Biological systems rely on recyclable materials resources such as amino acids, carbohydrates and nucleic acids. When biomaterials are damaged as a result of aging or stress, tissues undergo repair by a depolymerization-repolymerization sequence of remodelling. Integration of this concept into synthetic materials systems may lead to devices with extended lifetimes. Here, we show that a metastable polymer, end-capped poly(o-phthalaldehyde), undergoes mechanically initiated depolymerization to revert the material to monomers. Trapping experiments and steered molecular dynamics simulations are consistent with a heterolytic scission mechanism. The obtained monomer was repolymerized by a chemical initiator, effectively completing a depolymerization-repolymerization cycle. By emulating remodelling of biomaterials, this model system suggests the possibility of smart materials where aging or mechanical damage triggers depolymerization, and orthogonal conditions regenerate the polymer when and where necessary.

Self-immolative polymers (SIPs) are unique macromolecules that are able to react to multiple types of environmental influences by giving amplified response outputs. When triggering moieties installed at SIP chain ends are activated by their corresponding stimuli, a spontaneous headto-tail depolymerization ensues, often involving multitopic release of small molecules. SIP designs have evolved a high degree of modularity in each of their functional components, enabling a broad range of utility and applications-driven tuning. In this Perspective, we summarize and discuss recent progress in this nascent area of research, including (i) synthesis of different types of SIPs, (ii) design and evaluation of triggering moieties, (iii) depolymerization mechanisms and kinetics, (iv) applications of SIPs, and (v) outlook and challenges facing the field.

We describe the preparation and characterization of photo- and mechanochromic 3D-printed structures using a commercial fused filament fabrication printer. Three spiropyran-containing poly(ε-caprolactone) (PCL) polymers were each filamentized and used to print single- and multicomponent tensile testing specimens that would be difficult, if not impossible, to prepare using traditional manufacturing techniques. It was determined that the filament production and printing process did not degrade the spiropyran units or polymer chains and that the mechanical properties of the specimens prepared with the custom filament were in good agreement with those from commercial PCL filament. In addition to printing photochromic and dual photo- and mechanochromic PCL materials, we also prepare PCL containing a spiropyran unit that is selectively activated by mechanical impetus. Multicomponent specimens containing two different responsive spiropyrans enabled selective activation of different regions within the specimen depending on the stimulus applied to the material. By taking advantage of the unique capabilities of 3D printing, we also demonstrate rapid modification of a prototype force sensor that enables the assessment of peak load by simple visual assessment of mechanochromism.

Shape memory materials have emerged as an important class of materials in medicine due to their ability to change shape in response to a specific stimulus, enabling the simplification of medical procedures, use of minimally invasive techniques, and access to new treatment modalities. Shape memory polymers, in particular, are well suited for such applications given their excellent shape memory performance, tunable materials properties, minimal toxicity, and potential for biodegradation and resorption. This review provides an overview of biodegradable shape memory polymers that have been used in medical applications. The majority of biodegradable shape memory polymers are based on thermally responsive polyesters or polymers that contain hydrolyzable ester linkages. These materials have been targeted for use in applications pertaining to embolization, drug delivery, stents, tissue engineering, and wound closure. The development of biodegradable shape memory polymers with unique properties or responsiveness to novel stimuli has the potential to facilitate the optimization and development of new medical applications.

The effect of star versus linear polymer architecture on the rates of mechanochemically induced bond scission has been explored. We determined rate constants for chain scission of parent linear and star polymers, from which daughter fragments were cleanly resolved. These studies confirm a mechanistic interpretation of star polymer chain scission that is governed by the spanning rather than total molecular weight. We further demonstrate the preserved rate of site-selective mechanophore activation across two different polymer structures. Specifically, we observed consistent activation rate constants from three-arm star and linear polymer analogues, despite the M n of the star polymer being 1.5 times greater than that of the linear system. Letter pubs.acs.org/macroletters

We describe an efficient method to produce objects comprising spatially controlled and graded cross-link densities using vat photopolymerization additive manufacturing (AM). Using a commercially available diacrylate-based photoresin, 3D printer, and digital light processing (DLP) projector, we projected grayscale images to print objects in which the varied light intensity was correlated to controlled cross-link densities and associated mechanical properties. Cylinder and bar test specimens were used to establish correlations between light intensities used for printing and cross-link density in the resulting specimens. Mechanical testing of octet truss unit cells in which the properties of the crossbars and vertices were independently modified revealed unique mechanical responses from the different compositions. From the various test geometries, we measured changes in mechanical properties such as increased strain-to-break in inhomogeneous structures in comparison with homogeneous variants.

New polyphenylene-based dendronized polymers (denpols), exhibiting extended and rigid conformations, were prepared using ring-opening metathesis polymerization (ROMP). Their mechanochemical degradation was explored in ultrasound-induced elongational flow fields. Degradation rate constants were obtained for polyphenylene-based denpols, of varying generation, across a degree of polymerization (DP) range of ∼100-600. In general, it was found that larger side chains led to increased degradation rates and that the rate enhancement was proportional to the natural log of persistence length (Ln( l)) or the square root of monomer molecular weight ( M). These relationships led to the generation of "master curves" in which the rate constant trends for each polymer series converged, enabling accurate prediction of degradation rate constants for related polymers bearing long alkyl chains or ester-type dendrons. Furthermore, we observed evidence for, and used computational modeling to support, polymer chains undergoing multiple scissions during a single elongation event, leading to faster degradation of daughter fragments that come from parent polymers with large side chains.

Typical multimechanophore polymers (MMPs) are comprised of numerous mechanophores (force-responsive moieties) distributed throughout the backbone of linear polymers. We have developed a new MMP design based on graft polymers with mechanophores linking each arm to the backbone. By utilizing maleimide–anthracene cycloadducts, polymeric species containing anthracene were released from the parent polymer, enabling facile quantification of mechanophore activation. With pulsed ultrasound experiments, we observed that mechanophore activation was dependent on the arm length (a faster rate with longer arms), and we observed that 85% of the polystyrene (PS) arms underwent scission (64% specifically at the mechanophore site) for a graft polymer with 23 kDa arms. Solid-state activation was also investigated with hand-grinding experiments. Fast reactions were observed, with up to 96% of PS arms undergoing scission and 70–75% of mechanophores being activated, for all arm lengths studied. Multimechanophore graft polymers provide important insight into the distribution of forces in topologically complex polymers and may enable the development of new mechanoresponsive materials.