MADIX Polymerization: Same ... - ACS Publications

Chapter 17

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Aqueous RAFT/MADIX Polymerization: Same Monomers, New Polymers? Mathias Destarac,*,1 Issam Blidi,1 Olivier Coutelier,1 Aymeric Guinaudeau,1 Stéphane Mazières,1 Eric Van Gramberen,2 and James Wilson2 1Université

Paul Sabatier, Laboratoire Hétérochimie Fondamentale et Appliquée, UMR-CNRS 5069, Bât 2R1, 118 route de Narbonne, 31062 Toulouse cedex 9, France 2Rhodia Opérations, Centre de Recherches et Technologies d’Aubervilliers, 52 rue de la Haie Coq, 93308 Aubervilliers Cedex, France *E-mail : [email protected]

Thanks to their specific reactivity in RAFT/MADIX polymerization, O-ethyl xanthates are chain transfer agents of choice for controlling the polymerization of “less-activated” monomers (LAMs). In particular, hydrophilic LAMs can be polymerized in water by means of either water-soluble or hydrophobic xanthates in some appropriate conditions. Through the description of successful aqueous RAFT/MADIX polymerization of N-vinylpyrrolidone, diallydimethylammonium chloride and vinylphosphonic acid monomers, which strongly differ in their chemical nature and physicochemical properties of the resulting polymers, it is expected that various original water-soluble copolymers with complex architectures will be available in the future.

Introduction Since the early nineties, the unceasing development of reversible-deactivation radical polymerization technologies (1) (RDRP) has been offering synthetic chemists the ability to design an nearly limitless range of more or less complex macromolecular architectures. Among them, double hydrophilic block copolymers (DHBCs) are widely studied water-soluble copolymers that © 2012 American Chemical Society In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


can combine blocks with distinct physicochemical properties, e.g. neutral and polyelectrolyte blocks (2). These materials can exhibit stimuli-responsive properties, i.e. they either undergo morphological transitions induced by an external stimulus like changes in temperature, solvent polarity, ionic strength, pH, or can lead to nanostructured aggregates with oppositely charged surfactants (3), polymers (4) or inorganic nanoparticles (5). Although all the RDRP strategies can potentially be used to synthesize DHBCs, RAFT/MADIX (6, 7) is by far the most straightforward approach for controlling the polymerization of water-soluble monomers in aqueous media, thereby directly leading to a great variety of DHBCs in water (8–10). Whereas aqueous RAFT/MADIX solution polymerizations of many so-called “more-activated” monomers (MAMs) like hydrophilic (meth)acrylates, acrylamido and styrenic monomers have been widely reported in the literature (8), a very limited number of studies have dealt with “less-activated” monomers (LAMs) like diallyl (10) and vinyl monomers (11). As it is now well-established that O-ethyl xanthates are RAFT/MADIX agents of choice for controlling the polymerization of the main LAM monomers like vinyl acetate (12), N-vinylpyrrolidone (NVP) (13) and N-vinylcaprolactam (14) in organic media, we recently concentrated our efforts on their use for hydrophilic LAMs in aqueous solution. A first example consisted in the successful RAFT/MADIX polymerization of diallyldimethylammonium chloride (DADMAC) mediated by a O-ethyl xanthate-terminated low molar mass polyacrylamide (10). More recently, poly(N-vinylpyrrolidone) (PVP) of controlled Mn and low dispersities (Ð =Mw/Mn) was synthesized in water at room temperature with the Rhodixan A1 transfer agent, namely O-ethyl-S-(1-methoxycarbonyl)ethyldithiocarbonate (11). First promising results on RAFT/MADIX polymerization of vinyl phosphonic acid (VPA) in water mediated by a water-soluble carboxy-functional xanthate (15) encouraged us to further explore the strengths and limitations of this challenging system. This work aims to illustrate the great potential of O-ethyl xanthates for polymerizing the aforementioned LAM monomers by RAFT/MADIX in water with unprecedented control. The numerous resulting opportunities for the access to novel DHBCs will be exemplified through the synthesis of PDADMAC-based diblock copolyampholytes.

