Volume 56 Issue 4, April 2024

In this review article, an overview of recent studies on heat-responsive and photoresponsive adhesive materials and the characteristics of other external stimuli used for dismantlable adhesive systems are described. Then, research on dismantlable adhesive materials using polyperoxides is introduced as an example of early material design. Next, the development of a dual-stimuli responsive dismantlable adhesive material is interpreted as a material design for achieving stability during use and degradability during dismantling. Finally, recent studies on heat-responsive, dismantlable adhesive materials, which are thermally stable during use while responding quickly during disassembly by heating, are described.

Mechanochemistry is a promising technology to tackle current and future polymer waste streams for a sustainable future. With this review, we take into account synthetic, computational, technical, and engineering perspectives to converge trituration and polymer mechanochemistry with a particular focus on the fate of commodity polymers and potential technologies to monitor mechanochemical reactions while they occur. We highlight the need for future transdisciplinary research to tackle the high-leverage parameters governing an eventually successful mechanochemical polymer degradation approach for a circular economy.

Photoinduced transitions between the solid, glass, and liquid states based on molecular photoswitches promise an enormous variety of applications, such as photoswitchable adhesives, which contribute to material recycling for a sustainable future in the era of composite materials. In this review, we highlight recent progress in the photoinduced transitions of small molecules and polymers and systematically discuss the molecular designs, mechanisms, applications, merits and demerits, and future challenges in each photoswitch and the whole field.

Our recent studies on the synthesis, characterization, degradation and applications of vinyl polyperoxides are reviewed. Primarily, the recent achievements in the design, biocompatibility, thermal and enzymatic degradation of water-soluble vinyl polyperoxides and copolyperoxides are described. Finally, future development possibilities and challenges of vinyl polyperoxides for various potential applications are summarised.

Photochemically adaptable polymer materials are widely used in the fields of medicine, electronics, and engineering due to their precise and remote processability. Diverse designs of light-responsive units have been developed to fabricate various photocontrollable materials with low-energy, rapid, and reversible photoreactivity. Recently, multiple stimuli-responsive materials have been demonstrated to further control their photoreactivity by combining light with another stimulus, leading to advanced photocontrollable capabilities. This Focus Review summarizes the recent progress in developing photochemically adaptable polymer network materials by designing photoresponsive units, focusing on the chemical structures of cleavable moieties.

Monomer sequence transformation and reversible polymerization–depolymerization control were achieved in the cationic copolymerization of 2-methyl-1,3-dioxepane and γ-butyrolactone by vacuuming or temperature change. The removal of monomers from the polymerization solution with a vacuum pump promoted depolymerization, resulting in a transformation to pseudo-alternating copolymers and subsequent oligomer formation. Heating the polymerization solution also promoted a similar monomer sequence transformation and oligomer formation, while cooling led to the polymerization of monomer molecules generated via depolymerization.

Ester free type trimethylene carbonate derivatives bearing bulky aromatic groups were designed and synthesized, including one or two benzyl groups, diphenylmethyl groups, and triphenylmethyl groups. The mechanical properties of the polymers were estimated by compressive test, resulting in around 0.3MPa at most. The degradation behaviors were also evaluated under Lipase solution and 0.01M NaOHaq. for several weeks, revealing the substituents influenced on the prevention of degradation at most cases. The degradation behavior was slowly observed, which were analyzed by weight loss, SEM images, SEC, TGA, and DSC.

Oligoisoprene macromonomer, which bears a terminal vinyl group and cis-1,4 regularity, was prepared by the metathesis degradation of high-molecular-weight polyisoprene with ethylene in a high selectivity and yield. The ethenolysis is also applicable for the degradation of natural rubber-derived polyisoprene, although the catalytic activity decreased. The prepared oligoisoprene macromonomer was successfully copolymerized with ethylene similarly with 1-hexadecene using a phenoxyimine-ligated titanium catalyst. The oligoisoprene-grafted polyethylene showed a typical stress‒strain curve, of which the tensile modulus and yielding stress are comparable to those of linear low-density polyethylene.

To examine the behavior of degradation by a conjugate substitution reaction in aqueous media, the degradation of a poly(conjugated ester), prepared from 1,4-butylene bis[α-(bromomethyl)acrylate] and fluorescein, with aqueous solution of various nucleophiles was investigated using UV‒vis spectrometry and size-exclusion chromatography. At the earliest stage of the degradation, the affinity between the nucleophile and the polymer affected the reaction rate, leading to faster degradation for relatively hydrophobic amines. On the other hand, hydrophilic nucleophiles that produce water-soluble degradation products promoted complete chain scission.

The hydrogenolysis of PEG-modified soluble lignin was investigated with series of transition metal complexes to afford alkyl guaiacols. Although the introduced PEG moiety was also susceptible against the hydrogenolysis, in situ modification in PEG solvent was found effective for maintaining the lignin soluble and improved degradation efficiency.

