Recent research on reworkable network polymers, which can degrade after use, are reviewed. Using naturally occurring chemical substances can contribute to the realization of a low-carbon society, and the reworkable network polymers that can be removed by crosslinking after use should be useful as environmentally friendly materials. In this paper, recent researches to control polymer networks using reworkable network polymers containing structures derived from limonene, a naturally occurring chemical substance are reviewed. The synthesis, crosslinking, and degradation of reworkable network polymers and their application to functional materials are outlined.

The design of degradable polymeric materials on demand is highly desired. The excellent mechanical and thermally stability of the networked polymers prevents from recycling them after use. Although several technologies have been proposed for the degradable materials, photochemical method is useful because of easy spatiotemporal control and small damage on many kinds of substrates. Polyphthalaldehydes (PPAs) are well known as main chain of polymers that can be end-to-end depolymerized (unzipping). We have synthesized PPAs with oxime ether terminals, and their photoreactions were investigated on UV-irradiation both in solution and film states by spectral measurements. In addition, the results of nanoindentation measurement indicated that the films became elastic on irradiation. Also, we have prepared PPA copolymers with butyl acrylate (BA) and investigated the peel strength changes on irradiation to evaluate their pressure-sensitive adhesive properties. In this review, PPAs and their application as functionalized materials are reviewed, including our photodegradable PPAs.

Diacylhydrazine is a thermally and chemically stable functional group, although it is oxidatively degraded rapidly into a carboxylic acid by an artificial oxidizing agent such as sodium hypochlorite. Therefore, a network polymer bearing a diacylhydrazine moiety as a cross-linking site exhibits high thermal stability, chemical resistance, and weather resistance during use without being decomposed by natural stimuli such as heat, acid, water, and sun-light. However, it can be decrosslinked immediately after use when it is treated with sodium hypochlorite solution. The epoxy resin cured with a curing agent bearing a diacylhydrazine moiety could be used as an oxidatively degradable adhesive that can be oxidatively dismantled after use. The polymer bearing an ester moiety at the side-chain could be cross-linked by the formation of a diacylhydrazine moiety, and decrosslinked by its oxidative degradation to reproduce the original polymer. A transparent monolith crosslinked with a diacylhydrazine moiety was obtained by the reaction injection molding, and it could be solubilized with sodium hypochlorite solution. The superabsorbent polymer crosslinked with a diacylhydrazine moiety was immediately decomposed into sodium polyacrylate by sodium hypochlorite solution.

Biobased polyimides are synthesized from 4-aminocinnamic acid which was produced by a microorganism having genetically modified sugar metabolism. 4-Aminocinnamic acid can be photodimerized by ultraviolet, which is also a characteristic of cinnamate, and can be converted into biobased diamine. The obtained diamine can be employed into biobased polyimide syntheses by reacting with various tetracarboxylic dianhydrides, and exhibits the extreme high thermal resistivity among the existing bioplastics. In addition, the water-soluble polyimide was prepared by the treatment of biobased polyimide bearing ester side groups in an aqueous alkali metal solution. This water-soluble polyimide can also form a network structure by substitution with a polyvalent metal cation and impart water-insoluble properties. Furthermore, hydrogel is obtained by the condensation reaction of the side chain carbonyl group with water-soluble diamine.

In May 2019, the Ministry of Economy, Trade and Industry announced that it would promote the research and development of marine biodegradable plastics through industry, government, and academia as one of the solutions to the problems caused by marine plastic litter. At present, the number of candidate materials for marine biodegradable plastics is very limited, and their physical properties do not necessarily meet the requirements of the relevant applications. In order to achieve this, it is desirable to develop materials based on new ideas. In this article, we first introduce the science and technology behind the development of marine biodegradable plastics. In addition, the timing of the onset of degradation of biodegradable plastics in the ocean and the control of the rate of degradation to achieve timed biodegradability will be explained with examples.