Letter Cite This: ACS Macro Lett. 2017, 6, 1342-1346
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Micellar RAFT/MADIX Polymerization Cécile Barthet,†,‡ James Wilson,§ Arnaud Cadix,§ Mathias Destarac,† Christophe Chassenieux,*,‡ and Simon Harrisson*,† †
Laboratoire des IMRCP, Université de Toulouse, CNRS UMR 5623, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 9, France § Solvay Novecare, Research and Innovation Centre − Paris, 93306 Aubervilliers, France ‡ Le Mans Université, Institut des Molécules et Matériaux du Mans (IMMM) UMR 6283, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France S Supporting Information *
ABSTRACT: We apply the RAFT/MADIX technique to the micellar copolymerization of acrylamide and 2-acrylamido-2methylpropanesulfonic acid sodium salt with a hydrophobic monomer, 4-tert-butylstyrene. The resulting polymers have wellcontrolled molecular weight distributions. In the presence of sodium dodecyl sulfate, the polymerization is better controlled by an oligo-acrylamide chain transfer agent (PAm7-XA1) than by Rhodixan A1. The associative character of the polymer is maintained under RAFT polymerization conditions and chains can be extended to form block copolymers with associative segments.
W
(AMPS)16 to micellar copolymerization in the presence of sodium dodecyl sulfate (SDS) surfactant and 4-tert-butylstyrene (tBS) as hydrophobic monomer, as depicted in Scheme 1. For this purpose, we studied (1) the influence of SDS on a model system without tBS, (2) the control over the molecular weight distribution in a controlled micellar polymerization, (3) the rheological properties of the associative copolymers, and (4) their chain extension to form block copolymers containing an associative segment. We first investigated a model system containing 0.1 M SDS but without tBS and targeting a number-average molecular weight, Mn, of 100 kg mol−1. The SDS concentration was chosen because it allowed us to work at a reasonable Am/ AMPS concentration while incorporating sufficient tBS to have an effect on the properties of the polymer (typically the hydrophobic monomer content of associative polymers ranges from 1 to 5 mol %).3,17 Significant retardation (see section 3 and Table S2 of the Supporting Information) was observed in the presence of SDS for both Rhodixan A1 and an oligo-acrylamide chain transfer agent, PAm7-XA1.16 The presence of SDS also had a significant effect on the evolution of the molecular weight distributions with conversion. In the absence of SDS, the polymerization of the acrylamide/ AMPS mixture is fairly well controlled by both Rhodixan A1
ater-soluble polymers modified with small amounts of hydrophobic monomers are widely used as rheology modifiers.1−6 In aqueous solution, hydrophobic units from different polymer chains associate, leading to the formation of a physically cross-linked network that displays interesting rheological features.7 These polymers can be prepared by postmodification of water-soluble polymers,8 but the resulting hydrophobic segments are randomly distributed,8,9 which leads to limited thickening abilities. Hydrophilic and hydrophobic monomers are difficult to copolymerize directly, as they are rarely soluble in the same solvents. This problem can be solved using micellar copolymerization.10,11 In this process, micelles of surfactant are dispersed in an aqueous solution of hydrophilic monomer and loaded with a hydrophobic monomer. During polymerization, a blocky structure containing hydrophobic segments is formed as the locus of polymerization alternates between aqueous and dispersed phases. The properties of the resulting polymers can be tuned by adjusting parameters such as the molecular weight, the hydrophobic monomer content, and the hydrophobic monomer to surfactant ratio.12,13 In this contribution, we apply for the first time a reversible deactivation radical polymerization technique to micellar polymerization.14 The application of RAFT/MADIX technique15 to micellar polymerization opens new opportunities for the design of complex architectures with original properties in aqueous media. We adapted conditions developed for the aqueous RAFT/MADIX copolymerization of acrylamide (Am) and 2-acrylamido-2-methylpropanesulfonic acid sodium salt © XXXX American Chemical Society
Received: October 5, 2017 Accepted: November 15, 2017
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DOI: 10.1021/acsmacrolett.7b00791 ACS Macro Lett. 2017, 6, 1342−1346
Letter
ACS Macro Letters Scheme 1. Micellar RAFT/MADIX Copolymerization of Acrylamide (Am), AMPS, and 4-tert-Butylstyrene (tBS)
Figure 2. Evolution of the weight-average molecular weight Mw of P(Am80-co-AMPS20-co-tBS) and P(Am80-co-AMPS20) for Mn,th = 100 kg mol−1 and P(Am80-co-AMPS20-co-tBS) for Mn,th = 10 kg mol−1 with the conversion of hydrophilic monomers using PAm7-XA1 as chain transfer agent and with CSDS = 0.1 M. Solid lines correspond to fits of the data according to eq 2.