Experimental Unless otherwise stated, materials, experimental procedure and instrumentation are described in ref.10 for DADMAC, ref. 15 for VPA and ref. 11 for NVP.

Materials 2-[(ethoxythiocarbonyl)thio] propionic acid (X1) was prepared according to a procedure described elsewhere (16). 260 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Synthesis of O-ethyl-S-(1-carboxy)methyl Dithiocarbonate (X2)


Ethylxanthic acid potassium salt (7.2 g, 44.92 mmol) was dissolved in 60 mL of ethanol in a 250 mL round-bottomed flask. The reaction mixture was stirred overnight with 2-bromoacetic acid (6.24 g, 44.90 mmol) at room temperature and then dissolved in CH2Cl2 and washed with H2O. The organic phase was dried and evaporated to obtain the desired xanthate as a crystalline solid (4 g, 49.5%). 1H-NMR (CDCl3, 300.13 MHz): δ [ppm]: 1.36 (t, 3H, CH3CH2, 3JHH = 6.90 Hz), 3.91 (s, 2H, CH2COOH), 4.60 (q, 2H, CH3CH2, 3JHH = 6.90 Hz), 10.35 (s, COOH). 13C-NMR (CDCl3, 75.48 MHz) δ [ppm]: 13.68 (CH3CH2), 37.61 (CH2COOH), 70.92 (CH3CH2), 174.55 (CO), 211.97 (CS).

RAFT/MADIX Polymerization of VPA A typical polymerisation procedure is as follows: X2 (31.9 mg, 0.177 mmol), VPA (500 mg, 4.62 mmol), AIBA (9.35 mg, 0.034 mmol) and distilled water (615 mL) were put together in a Schlenk flask. The solution was then degassed by gently bubbling argon for 15 mins. After that, the reaction mixture was heated at 65°C for 24 hours in a thermostated oil bath. 54.7% of VPA was converted into polymer at the end of the reaction as determined by 31P NMR. Mn th=1640 g mol-1, Mn NMR=1460 g mol-1, Mn MALS=2270 g mol-1, Ð=1.32.

Synthesis of VPA-X1 1:1 Adduct X1 (719 mg, 3.7 mmol), VPA (200 mg, 1.85 mmol), AIBA (3.74 mg, 0.013 mmol) and distilled water (245 mL) were added to a Schlenk flask. The solution was then degassed by gently bubbling argon for 15 min. After that, the reaction mixture was heated at 65°C for 24 hours in a thermostated oil bath. The mixture was then purified by extraction of the residual amount of xanthate X1 with dichloromethane. The aqueous phase was then freeze-dried to remove water. The monoadduct was obtained as a mixture of diastereisomers (viscous oil, 280 mg, 51% yield). 1H-NMR (D2O, 300.13. MHz): δ (ppm) =1.05 (m, 3H, CH3-CH), 1.35 (m, 3H, CH3-CH2O), 1.62, 1.98 and 2.28 (m, 2H, CH-CH2-CH), 2.65 (m, 1H, CH-CO2H), 4.01 (m, 1H, CH-PO(OH)2), 4.55 (m, 2H, CH3 CH2-O). 31P-NMR (D2O, 121.50 MHz): δ (ppm) =19.5 (d). Instrumentation For PVPA samples, size exclusion chromatography (SEC) was performed on an Agilent 1100 HPLC system, a 18 angle Multi-Angle Light Scattering (MALS) DAWN-Heleos-II (Wyatt Technology), an OptilaRex Refractometer (Wyatt Technology) and a set of 2 columns (Shodex SB-806M and SB-802.5) thermostated at 30°C. Number-average molar masses (Mn MALS) and dispersities Ð were determined with the SEC-RI-MALS line described above. Water (NaCl 261 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

100 mmol.L-1, NaH2PO4 25 mmol.L-1, Na2HPO4 25 mmol.L-1, buffer solution at pH=7) was used as eluent with a flow rate of 1.0 mL.min-1. PAA-PDADMAC copolymer samples were eluted through three SB 806 M HQ Shodex columns in a 1 M NH4NO3 solution of water/acetonitrile 80/20 wt.-% containing 10 ppm PDADMAC (flow rate: 1 mL.mn-1) (10).

