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Dynamic covalent self-assembly processes often exhibit poor capacities for error-correction owing to the relatively low connectivity rearrangement rates of dynamic covalent interactions and the common use of reaction conditions where the equilibrium state remains fixed. Here, we report a dynamic covalent self-assembly technique employing temperature, a conventional, externally-applied stimulus, to mediate the hybridization of peptoid oligomers bearing maleimide- and furan-based pendant groups to afford molecular ladders incorporating Diels–Alder adduct-based rungs. By raising or lowing the reaction temperature, this system enables the equilibrium state to be readily varied without altering reagent concentrations. Both triethylamine and the Lewis acidic scandium triflate were examined as candidate reaction catalysts; however, only scandium triflate increased the rate of single strand conversion. As the Diels–Alder cycloaddition reaction does not liberate a small molecule, a registry-dependent mass change was effected by employing a base-catalyzed thiol-Michael addition reaction between any un-reacted maleimide pendant groups and a low molecular weight thiol to enable the number of Diels–Alder adduct rungs to be readily determined by mass spectrometry. Finally, by employing a slow temperature ramp from high to low temperature, approximating the thermal cycle employed for nucleic acid hybridization, sequence-selective hybridization between model, tetra-functional precursor strands was demonstrated.

Conventional photolithographic rapid prototyping approaches typically achieve reaction confinement in depth through patterned irradiation of a photopolymerizable resin at a wavelength where the resin strongly absorbs, such that only a very thin layer of material is solidified. Consequently, three-dimensional objects are fabricated by progressive, two-dimensional addition of material, curtailing fabrication rates and necessitating the incorporation of support structures to ensure the integrity of overhanging features. Here, we examine butyl nitrite as a UV-active photoinhibitor of blue light-induced photopolymerizations and explore its utilization to confine in depth the region polymerized in a volume of resin. By employing two perpendicular irradiation patterns at blue and near-UV wavelengths to independently effect either polymerization initiation or inhibition, respectively, we enable three-dimensional photopolymerization patterning in bulk resin, thereby complementing emergent approaches to volumetric 3D printing.

The self-assembly of long-sequence, peptoid-based molecular ladders with up to 16 rungs was demonstrated via the dimerization of oligomeric precursors bearing dynamic covalent pendant groups, with delineation of three potential mechanisms. These include a ‘molecular zipper’, whereby hybridization starts at one end followed by a series of imine condensation reactions to zip up complementary strands, or a ‘molecular hand-shake line’ or ‘toehold displacement’, whereby the molecular ladder formation would occur by shuffling or exchanging through bond rearrangement via cooperative transimination and imine metathesis reactions. MALDI mass spectrometry was used to determine the hybridization registry mechanism, with distance measurements using Forster resonance energy transfer (FRET) further confirming the registry mechanism, indicating that two complementary strands initially interact and bind by rapidly ‘zipping-up’ at any point generating molecular ladders with an arbitrary number of rungs, followed by slowly shuffling through a ‘molecular hand-shake line’ until these ladders come into registry.

Analogous to the thiol-ene and phosphane-ene polymerizations, radical-mediated iodo-ene reactions are described here that proceed via alternating propagation and chain transfer (i.e., APT) reactions between perfluoroiodide- and vinyl-bearing monomers. The thermal polymerization of a diiodo/tetraene formulation yielded a cross-linked, homogeneous polymer that was approximately seven times as radiopaque as aluminum owing to its high iodine content. Visible-light photopolymerizations of model iodo-ene monomers were monitored using mid-IR spectroscopy, revealing that the perfluoroiodide functional group consumption exceeded that of the vinyl, a discrepancy that decreased with increasing irradiation intensities and hence polymerization rates. The functional group conversions in resin formulations with a large initial perfluoroiodide excess exacerbated secondary side reactions that led to off-stoichiometric functional group consumption; nevertheless, photopolymerization of resin formulations with excess vinyl stoichiometry proceeded according to the ideal APT mechanism.

Autonomously healing materials that utilize thiol-ene polymerization initiated by an environmentally borne reaction stimulus are demonstrated by puncturing trilayered panels, fabricated by sandwiching thiol-ene-trialkylborane resin formulations between solid polymer panels, with high velocity projectiles; as the reactive liquid layer flows into the entrance hole, contact with atmospheric oxygen initiates polymerization, converting the liquid into a solid plug. Using infrared spectroscopy, we find that formulated resins polymerize rapidly, forming a solid polymer within seconds of atmospheric contact. During high-velocity ballistics experiments, additional evidence for rapid polymerization is provided by high-speed video, demonstrating the immediate viscosity increase when the thiol-ene-trialkylborane resins contact atmospheric oxygen, and thermal imaging, where surface temperature measurements reveal the thiol-ene reaction exotherm, confirming polymerization begins immediately upon oxygen exposure. While other approaches for materials self-repair have utilized similar liquid-to-solid transitions, our approach permits the development of materials capable of sealing a breach within seconds, far faster than previously described methods.

