Macromolecules最新文献

Mechanophores are a class of molecules that undergo changes in their optical properties, following a chemical reaction triggered by a force. Integrated in elastomeric materials, their ability to report molecular damage opens up a new toolbox to tackle diverse problems in polymer mechanics and durability. To be effective, mechanophores need to be included in a load-bearing position, i.e., as a cross-linker of networks. This is well-mastered in polyacrylates or polyurethanes but is still challenging in industrially relevant elastomers such as polydienes because of the harsh conditions of temperature associated with the typical fabrication of these materials. Here, we functionalize a damage-reporting π-extended anthracene-maleimide mechanophore (DACL) with nitrile oxide (CNO) in order to incorporate it as a cross-linker in a poly(styrene-co-butadiene) random copolymer. The incorporation occurs by click chemistry through a mild 1,3-cycloaddition between the CNO-functionalized DACL and the double bonds present in SBR that can take place at room temperature. Finally, we demonstrate the successful activation by force of the DACL mechanophore in the SBR by performing a fracture experiment. Given the industrial and scientific relevance of the highly entangled polydiene elastomers, our study opens up exciting possibilities for damage reporting in industrial rubbers.

The control over coordination geometry in metallo-supramolecular polymer networks not only helps developing new polymer structures but also provides new measures of metal complex characteristics. Herein, we compare multiscale characteristics of networks obtained by homo- and heteroleptic association of mono-, bi-, and tridentate ligands, as reflected in rheological measurements and DFT calculations. Accordingly, tetra-arm poly(ethylene glycol) (tetraPEG), functionalized with pyridine, phenanthroline, and terpyridine, form parent homoleptic networks, while their combination with a tetraPEG functionalized by the sterically demanding dimesitylene substituted phenanthroline forms heteroleptic networks. Among all employed transition metal ions, only Cu^+/2+ and Pd²⁺ comply with the coordination geometry necessities of all parent and mixed networks, ranging from trigonal to octahedral, providing good candidates for wide network topology rearrangements. Our rheological measurements and DFT calculations demonstrate that the stability of homoleptic complexes depends primarily on metal identity, whereas that of heteroleptic equivalents increases with increasing the denticity of the slim ligand, being a complex function of steric, electronic, and π–π interactions.

Polymer networks are widely used in engineering and biomedical applications because they can sustain large deformations. However, their mechanical properties, particularly at large strains, remain challenging to design within their molecular architecture through conventional synthetic methods, as these offer limited control over the kinetics and thermodynamics of gelation and, in turn, the connectivity of the polymers. In this work, we leverage recent advances in Reversible Deactivation Radical Copolymerizations (RDRPs) to tune the kinetics and thermodynamics of gelation and explore their impact on the molecular architecture and mechanical properties of polymer networks. We demonstrate that RDRPs lead to delayed gelation, phase separation, and softer and more extensible networks relative to conventional free radical copolymerizations. The reversible deactivation of the radical chain ends slows the kinetics of gelation, segregates the network precursors or clusters into cross-linker-rich and cross-linker-poor phases, and narrows the distribution of chain lengths within the polymers. This impact of the kinetics of gelation on the molecular architecture affects the load distribution among the constituent polymers and the interplay between the small- and large-strain mechanical properties. Overall, this work paves the way for rationally using polymer chemistry to design advanced polymer networks for emerging and more stringent applications.

Highly conjugated polymers possessing the NSS functional group were synthesized to investigate the colors obtained from polymers with this understudied functional group. Five aromatic diamines were polymerized with sulfur monochloride (S₂Cl₂) to yield high molecular weight cross-linked polymers. The polymerizations were completed with ratios of amine to sulfur monochloride of 1.5:1, 1:1, 1:1.5, and 1:2 to yield polymers with different levels of cross-linking and molecular weights from 15 to 5100 kg mol^–1. The compositions were investigated by elemental analysis to confirm that the amount of sulfur within the polymers agreed with predictions based on the ratio of monomers used in their synthesis. These polymers were brightly colored with colors from yellow, red, brown, green, and blue, and their UV–vis spectra were measured to determine the absorption maxima. Several control experiments were completed that demonstrated that the reactions between monomers occurred between the amines and S₂Cl₂ rather than electrophilic aromatic substitution reactions. The polymers were air stable but completely degraded in the presence of 2-mercaptoethanol. In each of these degradations, the diamine monomer was isolated without any evidence of a thiol on the ring, as would be expected for electrophilic aromatic substitution. These polymers were investigated for their ability to act as sensors for 2-mercaptoethanol by exposing them to 2-mercaptoethanol and measuring the change in the color. The reactions between the amines and S₂Cl₂ were rapid and used to print 2D objects with a variety of colors.

