PEO-b-PNIPAM copolymers via SARA ATRP and eATRP in aqueous media

Abstract

Supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) and electrochemically mediated atom transfer radical polymerization (eATRP) of N-isopropylacrylamide were investigated in aqueous media. The synthesis of poly(ethylene oxide)-block-poly(N-isopropylacrylamide) copolymers via SARA ATRP was carried out, utilizing 500 ppm of Cu^II species, rather than previously reported ca. 2,500–50,000 ppm of Cu^I/L used to pre-disproportionate in situ to form Cu⁰ and Cu^II/L. The copolymers were also prepared via eATRP under potentiostatic conditions. Various polymerization parameters were examined including the effect of the targeted degree of polymerization (DP) and, in the case of SARA ATRP, the influence of copper surface area on the polymerization. Under optimized reaction conditions, the DP of resulting copolymers increased linearly with monomer conversion. Successful formation of the block copolymers confirmed the living nature of the polymers prepared by SARA ATRP and eATRP of N-isopropylacrylamide in aqueous media.

Introduction

Atom transfer radical polymerization (ATRP) is one of the most often used reversible deactivation radical polymerization (RDRP) methods, allowing effective control over polymer molecular weight (MW), generating (co)polymers with narrow molecular weight distribution (MWD), site selected functionalities, controlled architectures, and well-defined compositions [1], [2], [3], [4], [5], [6], [7]. Monomers with various functionalities have been successfully polymerized by ATRP, including various styrenes, acrylates, methacrylates, acrylamides, and acrylonitrile as well as vinyl pyridine [6], [7], [8], [9], [10].

Poly(N-isopropylacrylamide) (PNIPAM) is widely used in cosmetics, for biomedical applications, wastewater treatment, and oil recovery [11], [12]. However, successful ATRP of acrylamides and its derivatives may be challenging with respect to the control of the polymerization when using water as the solvent. ATRP of NIPAM has relatively low value of ATRP equilibrium constant (K_ATRP) and some potential side reactions that include ligand displacement from the copper catalyst by the formed polymers, and loss of chain-end halogen (C-X) by solvolysis [6]. Since the X–Cu^II/L bond can easily dissociate in aqueous media, typically high concentration of catalyst is needed, or the presence of salts with halide anions is required [13], [14]. Hydrolysis of alkyl halides in water can be suppressed by conducting the polymerizations at lower temperatures [12], [15], [16]. The best control over an ATRP of (meth)acrylamides was obtained using one of the most active catalytic systems Cu^IBr/Me₆TREN (Me₆TREN = tris(2-(dimethylamino)ethyl)amine) due to its intrinsically high values of K_ATRP [2], [17]. PNIPAM with narrow MWD (M_w/M_n = 1.08) was prepared by employing water soluble initiator, however high catalyst loadings (ca. 10,000–15,000 ppm) were used [15]. Recently another procedure for synthesis of PNIPAM with narrow MWD (M_w/M_n = 1.13–1.22) was reported, but it typically required a large amount of organic solvent, at least 50% [18], [19]. NIPAM copolymers with poly(ethylene oxide) were reported, however the polymerizations either took long time (24 h) to reach high conversion and yielded polymers with broad MWD (M_w/M_n = 1.45–1.51) [20], required the use of high amounts of organic (co)solvent [21], or high amounts of catalyst [22].

The development of new ATRP systems, utilizing parts per million (ppm) concentrations of catalyst, can provide well-controlled polymerizations of various monomers using highly active copper catalysts under more environmentally benign and industrially scalable reaction conditions [6], [23]. These methods allow for significant reduction of the amount of catalyst used due to continuous regeneration of the Cu^I/L activator complex from excess Cu^II/L deactivator species. This can be achieved either by employing chemical reducing agents such as glucose [24], ascorbic acid [25], sulfites [26], hydrazine [27], Sn^II compounds [28], [29] or silver metal [30] in a process termed activators regenerated by electron transfer (ARGET) ATRP [28], [31], conventional thermal free radical initiators (e.g., azo-initiators) in initiators for continuous activator regeneration (ICAR) ATRP [27], [32], zerovalent metals including Cu⁰ in supplemental activator and reducing agent (SARA) ATRP [14], [33], [34], [35], [36], [37], [38], photochemical reduction of the Cu^II/L complex in photoATRP [23], [39], or by applying a reducing potential in electrochemically mediated ATRP (eATRP) [10], [40], [41], [42], [43], [44], [45], [46].

Among these low ppm Cu ATRP methods, SARA ATRP offers simple removal and reuse of unreacted Cu⁰ and control over the polymerization rate (R_p) by the surface area of Cu⁰ [14], [34], [35], [37]. On the other hand, eATRP stands out due to the possibility of recycling/reusing the transition metal (e.g., copper), in addition to facile control over the polymerization rate and the ability to intermittently switch a polymerization between “on” and “off” states [40], [42].

In addition to the ATRP equilibrium between Cu^I and Cu^II species, SARA ATRP employs a zerovalent metal, which introduces additional slow reactions, i.e. supplemental activation of alkyl halides by Cu⁰ and reduction of Cu^II/L back to Cu^I/L through comproportionation, as well as kinetically negligible disproportionation of Cu^I/L (Scheme 1). SARA ATRP is also termed as single electron transfer living radical polymerization (SET-LRP) [47]. Recent mechanistic studies [48], [49], [50] have shown that the mechanism of RDRP in the presence of Cu⁰ is consistent with SARA ATRP in dimethyl sulfoxide (DMSO), acetonitrile (MeCN) or even in water with Me₆TREN or tris(2-pyridylmethyl)amine (TPMA) [14].

In eATRP, the ratio of Cu^I/L to X–Cu^II/L concentration is precisely controlled by applied current (I), potential (E), and total charge passed (Q) at the electrode surface [41], [42]. The activator reduced by electrolysis is then distributed through the reaction mixture by vigorous stirring, and reacts with initiators (P_n-X) to form radical species (P_n^•) and the oxidized catalyst deactivator (X–Cu^II/L). Consequently, the radical species propagate to form polymeric chains by reacting with monomers (M), and/or are converted back to a dormant species (P_n-X) (Scheme 2) [10]. This allows for fast and controlled polymerizations and synthesis of polymers with complex architectures [7], [41], [42], [43], [44], [45], [51], [52].

This paper is focused on aqueous SARA ATRP and eATRP of N-isopropylacrylamide. Polymerization conditions were optimized to provide fast reactions employing low catalyst concentrations, while maintaining narrow molecular weight distributions. Successful preparation of poly(ethylene oxide)-block-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) diblock copolymers (Scheme 3) is reported.