A pyrrolopyrazine-thione derived from oltipraz, a compound that has been investigated as a chemopreventive agent, affords radicals in the presence of thiols and oxygen via a redox cycle, an attribute that suggests its suitability as an initiator for oxygen-mediated polymerization. Here, we explore the utilization of this pyrrolopyrazine-thione, generated in situ from a precursor, as an initiator for the radical-mediated thiol-ene polymerization. While the pyrrolopyrazine-thione was shown to be capable of generating radicals in the presence of atmospheric oxygen and thiol groups, the reaction extents achievable were lower than desired owing to the presence of unwanted side reactions that would quench radical production and, subsequently, suppress polymerization. Moreover, we found that complex interactions between the pyrrolopyrazine-thione, its precursor, oxygen, and thiol groups determine whether or not the quenching reaction dominates over those favorable to polymerization.

A new diepoxide monomer (ADTDE) containing an allylic dithioether moiety has been synthesized to extend the suite of monomers that form networks exhibiting radical-mediated photoplasticity. These cross-linked polymers show permanent shape change under load and stress relax only when under the stimulus of visible or UV radiation. The Lewis base-catalyzed reaction of the epoxide groups in ADTDE with tri- and tetrafunctional thiols was monitored by differential scanning calorimetry and infrared spectroscopy. These curing systems were found to be more efficient than anionic, cationic, or anhydride polymerizations. The dynamic mechanical thermal analysis of ADTDE with the tetrathiol showed that the network was rubbery at room temperature which is considered ideal for the demonstration of photoplasticity. The photoinduced creep behavior was shown to be controlled by the presence of active free radicals in the matrix—on cessation of irradiation, the creep was terminated. The stress relaxation of the stretched ne...

Allyl sulfide addition-fragmentation chain transfer was employed concurrently with the radical-mediated formation of a thiol-ene network to enable network adaptation and mitigation of polymerization-induced shrinkage stress. This result represents the first demonstration of simultaneous polymerization and network adaptation in covalently crosslinked networks with significant implications for the fabrication of low stress polymer networks. For comparison, analogous networks incorporating propyl sulfide moieties, incapable of addition-fragmentation, were synthesized and evaluated in parallel. At the highest irradiation intensity, the allyl sulfide-containing material demonstrated a more than 75% reduction in the final stress when compared with the propyl sulfide-containing material. Analysis of the conversion evolution revealed that allyl sulfide addition-fragmentation decreased the polymerization rate owing to thiyl radical sequestration. Slow consumption of the allyl sulfide functional group suggests that intramolecular homolytic substitution occurs by a step-wise, rather than concerted, mechanism. Simultaneous stress and conversion measurements demonstrated that the initial stress evolution was identical for both the allyl and propyl sulfide-containing materials but diverged after gelation. While addition-fragmentation chain transfer was found to occur throughout the polymerization, its effect on the stress evolution was concentrated towards the end of polymerization when network rearrangement becomes the dominant mechanism for stress relaxation. Even after the polymerization reaction was completed, the polymerization-induced shrinkage stress in the allyl sulfide-containing material continued to decrease, exhibiting a maximum in the stress evolution and demonstrating the potential for continuing, longer term stress relaxation.

Radical-mediated thiol-yne step-growth photopolymerizations are utilized to form highly cross-linked polymer networks. This reaction mechanism is shown to be analogous to the thiol-ene photopolymerization; however, each alkyne functional group is capable of consecutive reaction with two thiol functional groups. The thiol-yne reaction involves the sequential propagation of a thiyl radical with either an alkyne or a vinyl functional group followed by chain transfer of the radical to another thiol. The rate of thiyl radical addition to the alkyne was determined to be approximately one-third of that to the vinyl. Chain-growth polymerization of alkyne and vinyl functionalities was only observed for reactions in which the alkyne was originally in excess. Analysis of initial polymerization rates demonstrated a near first-order dependence on thiol concentration, indicating that chain transfer is the rate-determining step. Further analysis revealed that the polymerization rate scaled with the initiation rate to an exponent of 0.65, deviating from classical square root dependence predicted for termination occurring exclusively by bimolecular reactions. A tetrafunctional thiol was photopolymerized with a difunctional alkyne, forming an inherently higher cross-link density than an analogous thiol-ene resin, displaying a higher glass transition temperature (48.9 vs -22.3 degrees C) and rubbery modulus (80 vs 13 MPa). Additionally, the versatile nature of this chemistry facilitates postpolymerization modification of residual reactive groups to produce materials with unique physical and chemical properties.