A series of sulfur-free chain transfer agents (CTAs) for radical polymerization were synthesized to control the molecular weight of acrylic polymers for pressure-sensitive adhesion (PSA). PSAs are commonly used in daily life and industrial manufacturing, and their mechanical properties can be tuned by balancing adhesion and cohesion depending on the molecular weight of the polymer. The molecular weight is typically controlled by adding CTAs, such as n-dodecyl mercaptan (NDM), to the radical polymerization system, although sulfur compounds often cause problems, such as bad smell and metal corrosion. Therefore, sulfur-free CTAs were designed by substituting methacrylates and methacrylonitrile with phenyl groups and malonate skeletons. Among them, CTA containing a methacrylonitrile skeleton functioned efficiently in the solution and suspension polymerizations of (meth)acrylates, although it exhibited lower reinitiation efficiency than NDM. Nevertheless, the extension of the reaction time to 6 h resulted in monomer conversion and number-average molar mass (M_n) comparable to the polymerization in the presence of NDM. The resulting polymers did not lead to copper corrosion, while the PSAs prepared with these polymers exhibited mechanical properties comparable to those of conventional PSA in peel, creep, and probe-tack tests. PSAs that realize sufficient strength and no metal corrosion are particularly effective in applications involving LED elements and electronic circuit boards, where silver electrodes and wires are in direct contact with PSAs.

Crystalline–amorphous diblock copolymers (BCPs) comprising crystalline blocks of isotactic polypropylene (iPP) or polyethylene (PE) linked to amorphous blocks of random ethylene–propylene copolymers (EPR) (iPP-b-EPR and PE-b-EPR) of different block lengths and ethylene concentrations in EPR blocks, and crystalline–crystalline BCPs composed by iPP and PE blocks (iPP-b-PE) have been synthesized with different living catalysts. The effects of the presence of a linked EPR rubbery block of varying composition on the crystallization behaviors and kinetics of PE and iPP, and of crystalline PE or iPP block on the crystallization kinetics of linked iPP or PE, respectively, have been analyzed. All samples have been isothermally crystallized from melting at different temperatures, and the crystallization kinetics have been analyzed. In iPP-b-PE BCPs, the PE block crystallizes first from the melt during nonisothermal cooling or isothermal crystallization. The iPP block crystallizes after PE and nucleates over the PE crystals. In both iPP-b-EPR and PE-b-EPR BCPs, the linked amorphous EPR block slows down the crystallization of iPP and PE blocks with respect to their respective homopolymers. Furthermore, in iPP-b-EPR samples, a higher concentration of propylene in the EPR phase results in a more significant slowdown of the crystallization kinetics of iPP due to a higher solubility between the blocks. Analogously, in PE-b-EPR copolymers, the increase in the length of the EPR block results in a more pronounced slowdown of the crystallization rate of PE with respect to the PE homopolymer due to the significant dilution exerted by the long EPR block. All samples of iPP-b-PE copolymers show crystallization rates lower than that of the PE homopolymer but faster than that of the iPP homopolymer. In isothermal crystallization experiments, the iPP blocks do not crystallize, not even at low crystallization temperatures, but crystallize upon successive cooling, nucleating over the PE crystals formed in the isothermal step. Therefore, the linked iPP, which remains in the melt during the isothermal crystallization, slows down the crystallization kinetics of PE, in contrast to what happens in a sample of iPP/PE blend, where the crystallization kinetics of PE is not affected by the presence of the phase-separated iPP.