Results and Discussion


Vinylphosphonic Acid (VPA) In contrast to most of the main classes of functional monomers, RDRP of phosphonic acid-containing monomers was surprisingly never reported in the academic literature. Instead, the polymerization of alkyl ester derivatives of phosphonic monomers was considered in few cases (17, 18). Therefore, the possibility of directly synthesizing a poly(phosphonic acid) from an unprotected phosphonic monomer in water remains highly challenging and looks advantageous from both economic and environmental standpoints. In this respect, VPA (19) is a very attractive monomer because it is one of the few industrially available phosphonic monomer with a reasonable cost. Moreover, it polymerises under homogeneous aqueous conditions to yield watersoluble PVPA. It has been only recently that free-radical polymerization of VPA started to be studied in detail by several research groups (20–22). In this contribution, we aim to report RDRP of VPA by means of an aqueous RAFT/MADIX process mediated by the carboxy-functional O-ethyl-xanthate X2 and a O-ethyl xanthate/VPA 1/1 adduct (so-called monoadduct, Scheme1).

Scheme 1. Aqueous RAFT/MADIX polymerization of vinylphosphonic acid mediated by a carboxy-functional xanthate (X2 or monoadduct). Experimental conditions were defined from free-radical polymerization of VPA at various temperatures and for different concentrations of AIBA initiator and VPA. A polymerization temperature of 65 °C with AIBA and VPA concentrations of 56 mmol.L-1 and 7.52 mol.L-1, respectively, were found to be the best compromise to obtain both high VPA conversion (>80%) and sufficiently high molecular weight (Mn~9000 g.mol-1, table 1, entry 1-4) for efficient RAFT/MADIX polymerization. Bearing in mind the low Mn values obtained for 262 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


xanthate-free experiments, we considered VPA polymerization in the presence of two different concentrations of xanthate X2, corresponding to theoretical Mn values of 1000 and 3000 g.mol-1 (Table 1, entry 5-14). 31P NMR was found to be very useful to follow both conversion and polymer end-group. In fact, VPA has a characteristic chemical shift in deuterated water at 15.5 ppm and PVPA exhibits a broad signal in the range 29-32 ppm (Scheme 2). The characteristic signal of the VPA terminal unit next to the O-ethyl xanthate group has been identified by synthesising a xanthate:VPA monoadduct (Scheme 1) that mimics the PVPA-X2 chain end. A characteristic signal at 19.5 ppm can be observed (Scheme 2). In PVPA obtained in the presence of X2, a similar, but broader peak is visible at 19-20 ppm, which attests that X2 acts like a chain transfer agent during polymerization. Hence, VPA conversion and Mn of the xanthate-terminated chains (Mn NMR) could be easily determined by simple mathematical expressions using relative peak integrations for monomer, polymer and VPA terminal unit.

Scheme 2. VPA, PVPA-X2 and VPA/X1 monoadduct structures and corresponding chemical shifts in 31P NMR. VPA conversion (%)=(IP1+IP2)/( IP1+IP2+Iresidual VPA) and DPn NMR= (IP1+IP2)/IP1, with IP1, IP2 and Iresidual VPA the integration of the corresponding phosphorous signals. Compared to the xanthate-free VPA polymerization, polymerization in the presence of X2 is slowed down to a conversion of 49-60% after 24h (Table 1, entries 5-14). However, it can be clearly observed in Table 1 and on SEC-RI traces (Figure 1b) that X2 plays its role by regulating Mn. Assuming that all the chains are capped with the dithiocarbonate group, we could observe that Mn NMR values increase virtually linearly with VPA conversion for the two experiments in relatively good agreement with theoretical values (Figure 1a), except at low conversions (

MADIX Polymerization: Same ... - ACS Publications

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