Figure 1. Effect of 0.1 M SDS on the evolution of weight-average molecular weight (Mw) of P(Am80-co-AMPS20) for Mn,th= 100 kg mol−1 with conversion of AMPS and acrylamide in the presence of Rhodixan A1 or PAm7-XA1. Results are compared with data obtained without SDS for Rhodixan A1. Solid lines correspond to fits of the data according to eq 2.
Figure 3. Storage (G′, filled symbols) and loss (G″, open symbols) moduli of P(Am80-co-AMPS20) (triangles) and P(Am80-co-AMPS20-cotBS) (circles) for Mn,th = 100 kg mol−1 at 200 g L−1 in water and T = 25 °C.
(Figure 1) and PAm7-XA1.16 Both Mn and the weight-average molecular weight, Mw, increase linearly with conversion. We have concentrated our analysis on Mw as this is measured most accurately by size exclusion chromatography with multiangle light scattering detection (SEC-MALS);18 corresponding Mn versus conversion plots are given in the Supporting Information (Figure S3). Rearranging Müller’s equation19 for dispersity Đ as a function of conversion, p (eq 1), gives an expression for Mw as a linear function of conversion (eq 2).
Đ=
2−p 1 Mw =1+ . Mn p CS
Mw =
⎛ 2 1 ⎞ · M + ⎜1 − ⎟·p·M CS CS ⎠ ⎝
(1)
(2)
In eq 2, M represents the expected Mn at full conversion, while CS represents the chain transfer constant of the dormant polymer. It should be noted that eq 2 is only valid in the absence of irreversible termination reactions20 for chain transfer 1343
DOI: 10.1021/acsmacrolett.7b00791 ACS Macro Lett. 2017, 6, 1342−1346
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ACS Macro Letters
When tBS is added using PAm7-XA1 as chain transfer agent, the slope of the Mw versus conversion plot increases significantly, as does the final molecular weight (Figure 2). The relationship between Mn and conversion for these polymers is shown in Figure S5 (Supporting Information). For a target Mn of 100 kg mol−1 at full conversion, fits of the data with eq 2 suggest an apparent CS of 5 and M of 250 kg mol−1. For a target Mn of 10 kg mol−1, we obtain an apparent CS of 2.4 and M of 17 kg mol−1. In each case, the incorporation of tBS appears to improve the control over the polymerization (as expressed by the increase in apparent CS), but also to result in the irreversible loss of a substantial fraction (40−60%) of chain transfer agent. The increase in apparent CS may be a result of improved accessibility of the RAFT agent as polymerization now takes place within the micelles as well as in the aqueous phase. The increase in M, on the other hand, may be a result of the slow propagation of tBS (the propagation rate constants of styrene23 and acrylamide24 at 25 °C are 85 L mol−1s−1 and 15800 L mol−1s−1, respectively), causing an increase in the number of irreversible termination and transfer reactions. The distribution of monomers within the copolymer is fairly homogeneous. Acrylamide and AMPS are near their azeotropic composition (rAm = 0.85, rAMPS = 0.18, fAMPS = 0.155 at the azeotrope) and are consumed at approximately equal rates.25 The rate of consumption of tBS was approximately equal to that of the hydrophilic monomers. Using the Jaacks method,26,27 an apparent reactivity ratio, rAm/AMPS, of 1.1 was calculated, treating the Am/AMPS mixture as a single monomer (Figure S4). At high conversion, complete conversion of the hydrophobic monomer was observed, indicating its complete incorporation into the hydrophilic backbone. It is typically assumed that all monomers in a micelle polymerize to produce a single block of length equal to the number of monomers contained in the micelle. Although this is difficult to confirm experimentally, it is clear that