DOI: 10.25349/d9391z
title: Building tunable degradation into high-performance poly(acrylate) pressure-sensitive adhesives

description: Pressure-sensitive adhesives (PSAs) based on poly(acrylate) chemistry are
 common in a wide variety of applications, but the absence of backbone
 degradability causes issues with recycling and sustainability. Here, we
 report a strategy to create degradable poly(acrylate) PSAs using simple,
 scalable, and functional 1,2-dithiolanes as drop-in replacements for
 traditional acrylate comonomers. Our key building block is α-lipoic acid,
 a natural, biocompatible, and commercially available antioxidant found in
 various consumer supplements. α-Lipoic acid and its derivative ethyl
 lipoate efficiently copolymerize with n-butyl acrylate under conventional
 free-radical conditions leading to high–molecular-weight copolymers (Mn
 &gt; 100 kg mol–1) containing a tunable
 concentration of degradable disulfide bonds along the backbone. The
 thermal and viscoelastic properties of these materials are practically
 indistinguishable from non-degradable poly(acrylate) analogues, but a
 significant reduction in molecular weight is realized upon exposure to
 reducing agents such as tris(2-carboxyethyl)phosphine (e.g. Mn = 198 kg
 mol–1 → 2.6 kg mol–1). By virtue of the thiol chain ends
 produced after disulfide cleavage, degraded oligomers can be further
 cycled between high and low molecular weights through oxidative
 repolymerization and reductive degradation. Transforming otherwise
 persistent poly(acrylates) into recyclable materials using simple and
 versatile chemistry could play a pivotal role in improving the
 sustainability of contemporary adhesives.
description: <em><sup>1</sup>H nuclear magnetic resonance
 spectroscopy</em>: Solution state <sup>1</sup>H nuclear
 magnetic resonance (NMR) spectra were recorded on a Bruker Avance III HD
 400 MHz or Bruker Avance NEO 500 MHz. Chemical shifts (δ) are reported in
 ppm relative to residual protio solvent in CDCl<sub>3</sub>
 (7.26 ppm). <em>Size-exclusion chromatography
 instrumentation</em>: Size-exclusion chromatography (SEC) was
 performed on a Waters instrument using a differential refractive index
 detector and two Tosoh columns (TSKgel SuperHZM-N, 3 μm polymer, 150 × 4.6
 mm) with THF at 35 °C or chloroform containing 0.25% TEA at 35 °C for the
 mobile phase. Molar masses and molar mass dispersities (Đ) were determined
 against narrow PS standards (Agilent).
 <em>Mechanical analysis</em>: Frequency sweeps and
 isochronal temperature sweeps were obtained on a TA Instruments ARES G2 in
 oscillatory shear mode with an 8 mm parallel-plate geometry and a
 nitrogen-purged forced–convection oven. For all experiments, oscillatory
 shear was applied within the linear viscoelastic regime as verified from
 isochronal strain sweeps at a fixed temperature. Differential scanning
 calorimetry (DSC) was performed with a TA Instruments DSC Q2000 at a
 heating/cooling rate of 10 °C/min using 3–5 mg of sample in a sealed
 aluminum pan. Lap shear and 180° peel tests were performed on a TA.XTplusC
 texture analyzer equipped with A/MTG tensile grips. 180° Peel test samples
 were performed with a peel rate of 100 mm min<sup>–1</sup>
 over a 20 mm displacement. Lap shear experiments were extended at a rate
 of 0.5 mm min<sup>–1</sup> until cohesive failure.
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