We developed a novel method for synthesizing degradable polymers based on 1,5-shift radical isomerization polymerizations of vinyl ethers with transferable atoms or groups and in-between acid-cleavable ether linkages in the side chains. In particular, vinyl ethers with side chains composed of thiocyano and p-methoxybenzyl ether groups underwent radical isomerization polymerizations via 1,5-shifts, in which a vinyl ether radical abstracted the cyano group intramolecularly to generate a thiyl radical and result in a polymer with a p-methoxybenzyl ether linkage in the main chain.

The depolymerization of super engineering plastics such as polysulfone (PSU) smoothly proceeded in the presence of methanol mediated by sodium hydroxide in 1,3-dimethyl-2-imidazolidinone (DMI) at 80 °C to form bis(4-methoxyphenyl)sulfone and 4,4’-(propane-2,2-diyl)diphenol (bisphenol A) in high yields. These products were readily isolated by simple filtration. The DMI solvent effectively promoted depolymerization and allowed insoluble resins such as polyetheretherketone (PEEK) to undergo the reaction. This method was applicable to other alcohols, such as ethanol and isopropyl alcohol.

Photooxidation and fragmentation behaviors of itPP were studied. Photooxidation likely occurred in the amorphous regions of itPP due to the higher oxygen diffusion. Surface deterioration was observed on the UV-exposed itPP films. Pressed films exhibited much denser cracks compared to uniaxially oriented itPP films. Notably, cracks in the uniaxially oriented itPP films were formed along the direction of orientation and decreased with increasing draw ratio. The crystalline structure and oriented molecular chains notably inhibited the photooxidative degradation and fragmentation of the itPP films.

A degradable cross-linked polybutadiene was synthesized using a peptide-based cross-linker. As a degradable cross-linker, we synthesized poly(L-cysteine) (polyCys) by papain-catalyzed chemoenzymatic polymerization. The thiol-ene reaction between polybutadiene and polyCys was conducted in the presence of a radical initiator, resulting in the formation of an insoluble polybutadiene gel. The successful cross-linking and network formation was confirmed by Raman spectroscopy and viscoelastic analysis. Dynamic viscoelastic measurements of the cross-linked polybutadiene revealed the disappearance of slow relaxation mode. The polyCys-linked polybutadiene underwent degradation through acidic hydrolysis, allowing the recovery of soluble polybutadiene.

Overview of the reusable dismantlable adhesion interface system. Heating induces a cleavage reaction of the anthracene photodimer in the molecular layer at the adhesion interface, and the anthracene monomer remains on the substrate surface of the peeled specimen. Photoirradiation to the cleaved molecular layer induces the photodimerization of anthracenes, and the materials exhibit strong adhesion at the adhesion interface.

Amine-cured epoxy resins bearing ester moieties were synthesized, and their properties, hydrolytic degradation behavior, and biomineralization were investigated. Neopentyl glycol diglycidate (NPG) was used as the epoxide and was cured with diethylenetriamine and isophoronediamine at different ratios. The T_gs and degradability were controlled using the composition of amines. To demonstrate their potential application as degradable materials for bone and dental repair, composites containing hydroxyapatite were prepared by curing NPG and the amines in the presence of HA. Bone-like apatite was grown on a composite by immersion in synthetic biofluid.

We report the syntheses of novel marine biodegradable poly(ethylene succinate) (PES)- and poly(butylene succinate) (PBS)-based copolymers containing different dicarboxylic acid (DCA) units with various carbon numbers and different feed ratios. Specifically, the copolymers with longer-chain DCA units, even at low contents, exhibited marine biodegradability. The thermomechanical properties also varied with the DCA contents. These results indicated that the thermomechanical properties and the marine biodegradability of the PES- and PBS-based copolymers were regulated by controlling their structures and DCA contents.

Aliphatic polycarbonates with ether side groups linked by amide bonds exhibit high hydrolyzability and antiplatelet properties due to enhanced hydration resulting from strong interactions with amide and ether side groups.

To functionalize a poly(isosorbide carbonate) (PIC)-based polymer and evaluate its recyclability, several types of diol comonomers were copolymerized with isosorbide. The thermal and mechanical properties and decomposition behavior of the PIC copolymers were investigated. The thermal stability of PIC was retained, and its glass transition temperature was systematically controlled by copolymerization. The decomposition of the PIC copolymers upon treatment with aqueous ammonia yielded monomers and urea, and the decomposition rate was governed by the structure of the comonomer.

Steam decomposition of poly(lactic acid) (PLA) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) enhanced the recovery of chemical feedstock compared with simple pyrolysis. Steam enhanced the hydrolysis of PLA and resulted in the formation of short-chain compounds with hydroxyl end groups, and subsequent pyrolysis of them improved lactide recovery. Monomer production from PHBH was also enhanced by simultaneous hydrolysis and pyrolysis under steam decomposition.