micellar polymerization produces polymers that contain blocks of hydrophobic monomers dispersed in a hydrophilic backbone.28,29 The incorporation of tBS strongly modifies the rheological properties of the polymer (Figure 3). While the unmodified polymers display viscous behavior (G′ ∼ ω2 ≪ G″ ∼ ω1) over the whole frequency range investigated, their hydrophobically modified homologues display viscoelastic properties when dispersed in water at the same concentration. The associative polymer displays a crossover point, with G′ > G″ at frequencies above 0.3 Hz. The associations between hydrophobic blocks create physical cross-links connecting several polymer chains resulting in a slowdown of the overall dynamics of the polymer chains, which is known as sticky reptation.7,30 This rheological feature provides evidence for the incorporation of the hydrophobic blocks within the polymeric chains. Finally, a P(Am-co-AMPS-co-tBS) associative polymer with a theoretical molecular weight of 10 kg mol−1, containing 20 mol % AMPS and 1 mol % tBS, was extended with acrylamide to molecular weights of 90, 176, 922, and 1751 kg mol−1. The reactions were carried out for 24 h to ensure that full conversion was reached. Figure 4 displays the chromatograms of the extended polymers starting from P(Am80-co-AMPS20-co-tBS) at 10 kg mol−1. They are monomodal and symmetrical for all targeted molar masses. Over the range of molar masses investigated, the distributions are narrow (Đ ≤ 1.2) and the experimental
Figure 4. (a) Overlay of RI-SEC chromatograms of the chain extensions of P(Am80-co-AMPS20-co-tBS)10k to PAm targeting final molecular weight Mn,th of 90, 176, 922, and 1751 kg mol−1. (b) Evolution of the experimental molecular weight determined at full conversion of the chain extensions of P(Am80-co-AMPS20-co-tBS)10k to PAm compared to the targeted molecular weights.
constants that are independent of molecular weight20 and when the molecular weight of the chain transfer agent is negligible compared to that of the polymer.21 In the system studied here, only the last of these conditions is fulfilled. Nevertheless, the linear evolution of Mw with conversion that is observed suggests that deviations from eq 2 as a result of irreversible termination and variable chain transfer constants are relatively small. The results of Figure 1 are consistent with M of 90 kg mol−1 and an apparent CS of 3.4. These figures compare well with the targeted Mn of 100 kg mol−1 (based on an initial ratio of 100 g Am/AMPS per mmol of RAFT agent) and the reported value of CS of 2.3 for Rhodixan A1 in the polymerization of N,Ndimethylacrylamide.22 In the presence of SDS, significantly higher molecular weights are obtained compared to polymerizations without SDS. For polymerizations controlled by Rhodixan A1, Mw is roughly constant at 235 kg mol−1, corresponding to M of 117 kg mol−1 and apparent CS of 1, while for those controlled by PAm7-XA1, Mw increases linearly from 120 kg mol−1 at nearzero conversion to 180 kg mol−1 at full conversion, giving M of 120 kg mol−1 and an apparent CS of 2. In both cases, M remains close to 100 kg mol−1, but apparent CS decreases significantly. This may be due to sequestration of the chain transfer agent in the SDS micelles, from which it is slowly released during the polymerization. The effect is more pronounced for Rhodixan A1, which is more hydrophobic than PAm7-XA1. 1344
