Simple and Rapid Adhesion of Commodity Polymer Substrates under Ambient Conditions Using Complexed Alkylboranes.

Adhesives are ubiquitous in manufacturing spanning nearly all sectors from healthcare and photovoltaics to aerospace and electronics. Yet many commercial polymers remain challenging to adhere, necessitating either pretreatment, mechanical fastening, or adhesive processes that involve specialized equipment, high temperature/vacuum, and long cure times. Thus, rapid-cure adhesives for polymers that can set under ambient conditions using simple procedures are desirous because they offer cost savings, faster production, and greater design freedom to producers. Herein, we report a powerful adhesive platform that bonds a wide scope of commodity polymers via (hydrogen) atom transfer and free-radical (graft) polymerization initiated with a trialkylborane-ligand complex and isocyanate decomplexing agent. The developed adhesive formulation is air-stable, bulk, and operates in air at room temperature using a high-glass-transition temperature polyacrylate, i.e., poly(isobornyl acrylate). The alkylborane-initiated bonding process is rapid (∼30 min), requires minimal surface preparation (cleaning and mild roughening), and successfully bonds seven diverse substrates including polytetrafluoroethylene, polyethylene, polypropylene, polycarbonate, nylon, polymethylmethacrylate, and polyvinylchloride. This contribution uniquely investigates the process-property relationships for the adhesive formulation, lap-shear performance, mechanism of failure, and a reactive additive for enhancing the adhesive's glass-transition temperature to ∼120 °C (polyhedral oligomeric silsesquioxane or POSS) to widen its operation temperature. We envision that the reported alkylborane-initiated adhesion platform could hold promise in the automotive, aerospace, and marine sectors as means for rapid manufacturing and structural adhesion.

Alkylborane-Initiated Thiol-Ene Networks for the Synthesis of Thick and Highly Loaded Nanocomposites.

Thiol-ene nanocomposites were synthesized for the first time using an alkylborane-ligand initiator complex under bulk and ambient conditions without external light or thermal stimuli. Initiation was triggered by the in situ decomplexation of an air-stable trialkylborane-amine complex to liberate trialkylborane, which rapidly autoxidizes with atmospheric oxygen and generates free radicals to drive thiol-ene polymerization. This chemically activated mode of initiation uniquely affords thiol-ene nanocomposites with an unrivaled carbon nanotube (CNT) loading of 1.3 wt % and thicknesses of ∼6.7 mm by circumventing restrictions imposed by long pathlengths and light-impeding fillers during photoinitiation. Alkylborane initiation also exhibited advantageous polymerization rates, equivalent to photoinitiation, resulting in network formation and gelation within minutes. Systematic studies were conducted to evaluate comparable alkylborane- and photo-initiated nanocomposites under progressively higher loadings and larger specimen thicknesses, revealing an enhancement or better retainment of mechanical performance in alkylborane-initiated nanocomposites.

Oxygen Tolerant and Room Temperature RAFT through Alkylborane Initiation.

A reversible addition-fragmentation chain transfer (RAFT) process was developed capable of being performed at room temperature and in the presence of oxygen by initiating polymerization through an alkylborane-amine complex. This air-stable alkylborane-amine complex was chemically deblocked with carboxylic acid or isocyanate functionalities to liberate a reactive trialkylborane that consumes oxygen and generates radicals to mediate RAFT. Alkylborane-initiated RAFT (AI-RAFT) was demonstrated to allow the synthesis of a wide range of polymer molecular weights with narrow distributions. Rapid polymerization was also possible within minutes under an ambient environment without any prior deoxygenation. Optimal conditions were investigated revealing that carboxylic acids are required in larger excess to alkylborane versus isocyanates and that deblocker functionality can have an impact on polymerization kinetics, achievable molecular weight, and dispersity. Living chain-ends were confirmed by synthesizing block copolymers using AI-RAFT-derived macro-chain transfer agents. In this preliminary study, a chemically induced RAFT process is introduced without requirement of any thermal, photochemical, electrical, or mechanical stimulus capable of polymerizing acrylamide, acrylate, and methacrylate monomers in limited amounts of oxygen at room temperature.

Functionalization-Induced Self-Assembly of Block Copolymers for Nanoparticle Synthesis.

Nanoparticle synthesis was demonstrated via functionalization-induced self-assembly (FISA) of block copolymers using Suzuki-Miyaura cross-coupling. In situ self-assembly was triggered in organic media by the progressive installation of solvophobic pendant groups onto an initially soluble diblock copolymer, rendering the reactive block insoluble and causing the formation of spherical polymeric micelles. Self-assembly was found to depend on the percent functionalization (f%), where after a critical threshold micelles were accessible that increased in size with increasing f% values. We found the chemical nature of the installed functional group to be crucial for conducting FISA and for controlling the solution morphology, with relatively solvophilic adducts remaining as unimers and increasingly solvophobic adducts trending toward larger micelles, from ca. 40 to 100 nm in diameter. The core and corona of the anticipated micellar structure were visualized using fluorine mapping through electron energy loss spectroscopy, in conjunction with FISA achieved through pendent trifluorophenyl functionality. This work establishes FISA as a new, versatile synthetic strategy to create nanoparticles having tunable morphologies with potential application as molecular payload delivery vehicles.

