the pAN− pBA−pAN prepared with the same [pAN]/[diene]/[acrylate] molar ratios (compare experiments 3 and 6 in Table 1), which fits well with the increase in the propagation rate coefficient between BA and PhCEA from kp(BA)25 °C = 16 200 L·mol−1· s−1 to kp(PhCEA)25 °C = 37 500 L·mol−1·s−1.38,45 Finally, this convergent synthetic strategy based on the simultaneous chain extension and coupling reactions was tested for the synthesis of well-defined symmetrical ABCBA pentablock copolymers. In this case, we followed a three-step process as represented in Figure 6. First, a well-defined pVAc− Co(acac)2 sample was synthesized following a previously
Figure 6. General strategy and overlay of the SEC chromatograms in DMF for the synthesis of a symmetrical ABCBA pentablock by treatment of a pVAc-b-pAN−Co(acac)2 diblock with a BA/1,3-CHD mixture. Conditions: r.t., DMSO, [pVAc-b-pAN−Co(acac)2]/[1,3CHD]/[BA] = 0.01 M/0.024 M/4.18 M.
disappeared yielding a higher molar mass product with very low molar mass distribution (Đ ∼ 1.15) (Figure 4B). To stay on the side of caution, we must, however, exclude the hypothesis that isoprene itself induced a chain length control in the BA chain extension (by establishing an equilibrium with the cobalt species) producing a well-defined pAN-b-pBA diblock. Such control equilibrium may not be active for two reasons: First, the low BA conversion cannot account alone for the increase of the molar mass observed by SEC upon addition of BA/IP (Mn SEC PS changed from 14000 to 33200 g·mol−1). Moreover, the extreme low bond dissociation energy of the PIP-cobalt bond is clearly not in favor of a controlled CMRP system but rather promotes massive termination of the chains by coupling. The copolymerization parameters also predict that no copolymerization of BA and IP should really take place under the chosen reaction conditions. Instead, subsequent chain extension of the polymers with BA and isoprene-assisted coupling is the only feasible explanation for a substantial molar 8928
dx.doi.org/10.1021/ma401918t | Macromolecules 2013, 46, 8922−8931
Macromolecules
Article
Figure 7. 1H NMR spectrum in DMSO-d6 of a symmetrical ABCBA pentablock by treatment of a pVAc-b-pAN−Co(acac)2 diblock with a BA/1,3CHD mixture. Conditions: room temperature, DMSO, [pVAc-b-pAN−Co(acac)2]/[1,3-CHD]/[BA] = 0.01 M/0.024 M/4.18 M.
units prior coupling and so the formation of the targeted pentablock (Figure 7). Indeed, between signal i at 4.8 ppm corresponding to the −CH(OAc)− protons of the pVAc blocks and signal a at 3.2 ppm corresponding to the −CH(CN)− protons of the pAN segments, we found the signal c at 4.1 ppm characteristic of the −CH2−O(CO)− protons of the pBA. Considering the relative intensity of these signals and the molar mass of the pVAc, we could evaluate the DPn of each block of the pentablock, i.e., pVAc 102 −pAN 107 −pBA 75 −pAN 107 − pVAc102.
reported CMRP procedure which consists in the bulk polymerization of VAc initiated at 40 °C by [R 0 − (CH2CHOAc)