A network polymer, incorporating dynamic and reversible cross-links, was synthesized using the Diels−Alder reaction. Fourier transform infrared (FTIR) spectroscopy was used to characterize the reaction rate and thermodynamic equilibrium over a broad temperature range. Equilibrium conversion of the furan and maleimide varied from 74% at 85 °C to 24% at 155 °C, demonstrating significant depolymerization via the retro-Diels−Alder reaction. The gel point temperature, as determined by rheometry using the Winter−Chambon criterion, was 92 °C, corresponding to a gel-point conversion of 71%, consistent with the Flory−Stockmayer equation. The scaling exponents for the complex moduli, viscosity, and plateau modulus, in the vicinity of the gel-point, were determined and compared with experimental and theoretical literature values. Further, the material exhibited a low frequency relaxation owing to dynamic rearrangement of cross-links by the Diels−Alder and retro-Diels−Alder reactions.

BACKGROUND: Thiol–ene networks are of interest due to their facile photopolymerization and their open network structure. In this work, an allylic disulphide divinyl ether monomer is reacted with tetrathiol and divinyl ether monomers, which allows the network structure to permanently change in shape if stressed while under irradiation. We also study the photo-differential scanning calorimetry (DSC) kinetics and photorheology during cure and the dynamic mechanical properties after cure. RESULTS: The heat of polymerization is similar for the thiol–ene systems and suggests ca 80% conversion of the vinyl ether groups. An increase in the initiator concentration increases the photocure rate as expected. The activation energy for photopolymerization is 7.6 kJ mol−1. DSC and rheometry studies show that the polymerization kinetics is slowed by the addition of the allylic disulfide divinyl possibly due to the formation of less reactive radicals. However, as shown by dynamic mechanical thermal analysis, the network structure is not changed very much by addition of this monomer. If radicals are generated by irradiation of a photoinitiator in the network while a stress is being applied, the polymer will permanently deform depending on the fraction of 2-methylenepropane-1,3-di(thioethyl vinyl ether) in the network, due to a bond interchange reaction. CONCLUSION: The rate of thiol–ene reaction is slowed by the addition of the allylic disulfide divinyl ether. Photoplasticity is observed in the networks containing the allylic disulfide groups. Further work is required to optimize the extent of photoplasticity in these systems. Copyright © 2007 Society of Chemical Industry

The temperature and frequency dependence of the dynamic mechanical properties in the glass transition was studied for a series of partially cured thermally reactive networks (low temperature cured epoxy and a dimethacrylate photocured with a conventional initiator) and more thermally stable networks (high temperature cured epoxy and two dimethacrylates photocured with a photoiniter1). The viscoelastic behavior in the transition region of the former networks changed during the experiment due to additional cure, whereas the thermally stable networks enabled the study of the effect of conversion on the transition region. The glass transition temperatures showed 1 : 1 relationships with the isothermal curing temperatures and were correlated with the degree of conversion. The breadth of the glass transition, as determined from the real and loss moduli, tan δ, and the loss compliance in the temperature and frequency domains, was found to be greater for the dimethacrylate networks than for the epoxy networks. The breadth of the transition for the dimethacrylates was not significantly dependent on the degree of cure. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1348–1359, 2004

A range of dimethacrylates with varying backbone flexibility were partially photocured to various conversions using p-xylylene bis-(N,N-diethyldithiocarbamate) as a photoiniferter and their glass transition region investigated by dynamic mechanical thermal analysis. For isothermally cured samples, the final degree of conversion was found to increase as the length of the spacer group in the monomer was increased or as the crosslink density in the resin was lowered due to the reduced glass transition temperature which allowed greater mobility and, hence, higher cure. Increasing the curing temperature also resulted in a higher degree of conversion as the network was able to polymerize further before vitrification occurred. For the partially photocured samples, the glass transition temperature was raised as the degree of conversion was increased. Most of the measures of the breadth of the glass transition were found to increase with increased conversion for dimethacrylates with short or stiff backbones (TETDMA and bisGMA) while the transition breadth was independent of conversion for either a more flexible dimethacrylate (NEGDMA) or a dimethacrylate network with a lower crosslink density (50 wt % bisGMA/50 wt % PGEMA). This conclusion was generally confirmed by analysis of the viscoelastic parameters in the frequency domain. It is not clear whether these behaviors resulted from differences in the range of molecular motions available in tight networks or if they were due to spatially heterogeneous regions in the network. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3753–3766, 2003