Genetically targeted fluorogenic macromolecules for subcellular imaging and cellular perturbation.

The alteration of cellular functions by anchoring macromolecules to specified organelles may reveal a new area of therapeutic potential and clinical treatment. In this work, a unique phenotype was evoked by influencing cellular behavior through the modification of subcellular structures with genetically targetable macromolecules. These fluorogen-functionalized polymers, prepared via controlled radical polymerization, were capable of exclusively decorating actin, cytoplasmic, or nuclear compartments of living cells expressing localized fluorgen-activating proteins. The macromolecular fluorogens were optimized by establishing critical polymer architecture-biophysical property relationships which impacted binding rates, binding affinities, and the level of internalization. Specific labeling of subcellular structures was realized at nanomolar concentrations of polymer, in the absence of membrane permeabilization or transduction domains, and fluorogen-modified polymers were found to bind to protein intact after delivery to the cytosol. Cellular motility was found to be dependent on binding of macromolecular fluorogens to actin structures causing rapid cellular ruffling without migration.

Highly Active Bipyridine-Based Ligands for Atom Transfer Radical Polymerization.

A series of 2,2'-bipyridines with 4,4'-substituents (R-bpy) were investigated for atom transfer radical polymerization (ATRP) of methyl acrylate (MA) and methyl methacrylate (MMA). Ligand substituents with a large range of Hammett parameters (R = Cl, H, Me, dinonyl (dN), MeO, and (Me)2N) were studied with cyclic voltammetry (CV), revealing that increasing the strength of electron donating groups (EDGs) resulted in more stable CuII complexes and larger ATRP equilibrium constants. Normal ATRP experiments confirmed the obtained CV data by showing the fastest rates of polymerization with R-bpy ligands containing EDGs ((Me)2N and MeO) and the slowest with electron withdrawing Cl. A 400-fold increase in the polymerization rate was observed with bpy ligands containing p-(Me)2N compared to H substituents. Linear increases in molecular weight with monomer conversion, and narrow molecular weight distributions were obtained with (Me)2N-bpy and MeO-bpy ligands. Low catalyst concentrations of 50 to 100 parts-per-million (ppm) were successfully employed with highly active R-bpy ligands (R = MeO and (Me)2N) and found to be effective in polymerizing MA and MMA, respectively, with narrow molecular weight distributions <1.3.

ATRP under Biologically Relevant Conditions: Grafting from a Protein.

Atom transfer radical polymerization (ATRP) methods were developed in water-based media, to grow polymers from proteins under biologically relevant conditions. These conditions gave good control over the resulting polymers, while still preserving the protein's native structure. Several reaction parameters, such as ligand structure, halide species, and initiation mode were optimized in water and PBS buffer to yield well-defined polymers grown from bovine serum albumin (BSA), functionalized with cleavable ATRP initiators (I). The CuCl complex with ligand 2,2'-bipyridyne (bpy) provides the best conditions for the polymerization of oligo(ethylene oxide) methacrylate (OEOMA) in water at 30 °C under normal ATRP conditions (I/CuCl/CuCl2/bpy = 1/1/9/22), while the CuBr/bpy complex gave better performance in PBS. Activators generated by electron transfer (AGET) ATRP gave well-controlled polymerization of OEOMA at 30 °C with the ligand tris(2-pyridylmethyl)amine (TPMA), (I/CuBr2/TPMA = 1/10/11). The AGET ATRP reactions required slow feeding of a very small amount of ascorbic acid into the aqueous reaction medium or buffer. The reaction conditions developed were used to create a smart, thermoresponsive, protein-polymer hybrid.

Substituted Tris(2-pyridylmethyl)amine Ligands for Highly Active ATRP Catalysts.

The synthesis and application of a very active catalyst for copper-catalyzed atom transfer radical polymerizations (ATRP) with tris([(4-methoxy-2,5-dimethyl)-2-pyridyl] methyl)amine (TPMA*) ligand is reported. Catalysts with TPMA* ligands are approximately 3 orders of magnitude more active than those with tris(2-pyridylmethyl)amine (TPMA). Catalyst activity was evaluated by cyclic voltammetry, stopped-flow, and ATRP kinetics. Catalysts with TPMA* ligands perform better than those with TPMA ligands, especially at low catalyst concentrations.

Electrochemically mediated atom transfer radical polymerization.

Atom transfer radical polymerization is a versatile technique for exerting precise control over polymer molecular weights, molecular weight distributions, and complex architectures. Here, we show that an externally applied electrochemical potential can reversibly activate the copper catalyst for this process by a one-electron reduction of an initially added air-stable cupric species (Cu(II)/Ligand). Modulation of polymerization kinetics is thereby tunable in real time by varying the magnitude of applied potential. Application of multistep intermittent potentials successfully triggers initiation of polymerization and subsequently toggles the polymerization between dormant and active states in a living manner. Catalyst concentrations down to 50 parts per million are demonstrated to maintain polymerization control manifested in linear first-order kinetics, a linear increase in polymer molecular weight with monomer conversion, and narrow polymer molecular weight distributions over a range of applied potentials.