Monomers derived from glucose and galactose, which contain an endocyclic alkene (in the sugar ring) and a terminal alkyne, underwent a cascade polymerization to prepare new polymers with the ...ring-opened sugar incorporated into the polymer backbone. Polymerizations were well-controlled, as demonstrated by a linear increase in molecular weight with monomer-to-initiator ratio and generally narrow molecular weight dispersity values. The living nature of the polymerization was supported by the preparation of a block copolymer from two different sugar-based monomers. The resulting polymers were also fully degradable. They underwent fast and complete depolymerization to small molecules under acidic conditions.
We explored the mechanochemical degradation of bottlebrush and dendronized polymers in solution (with ultrasonication, US) and solid states (with ball‐mill grinding, BMG). Over 50 polymers were ...prepared with varying backbone length and arm architecture, composition, and size. With US, we found that bottlebrush and dendronized polymers exhibited consistent backbone scission behavior, which was related to their elongated conformations in solution. Considerably different behavior was observed with BMG, as arm architecture and composition had a significant impact on backbone scission rates. Arm scission was also observed for bottlebrush polymers in both solution and solid states, but only in the solid state for dendronized polymers. Motivated by these results, multi‐mechanophore polymers with bottlebrush and dendronized polymer architectures were prepared and their reactivity was compared. Although dendronized polymers showed slower arm‐scission, the selectivity for mechanophore activation was much higher. Overall, these results have important implications to the development of new mechanoresponsive materials.
The mechanochemical degradation of bottlebrush and dendronized polymers with ultrasonication (solution state) and ball‐mill grinding (solid state) was explored. This study provides important insight into the influence of polymer architecture on mechanochemical reactions and the difference in reactivity that is observed in solution and solid states, which has important implications in the development of new mechanoresponsive materials.
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.
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 head-to-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.
Conspectus Metathesis cyclopolymerization (CP) of α,ω-diynes is a powerful method to prepare functional polyacetylenes (PAs). PAs have long been studied due to their interesting electrical, optical, ...photonic, and magnetic properties which make them candidates for use in various advanced applications. Grubbs catalysts are widely used throughout synthetic chemistry, largely due to their accessibility, high reactivity, and tolerance to air, moisture, and many functional groups. Prior to our entrance into this field, only a few examples of CP using modified Grubbs catalysts existed. Inspired by these works, we saw an opportunity to expand the accessibility and utility of Grubbs-catalyzed CPs. We began by exploring CP with popular and commercially available Grubbs catalysts. We found Grubbs third-generation catalyst (G3) to be an excellent catalyst when we used strategies to stabilize the propagating Ru carbene, such as decreasing the polymerization temperature or using weakly coordinating solvent or ligands. Controlled living polymerizations were demonstrated using various 1,6-heptadiyne monomers and yielded polymers with exclusively 5-membered rings (via α-addition) in the polymer backbone. The strategy of stabilizing the Ru carbene was also critical to successful CP with Hoveyda-Grubbs second-generation (HG2) and Grubbs first-generation (G1) catalysts. We found that decomposed Ru species were catalyzing side reactions which could be completely shut down by decreasing the reaction temperature or using weakly coordinating ligands. While HG2 generally led to uncontrolled polymerizations, we found it to be an effective catalyst for monomers with very large side chains. G1 displayed broader functional group tolerance and thus broader monomer scope than G3. We next looked at our ability to change the regioselectivity of the polymerization by using Z-selective catalysts which favor β-addition and the formation of 6-membered rings in the polymer backbone. While modest β-selectivity could be obtained using Grubbs Z-selective catalyst at low temperatures, we found that by using one of Hoveyda and co-workers’ catalysts with decreased carbene electrophilicity, we could achieve exclusive formation of 6-membered rings. We also pursued alternative routes to achieve 6+-membered rings in the polymer backbone by using diyne monomers with increased distance between alkynes. We found that optimizing the monomer structure for CP was an effective strategy to achieve controlled polymerizations. By using bulky substituents (maximizing the Thorpe-Ingold effect) and/or using heteroatoms (shorter bonds) to bring the alkynes closer together, controlled living CP could be achieved with various 1,7-octadiyne and 1,8-nonadiyne monomers. Finally, we took advantage of several inherent properties of controlled CP techniques to prepare polymers with advanced architectures and nanostructures. For instance, the living nature of the polymerization enabled production of block copolymers, the tolerance of very large substituents enabled production of dendronized and brush polymers, and the insolubility or crystallinity of some monomers was utilized for the spontaneous self-assembly of polymers into various one- and two-dimensional nanostructures. Overall, the strategies of stabilizing the propagating Ru carbene, modulating the selectivity and reactivity of the Ru carbene, and enhancing the inherent reactivity of monomers were key to improving the utility and performance of CP with Grubbs-type catalysts. The insight provided by these studies will be important for future developments of CP and other metathesis polymerizations utilizing ring-closing steps.
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.
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.
The influence of ball-mill grinding process parameters and polymer properties on the mechanochemical degradation of amorphous polymers was explored. For process parameters, the grinding frequency was ...found to have the greatest impact on the degradation rates of polystyrene (PS), with ca. 17 times difference in rate constants between the lowest and the highest frequencies studied. For polymer properties, molecular weight and molecular weight dispersity were shown to both influence degradation rates but in an indirect manner. Linear relationships were found between degradation rate constants and the initial glass transition temperature (T g) for PS and poly(methyl methacrylate) samples, suggesting that the T g was a better predictor of the degradation rate than molecular weight (above a limiting molecular weight value). Ultrasonication experiments also further highlighted that polymer properties can have a disparate impact on mechanochemical reactivity depending on the method used to apply mechanical forces to polymers.
An unsaturated polymer's
/
-olefin content has a significant influence on its properties. For polymers obtained by ring-opening metathesis polymerization (ROMP), the
/
-olefin content can be tuned by ...using specific catalysts. However,
-selective ROMP has suffered from narrow monomer scope and lack of control over the polymerization (giving polymers with broad molecular weight distributions and prohibiting the synthesis of block copolymers). Herein, we report the versatile
-selective controlled living ROMP of various
-tricyclo4.2.2.0
deca-3,9-diene and various norbornene derivatives using a fast-initiating dithiolate-chelated Ru catalyst. Polymers with
-olefin content as high as 99% could be obtained with high molecular weight (up to
of 105.1 kDa) and narrow dispersity (<1.4). The living nature of the polymerization was also exploited to prepare block copolymers with high
-olefin content for the first time. Furthermore, owing to the successful control over the stereochemistry and narrow dispersity, we could compare
- and
-rich polynorbornene and found the former to have enhanced resistance to shear degradation.
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 p)) or the square root of monomer molecular weight (M mon 0.5). 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.
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