The reaction kinetics during photopolymerization of bisGMA/styrene blends were investigated using FTIR and ESR spectroscopy. Increased styrene concentration reduced the polymerization rate of both methacrylate and styrene due to an increase in the termination rate and due to the stability of the styryl radical. Raised styrene concentrations also increased the final methacrylate conversion, but the final styrene conversion decreased because styrene plasticized the network, allowing methacrylate conversion to rise at higher styrene concentrations; however, the copolymerization did not proceed far enough to achieve increased conversions of styrene. The final concentration of radicals was reduced at higher styrene concentrations because of an increase in the bimolecular termination rate for networks with low cross-link densities. The proportion of styryl radicals trapped in the vitrified matrix was found to be markedly higher than the proportion predicted from the proportion of styrene monomer in the feed res...

The photopolymerization kinetics of two commercial vinyl ester resins (VERs) and a model VER photoinitiated by the camphorquinone/amine photoinitiator system were monitored using isothermal DSC. A decrease in styrene concentration in model VERs was found to raise the rate of photopolymerization. In contrast, when the styrene was replaced by a monomethacrylate diluent, the photopolymerization rate passed through a maximum near 70 wt% diluent monomer. This difference in the variation of the rate of polymerization with decreased monomer concentration was attributed to the competition of the effects of the higher reactivity of the methacrylyl radical relative to the styryl radical and the lower termination rate for divinyl-rich systems (both of which tend to raise the maximum polymerization rate) and the effects of the reduction in the initiation efficiency and decrease in k p due to increased fraction of pendant double bonds (which lower the polymerization rate) when the concentration of diluent monomer was reduced. Subsequent dark polymerization was observed during a temperature ramp and the onset of polymerization was independent of resin composition due to vitrification effects during the isothermal photocuring stage. The kinetics during the dark polymerization stage was discussed in terms of the radical concentration and the propagation rate constant. Increases in the concentration of either diluent monomer raised the extent of isothermal cure during the isothermal polymerization because vitrification was delayed by the lower crosslink density and the plasticizing effect of the diluent. Higher levels of diluent also raised the maximally attainable conversion due to reduced topological restrictions for reaction in networks of lower crosslink density.

Bisphenol-A diglycidyl ether dimethacrylate was blended with styrene at varying concentrations and this model vinyl ester resin (VER) was compared with two commercial VERs. The VERs were characterized using gravimetry, FTIR spectroscopy, NMR spectroscopy, differential scanning calorimetry (DSC) and DMTA. NMR spectroscopy differentiated between a novolac epoxy-based multimethacrylate oligomer and the two bisphenol-A epoxy-based dimethacrylate oligomers. Reaction kinetics were studied using scanning and isothermal DSC and isothermal FTIR spectroscopy using benzoyl peroxide as the thermal initiator. The presence of oxygen was found to inhibit significantly the polymerization. Increased initiator concentration raised the rate of isothermal polymerization, but did not affect the final conversion while increased styrene concentration reduced the polymerization rate constant and increased the total conversion. This was interpreted in terms of the variations in the termination rate and the stability of the styryl radical on the cure rate and the effect of vitrification on the extent of cure. From measurements of the dynamic mechanical properties as a function of temperature, the breadth of the glass transition tan δ curve and the magnitude of the rubbery modulus was found to increase while the tan δ maximum decreased with increased crosslink density. The T g , as measured by DSC, and the temperature of the tan δ maximum, as measured by DMTA, were not significantly affected by the styrene content in the resin per se , but were dependent on the combined effects of composition and crosslink density of the network.

Isothermal DSC was employed to monitor the photopolymerization kinetics of a commercial vinyl ester resin (VER) photoinitiated by the camphorquinone/amine photoinitiator system. The maximum rate of photopolymerization was found to increase tenfold as the isothermal cure temperature was varied from −30 to 90 °C due to faster propagation and greater initiator efficiency counterbalanced by a faster termination rate. After photopolymerization, the samples were temperature ramped in the DSC to monitor the dark polymerization reaction. The reduced polymerization rate increased with the scan temperature due to an increase in propagation rate. The summation of the isothermal and residual polymerization heats indicated that the final degree of cure after postcure was independent of the isothermal cure temperature. The onset temperatures for the recommencement of cure during the dark temperature scanning experiments were found to correspond closely with their respective isothermal cure temperatures as expected due to vitrification during the isothermal cure stage.

