Mechanical properties of network polymers formed from monodisperse oligo(ethylene glycol)s of different molecular weights through Thiol–Yne reaction

Abstract

To improve the properties of network polymers, it is important to synthesize homogeneous network polymers. Aiming at easy formation of homogeneous network polymers, we have synthesized network polymers from monodisperse oligo(ethylene glycol) (MD-OEG) through thiol–yne reaction in this study. MD-OEG samples were first functionalized with thiol and yne moieties on both ends to prepare dithiol and diyne derivatives (DT-EGn and DY-EGn, where n is the degree of polymerization (DP) of MD-OEG). Thiol–yne reaction of DT-EGn and DY-EGn of n = 2, 3, or 4 was then carried out to form network polymers, which were characterized by compression tests. Young's modulus of the network polymers decreased with increasing n due to the decrease in crosslink density.

Introduction

Network polymers are an important class of polymers acting as resins, elastomers, and gels, and are thus utilized in a wide variety of fields, e.g., building materials, civil engineering, automotive industry, electrical industry, and biomedical applications [[1], [2], [3], [4], [5]]. Network polymers are usually prepared by crosslinking of linear polymers or by copolymerization of bifunctional and multifunctional monomers [6]. A typical example of the former is vulcanization of natural rubber, while the latter includes copolymerization of vinyl and divinyl monomers. However, these methods usually yield inhomogeneous network polymers, in which crosslinks are somehow localized [[7], [8], [9]]. Stress can concentrate at these dense localized crosslinks, resulting in reduced mechanical properties and swelling characteristics.

To prepare homogeneous network polymer, some research groups have reported coupling of branched polymers possessing reactive groups at the chain ends. Sakai et al. [[10], [11], [12]] have coupled four-arm star-shaped poly(ethylene glycol)s (tetra-PEG) possessing amino and N-hydroxysuccinimide terminals through amide formation to obtain homogeneous “tetra-PEG gel”. The structure and mechanical properties of tetra-PEG gel have been well characterized to confirm the high uniformity and mechanical strength of tetra-PEG gel. Some other groups have also reported the formation of homogeneous network polymers using a combination of branched polymers and click chemistry, e.g., copper(I)-catalyzed azide–alkyne cycloaddition [[13], [14], [15], [16]], thiol–ene reaction [[17], [18], [19]], and thiol–yne reaction [[20], [21], [22], [23], [24]]. The network polymers obtained have been characterized by glass transition temperature and rheological measurements [[13], [14], [15], [16], [17],[20], [21], [22], [23], [24]].

Among the reactions for click chemistry, thiol–yne reaction forms branched structures, i.e., 1,2-dithioether, through the addition of two equivalents of thiol to one equivalent of alkyne (Scheme 1) [[25], [26], [27]]. Thus, through the thiol–yne reaction, network polymers are prepared from dithiol and diyne compounds [28,29]. We focus on this simple system for preparation of homogeneous network polymers. In this study, we modify monodisperse oligo(ethylene glycol) (MD-OEG) samples with thiol and yne moieties on both ends, and carry out thiol–yne reaction of these modified MD-OEG samples to form homogeneous network polymers. We also study the effect of the degree of polymerization of MD-OEG on the mechanical properties of the network polymers obtained. The thiol–yne reaction of similar components has been reported by Han et al. [30,31] for the formation of hyperbranch polymers, in which dilute conditions were utilized to avoid the formation of network polymers. In this study, however, we conduct the thiol–yne reaction under bulk conditions.