Novel monomers for photopolymer networks

Abstract

Photopolymerization technology to transform monomers into polymer network materials has been widely applied to enable various conventional and emerging applications such as dentistry, coatings, adhesives, holography, optics, microelectronics, and 3D printing, to name but a few. Monomers play a critical role both in determining the properties of photopolymers and, thus, their ultimate applications. This review article highlights the recent progress in the development of novel monomers which can be used in photopolymerization for advanced materials design. An overview on some of the fundamental concerns for the demand of novel monomers is given first. Subsequently, recent progress towards novel (meth)acrylate monomers for free radical polymerization, monomers for thiol-ene polymerization, and epoxide monomers for ring-opening cationic polymerization are summarized and the structure-property relationships of the resulting photo-crosslinked polymers are discussed. Furthermore, recent innovations of novel photo-controlled reaction types for photopolymer network formation such as polymerization initiated by photobase generation and photo-click reactions are discussed. The review aims to not only provide researchers an up-to-date survey of monomers for the design of functional photopolymer materials but also inspire advances in the development of monomers that will enable the development of high-performance functional photopolymer network materials.

Introduction

Polymer networks formed by the photopolymerization approach have found wide applications in dentistry, coatings, adhesives, holography, optics, microelectronics, and 3D printing, etc [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18].Photopolymerization resins are commonly formulated by photoinitiators (PIs), polymerizable monomers/oligomers, fillings, and additives. Over the past decade, the rapid development of novel PIs has fostered the production of a series of PIs with the fast initiation and unprecedented ability for photopolymerization reactions under visible light. The progress in developing novel PIs has been well summarized in the previously published reviews and books [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31].Compared to the small amount and solely initiating function of PIs in the photopolymer matrices, monomers generally constitute the vast majority of the organic components, which primarily affect the properties of the photocured composites, including the mechanical strength, sorption, polymerization shrinkage, abrasion resistance, and biocompatibility, etc [32].Numerous efforts have been devoted to developing novel monomers to address the issues related to the existing monomer systems and explore monomers with interesting properties which have never been reported before. For instance, 2,2-Bis [4–2-hydroxy-3-methacryloyloxypropyl)phenyl]propane (Bis-GMA) has been widely used as a component in dentistry since its introduction in 1960s [33].Concerns have been raised about the potential for Bis-GMA to release bisphenol A (BPA), which can induce estrogenicity and cause endocrine disruption [34].The exploration of BPA-free monomers to reduce possible adverse health issues is presently under active investigation. Besides, researchers in academy and industry have been constantly unraveling the potential for light-assisted additive manufacturing (also known as VAT photopolymerization or photopolymerization-based 3D printing) in advanced material engineering, rapid prototyping and flexible production [35], [36], [37], [38], [39], [40], [41], [42].However, these advancements call for new materials which can meet the demands to match these 3D manufacturing processes and allow the easy tailoring of structural and functional properties of the 3D printed parts [43].Moreover, the new materials still require being comparable in cost and properties to those of traditional manufacturing. In addition, the photopolymerization of monomers with stimuli-responsive linkage to fabricate smart materials are emerging [44].Strong efforts have also been devoted into the implementation of renewable resources into photopolymerization resins to lower the petroleum-based chemical dependency [45].However, these researches mentioned above are not well summarized as the development of the newly developed efficient PIs.

Within the context of the rapid development in the above-mentioned areas, the purpose of this review is to summarize the work in the past decade in designing and developing novel monomers to fabricate polymer network materials under light irradiation and expanding the applications of developed monomers, to elucidate the structure-function relationship in the reported monomers, and to extract the principle in designing future monomer systems with specific functions. Accordingly, we will focus our attention on discussing monomers which were reported to form polymer network materials under light irradiation. However, some newly developed epoxide monomers operating in different modes such as thermal curing and exhibiting potential to be adapted in photopolymerization will also be introduced in this article. This review will be organized into sections according to polymerizable units in the monomers which include prevailing (meth)acrylate, allyl and thiol monomers, epoxides, and new monomers polymerized via other chain growth or step growth polymerization pathways which are initiated by light. This review is expected to provide the guidance for researchers working in chemistry and material areas in designing and selecting monomer systems to fabricate photopolymer materials with desirable performance.