Radical mediated addition-fragmentation chain transfer of mid-chain allyl sulfide functional groups was utilized to reduce polymerization-induced shrinkage stress in thiol-yne step-growth photopolymerization reactions. In previous studies, the addition-fragmentation of allyl sulfide during the polymerization of a step-growth thiol-ene network demonstrated reduced polymerization stress; however, the glass transition temperature of the material was well below room temperature (~ -20°C). Many applications require super-ambient glass transition temperatures, such as microelectronics and dental materials. Polymerization reactions utilizing thiol-yne functional groups have many of the advantageous attributes of the thiol-ene-based materials, such as possessing a delayed gel-point, resistant to oxygen inhibition, and fast reaction kinetics, while also possessing a high glass transition temperature. Here we incorporate allyl sulfide functional groups into a highly crosslinked thiol-yne network to reduce polymerization-induced shrinkage stress. Simultaneous shrinkage stress and functional group conversion measurements were performed during polymerization using a cantilever-type tensometer coupled with a FTIR spectrometer. The resulting networks were highly crosslinked, possessed super-ambient glass transition temperatures, and exhibited significantly reduced polymerization-induced shrinkage stress when compared with analogous propyl sulfide-containing materials that are incapable of addition-fragmentation.

In this study we introduce the use of thiol-ene photopolymers as shape memory polymer systems. The thiol-ene polymer networks are compared to a commonly utilized acrylic shape memory polymer and shown to have significantly improved properties for two different thiol-ene based polymer formulations. Using thermomechanical and mechanical analysis, we demonstrate that thiol-ene based shape memory polymer systems have comparable thermomechanical properties while also exhibiting a number of advantageous properties due to the thiol-ene polymerization mechanism which results in the formation of a homogeneous polymer network with low shrinkage stress and negligible oxygen inhibition. The resulting thiol-ene shape memory polymer systems are tough and flexible as compared to the acrylic counterparts. The polymers evaluated in this study were engineered to have a glass transition temperature between 30 and 40 °C, exhibited free strain recovery of greater than 96% and constrained stress recovery of 100%. The thiol-ene polymers exhibited excellent shape fixity and a rapid and distinct shape memory actuation response.

In this paper we briefly overview the area of stimuli responsive polymers and the subset of photo-plastic polymers which change shape or undergo stress relaxation during irradiation. The chemistry, mechanism and properties of a new class of photo-plastic polymers is presented in detail - these tetrathiol-diene networks contain an allylic dithioether (also termed allylic disulphide) unit in the network strands and when this moiety is attacked by a photogenerated radical, the allylic dithioether undergoes β-cleavage which breaks one network bond and reforms another. Two different methods of integration of the allylic dithioether unit into the backbone are investigated - the use of a cyclic allylic dithioether which copolymerizes with a thiol-ene system or the incorporation of the allylic dithioether moiety in a divinyl ether monomer which can directly copolymerize with the thiol. The photo-plasticity effect of these thiol-ene networks is revealed in both creep and stress relaxation experiments, however, the extent of photo-plasticity is less extensive than expected. The polymerization kinetics and evolution of the rheological properties during photocuring are studied and the slower copolymerization of the monomer containing the allylic dithioether is discussed. The photo-rheology and dynamic mechanical properties of the thiol-ene networks are interpreted in terms of the network structure.

Polymer networks possessing reversible covalent crosslinks constitute a novel material class with the capacity for adapting to an externally applied stimulus. These covalent adaptable networks (CANs) represent a trend in polymer network fabrication towards the rational design of structural materials possessing dynamic characteristics for specialty applications. Herein, we discuss the unique attributes of CANs that must be considered when designing, fabricating, and characterizing these smart materials that respond to either thermal or photochemical stimuli. While there are many reversible reactions which to consider as possible crosslink candidates in CANs, there are very few that are readily and repeatedly reversible. Furthermore, characterization of the mechanical properties of CANs requires special consideration owing to their unique attributes. Ultimately, these attributes are what lead to the advantageous properties displayed by CANs, such as recyclability, healability, tunability, shape changes, and low polymerization stress. Throughout this perspective, we identify several trends and future directions in the emerging field of CANs that demonstrate the progress to date as well as the essential elements that are needed for further advancement.