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(4) Abdollahi, M.; Khakpour, H. Synthesis Of Polyacrylamides Hydrophobically Modified With Butyl Acrylate Using A Nanoclay With Interlayer Spaces For Butyl Acrylate Aggregation - Studies On The Microstructure And Aqueous Solution. RSC Adv. 2015, 5, 102844−102855. (5) Xue, W.; Hamley, I. W.; Castelletto, V.; Olmsted, P. D. Synthesis And Characterization Of Hydrophobically Modified Polyacrylamides And Some Observations On Rheological Properties. Eur. Polym. J. 2004, 40, 47−56. (6) Li, F.; Luo, Y.; Hu, P.; Su, G.; Zheng, M. Preparation And Evaluation Of Fluorinated Hydrophobically Associating Polyacrylamide. J. Polym. Res. 2016, 23, 1−9. (7) Castillo-Tejas, J.; Castrejon-Gonzalez, O.; Carro, S.; GonzalezCoronel, V.; Alvarado, J. F. J.; Manero, O. Associative Polymers. Part III: Shear Rheology From Molecular Dynamics. Colloids Surf., A 2016, 491, 37−49. (8) Bastiat, G.; Grassl, B.; François, J. Micellar Copolymerization Of Associative Polymers: Study Of The Effect Of Acrylamide On Sodium Dodecyl Sulfate-poly(propylene Oxide) Methacrylate Mixed Micelles. J. Colloid Interface Sci. 2005, 289, 359−370. (9) Magny, B.; Iliopoulos, I.; Zana, R.; Audebert, R. Mixed Micelles Formed By Cationic Surfactants And Anionic Hydrophobically Modified Polyelectrolytes. Langmuir 1994, 10, 3180−3187. (10) Evani, S. Water-Dispersible Hydrophobic Thickening Agent. Patent No WO4432881, 1984. (11) Turner, R. S.; Siano, D. B.; Bock, J. Microemulsion Process For Producing Acrylamide−Alkyl Acrylamide Copolymers. Patent No WO4521580, 1985. (12) Candau, F.; Regalado, E.; Selb, J. Scaling Behavior Of The Zero Shear Viscosity Of Hydrophobically Modified Poly(acrylamide)s. Macromolecules 1998, 31, 5550−5552. (13) Regalado, E. J.; Selb, J.; Candau, F. Viscoelastic Behavior Of Semidilute Solutions Of Multisticker Polymer Chains. Macromolecules 1999, 32, 8580−8588. (14) Wilson, J.; Destarac, M.; Cadix, A. Preparation Of Amphiphilic Block Polymers By Controlled Radical Micellar Polymerization. Patent No US2014/0378617A1, 2014. (15) Corpart, P.; Charmot, D.; Zard, S. Z.; Biadatti, T.; Michelet, D. Method For Block Polymer Synthesis By Controlled Radical Polymerization. Patent No US006153705A, 2000. (16) Read, E.; Guinaudeau, A.; James Wilson, D.; Cadix, A.; Violleau, F.; Destarac, M. Low Temperature RAFT/MADIX Gel Polymerisation: Access To Controlled Ultra-high Molar Mass Polyacrylamides. Polym. Chem. 2014, 5, 2202−2207. (17) Volpert, E.; Selb, J.; Candau, F. Influence Of The Hydrophobe Structure On Composition, Microstructure, And Rheology In Associating Polyacrylamides Prepared By Micellar Copolymerization. Macromolecules 1996, 29, 1452−1463. (18) Podzimek, S. Truths And Myths About The Determination Of Molar Mass Distribution Of Synthetic And Natural Polymers By Size Exclusion Chromatography. J. Appl. Polym. Sci. 2014, 131, 1−9. (19) Muller, A. H. E.; Litvinenko, G.; Yan, D. Kinetic Analysis Of “Living” Polymerization Systems Exhibiting Slow Equilibria. 3. “Associative” Mechanism Of Group Transfer Polymerization And Ion Pair Generation In Cationic Polymerization. Macromolecules 1996, 29, 2339−2345. (20) Monteiro, M. J. Design Strategies for Controlling the Molecular Weight and Rate Using Reversible Addition−Fragmentation Chain Transfer Mediated Living Radical Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3189−3204. (21) Monteiro, M. J. Modeling the Molecular Weight Distribution of Block Copolymer Formation in a Reversible Addition−Fragmentation Chain Transfer Mediated Living Radical Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 5643−5651. (22) Girard, E.; Tassaing, T.; Marty, J.-D.; Destarac, M. Influence Of Macromolecular Characteristics Of RAFT/MADIX Poly(vinyl Acetate)-based (Co)polymers On Their Solubility In Supercritical Carbon Dioxide. Polym. Chem. 2011, 2, 2222.
