Phenylboronate-diol crosslinked polymer gels with reversible sol-gel transition

Abstract

Covalent dynamic gels based on reversible phenylboronic acid-diol ester bonds were prepared by crosslinking of N,N-dimethylacrylamide-4-((4-vinylbenzyloxy)carbonyl)phenylboronic acid copolymer, poly(VPB-co-DMA), and poly(2,2-bis(hydroxymethyl)butyl acrylate) (PHBA) in several organic solvents at ambient temperature. These gels formed rapidly and revealed typical properties of chemical gels. Analysis of the composition–property relationships of these polymer gels, specifically considering the effects of pH, molar ratio of HBA to VPB units, and gelator concentrations on dynamic rheological properties, were performed. Additionally, the polymer gels can be switched into their starting polymer solutions by adjusting pH of the system. The reversible sol-gel phase transition can be performed for several cycles in a similar way of supramolecular gel. Moreover, the gel revealed interesting self-healing property which occurred autonomously without any outside intervention. Employing this dynamic character, it is possible to regenerate the used gel, and thus has the potential to perform in a range of dynamic or bioresponsive applications.

Introduction

In recent years, polymer gel has been the focus of considerable interest, from both fundamental and applied perspectives [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. For example, polymer gels have played many technologically important roles in chemical separations, biomedical devices and absorbent products [1], [2]. Generally, polymer gels are understood to be polymeric networks which absorb enough solvent to cause macroscopic changes in the sample dimensions [4]. According to their crosslinkage, polymer gels can be classified into chemical gels and physical gels. The three dimensional network can be stabilized by crosslinks which may be provided either by covalent bonds in chemical gels or by noncovalent interactions in physical gels, such interactions including van der Waals, hydrogen bonding, or hydrophobic and ionic interactions [12]. Therefore, it is easy for a physical gel to perform a sol-gel transition in response to temperature, pH, and solvent. Chemical gels show relatively high mechanical strength, however, cannot carry through such a reversible transition. In order to yield a gel that owns good perspective applications, an efficient route is proposed to integrate the stimuli-responsibility of the physical gels and stability of the chemical gels into a single system.

Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control [13]. Polymer gels crosslinked with dynamic covalent bonds would provide an energetically favorable, specific and controlled mechanism for engineering functional dynamic networks [14]. The most significant advantage of such polymer gels is that they are not only flexible in altering their constitutional structures, as in the case of supramolecular polymers with noncovalent interactions, but also stable enough to maintain their structure in the absence of external stimuli, as in the case of conventional covalent polymers [15], [16], [17]. As one of the dynamic covalent bonds, boronate ester bond has been successfully used by Kataoka to prepare glycol-sensors [18]. Boronic acid undergoes a well-known condensation reaction with 1,2- or 1,3-diols to form five- or six-membered cyclic esters. This reaction is reversible and highly influenced by the pH and chemical structure of the diols [19], [20], [21], [22], [23], [24]. Taking advantage of this transition, growing attention has focused on the use of phenylboronic acid functional groups in the design of polymer gels [25], [26], [27], [28], [29], [30], [31], [32]. In previous articles dealing with the preparation of boronic acid-based polymer gels, experimental observations were interpreted as two aspects. One involves direct radical copolymerization of acrylamides with phenylboronic acid-functionalized monomers using bis-(acrylamide)s as a crosslinker in aqueous solution [25], [26], [27], [28], [29]. However, this uncontrolled crosslinking technique often requires prolonged reaction time, thus limiting its applications. Moreover, this method involves the use of toxic reagents like initiators, and crosslinkers, therefore become less popular. On the other hand, polymer gels can also be achieved by crosslinking of boronic acid-containing polymers with hydroxyl-containing polymers such as poly(vinyl alcohol) (PVA) [30], [31], [32]. The use of PVA or other hydroxyl-containing polymers prepared by conventional radical polymerization, however, poses problems due to its poorly defined architecture, increasing the difficulty in relating the network structure to the final physical properties of the gel. For this reason, it would be highly desirable to develop alternative strategies for the synthesis of boronic acid-based gels while maintaining precise control over network structures. In order to realize this goal, polymeric precursors with well-defined architecture are usually required. Recently, reversible addition-fragmentation chain-transfer (RAFT) polymerization has attracted significant attention in polymer science because of its applicability to a wide range of monomers, suitability for a broad range of functional groups, and tolerance to various organic and aqueous solvents [33], [34], [35], [36], [37]. It was therefore postulated that the use of RAFT polymerization would be an ideal methodology for the preparation of a wide variety of polymers that could be used for the formation of crosslinked polymer gels.

Herein, we report a strategy for constructing reversible polymer gels based on dynamic covalent chemistry in organic solvents. Firstly, polymeric precursors, poly(2,2-bis(hydroxymethyl)butyl acrylate) (PHBA) and N,N-dimethylacrylamide-4-((4-vinylbenzyloxy)carbonyl)phenylboronic acid copolymer (poly(VPB-co-DMA)), with well-defined architecture were separately prepared by RAFT polymerization. Subsequent construction of polymer gels was achieved by the reversible covalent interaction of polymer-bound phenylboronic acid and 1,3-diols as shown in Scheme 1. The reversible sol-gel phase transition of this polymer gel can be accomplished by adjusting pH of the system due to the dynamic nature of boronic acid-diol ester bonds. The VPB-HBA complex is unique in its stability at slightly acidic pH values, compared to other boronate-diol complexes, which can only stably exist at alkaline pH [19]. Additionally, reversible crosslinks allow these networks to restructure dynamically and self-healing after mechanical disruption. This network structure may produce gels with novel properties such as stimuli responsive and, therefore, have potential in biological and biomedical applications, including therapeutic agents, self-regulated drug delivery systems, and sensors for sugars and glycoproteins.