number-average molecular weights, Mn,exp, are linearly correlated with the targeted molecular weights, Mn,th. In conclusion, for the first time, reversible deactivation radical polymerization has been combined with micellar copolymerization to prepare block copolymers with associative segments displaying controlled molar masses and homogeneous composition. This was achieved by carrying out micellar polymerization in the presence of xanthate chain transfer agents. The associative polymer chains could be further extended with acrylamide to form high molar mass block copolymers containing an associative segment. This result will allow the production of complex architectures containing associative segments, opening the way to a wide variety of associative block, star, and comb copolymers.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00791. Additional information regarding materials and methods, synthesis details, the influence of SDS on the reaction rate, and the determination of the reactivity ratio (PDF).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Mathias Destarac: 0000-0002-9718-2239 Christophe Chassenieux: 0000-0002-3859-8277 Simon Harrisson: 0000-0001-6267-2599 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Solvay Novecare is gratefully acknowledged for its financial support. ABBREVIATIONS RAFT/MADIX, reversible addition−fragmentation chain transfer/macromolecular design by interchange of xanthates; Am, acrylamide; AMPS, 2-acrylamido-2-methylpropanesulfonic acid sodium salt; SDS, sodium dodecyl sulfate; tBS, 4-tertbutylstyrene; SEC-MALS, size exclusion chromatography with multiangle light scattering detection
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REFERENCES
(1) Hill, A.; Candau, F.; Selb, J. Properties Of Hydrophobically Associating Polyacrylamides: Influence Of The Method Of Synthesis. Macromolecules 1993, 26, 4521−4532. (2) Lacik, I.; Selb, J.; Candau, F. Compositional Heterogeneity Effects In Hydrophobically Associating Water-soluble Polymers Prepared By Micellar Copolymerization. Polymer 1995, 36, 3197− 3211. (3) Candau, F.; Selb, J. Hydrophobically-modified Polyacrylamides Prepared By Micellar Polymerization. Adv. Colloid Interface Sci. 1999, 79, 149−172. 1345
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ACS Macro Letters (23) Buback, M.; Gilbert, R. G.; Hutchinson, R. A.; Klumperman, B.; Kuchta, F.; Manders, B. G.; O’Driscoll, K. F.; Russell, G. T.; Schweer, J. Critically Evaluated Rate Coefficients For Free-radical Polymerization, 1. Propagation Rate Coefficient For Styrene. Macromol. Chem. Phys. 1995, 196, 3267−3280. (24) Pascal, P.; Winnik, M. A.; Napper, D. H.; Gilbert, R. G. Pulsed Laser Study Of The Propagation Kinetics Of Acrylamide And Its Derivatives In Water. Macromolecules 1993, 26, 4572−4576. (25) Scott, A.; Riahinezhad, M.; Penlidis, A. Optimal Design For Reactivity Ratio Estimation - A Comparison Of Techniques For AMPS/Acrylamide And AMPS/Acrylic Acid Copolymerizations. Processes 2015, 3, 749−768. (26) Lad, J.; Harrisson, S.; Mantovani, G.; Haddleton, D. M. Copper Mediated Living Radical Polymerisation: Interactions Between Monomer And Catalyst. Dalt. Trans. 2003, 6, 4175−4180. (27) Jaacks, V. A Novel Method Of Determination Of Reactivity Ratios In Binary And Ternary Copolymerizations. Makromol. Chem. 1972, 161, 161−172. (28) Candau, F.; Selb, J. Hydrophobically-modified Polyacrylamides Prepared By Micellar Polymerization. Adv. Colloid Interface Sci. 1999, 79, 149−172. (29) Kujawa, P.; Rosiak, J. M.; Selb, J.; Candau, F. Micellar Synthesis And Properties Of Hydrophobically Associating Polyampholytes. Macromol. Chem. Phys. 2001, 202, 1384−1397. (30) Rubinstein, M.; Semenov, A. N. Dynamics Of Entangled Solutions Of Associating Polymers. Macromolecules 2001, 34, 1058− 1068.
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