Nathalie K. Guimard

Cycloadditions in Modern Polymer Chemistry

Synthetic polymer chemistry has undergone two major developments in the last two decades. About 20 years ago, reversible-deactivation radical polymerization processes started to give access to a wide range of polymeric architectures made from an almost infinite reservoir of functional building blocks. A few years later, the concept of click chemistry revolutionized the way polymer chemists approached synthetic routes. Among the few reactions that could qualify as click, the copper-catalyzed azide-alkyne cycloaddition (CuAAC) initially stood out. Soon, many old and new reactions, including cycloadditions, would further enrich the synthetic macromolecular chemistry toolbox. Whether click or not, cycloadditions are in any case powerful tools for designing polymeric materials in a modular fashion, with a high level of functionality and, sometimes, responsiveness. Here, we wish to describe cycloaddition methodologies that have been reported in the last 10 years in the context of macromolecular engineering, with a focus on those developed in our laboratories. The overarching structure of this Account is based on the three most commonly encountered cycloaddition subclasses in organic and macromolecular chemistry: 1,3-dipolar cycloadditions, (hetero-)Diels-Alder cycloadditions ((H)DAC), and [2+2] cycloadditions. Our goal is to briefly describe the relevant reaction conditions, the advantages and disadvantages, and the realized polymer applications. Furthermore, the orthogonality of most of these reactions is highlighted because it has proven highly beneficial for generating unique, multifunctional polymers in a one-pot reaction. The overview on 1,3-dipolar cycloadditions is mostly centered on the application of CuAAC as the most travelled route, by far. Besides illustrating the capacity of CuAAC to generate complex polymeric architectures, alternative 1,3-dipolar cycloadditions operating without the need for a catalyst are described. In the area of (H)DA cycloadditions, beyond the popular maleimide/furan couple, we present chemistries based on more reactive species, such as cyclopentadienyl or thiocarbonylthio moieties, particularly stressing the reversibility of these systems. In these two greater families, as well as in the last section on [2+2] cycloadditions, we highlight phototriggered chemistries as a powerful tool for spatially and temporally controlled materials synthesis. Clearly, cycloaddition chemistry already has and will continue to transform the field of polymer chemistry in the years to come. Applying this chemistry enables better control over polymer composition, the development of more complicated polymer architectures, the simplification of polymer library production, and the discovery of novel applications for all of these new polymers.

Light-Induced Modular Ligation of Conventional RAFT Polymers†

Making light work of RAFT conjugation: a non-activated RAFT agent at the end of RAFT polymers can readily be coupled with ortho-quinodimethanes (photoenols) in a photo-triggered Diels-Alder reaction under mild conditions without catalyst. The method is universal and opens the door for the conjugation of a large number of RAFT-prepared polymers with photoenol-functionalized (macro)molecules. (RAFT=reversible addition-fragmentation chain transfer.).

Thermally reversible Diels-Alder-based polymerization: an experimental and theoretical assessment

A pair of monomers capable of undergoing reversible polymerization—based on reversible Diels–Alder (DA) chemistry—as a function of the applied reaction temperature is presented. Specifically, the reaction of isophorone bis(sorbic carbamate), a difunctional diene, with 1,4-phenylenebis(methylene)bis((diethoxyphosphoryl)methanedithioformate), a difunctional dithioester, was studied in detail. Various factors, including the monomer concentration, the type of solvent, and the presence of a Lewis acid, that influence this step-growth polymerization were evaluated. The solvent type was found to have a significant effect on the DA reaction rate. Under the optimized conditions, which are 1.8 g mol−1 of each monomer in acetonitrile with 1.1 equivalents of zinc chloride at 50 °C for 4 h, a polymer with a peak molecular weight of 9600 g mol−1 (relative to poly(styrene) standards) was obtained. The resulting polymer was employed to investigate the correlation between time, temperature, and percentage of debonded monomers achieved during the retro DA (rDA) reaction. In addition, theoretical predictions of the rDA temperature were obtained via ab initio quantum chemical calculations. The monomeric diene and dienophile system was employed for the calculations of the equilibrium constants at various rDA reaction temperatures to correlate the percentage of bonded molecules with the applied temperature. It was calculated that 60% of the polymer becomes debonded at a temperature (Tqc) of around 220 °C, a result that agrees well with that obtained experimentally (Texp = 219 °C).

Sunlight-induced crosslinking of 1,2-polybutadienes: access to fluorescent polymer networks

The efficient sunlight-induced crosslinking of 1,2-polybutadienes to generate fluorescent patterns with spatial resolution is reported. The photochemical conjugation method employed is based on a nitrile imine-mediated tetrazole–ene cycloaddition (NITEC) reaction, which proceeds under UV-light irradiation (λmax = 312 nm) at ambient temperature in the absence of any catalyst. The NITEC reaction between 1-pentene and a newly designed di-linker, consisting of two photosensitive diaryl-substituted tetrazoles joined by a tetraethylene glycol spacer, was investigated in an initial study. Detailed characterization of a small molecule model study was performed by size exclusion chromatography (SEC), UV-vis and fluorescence spectroscopy as well as electrospray-ionization mass spectrometry (ESI-MS), which was also employed for monitoring the progress of the reaction (100% conversion in 20 min). Finally, two 1,2-polybutadienes of disparate molar masses were each photocrosslinked with the di-linker. The crosslinking reaction parameters, such as concentration, di-linker fraction and reaction time were optimized via SEC analysis and gravimetric determination of gel fractions. The applicability of the novel crosslinking technology for generating spatially controlled highly fluorescent gel patterns is demonstrated in a solvent-free reaction for 2 h under sunlight. In summary, the current study introduces an efficient light-triggered technology platform for crosslinking polymers carrying non-activated double bonds.

Fast and catalyst-free hetero-Diels-Alder chemistry for on demand cyclable bonding/debonding materials†

A new dithioester possessing a cyano Z-group (cyano-dithioester (CDTE)) has been synthesized via a 2-step, one-pot reaction. The cyano-substituted dithioester has been found to undergo fast reversible hetero-Diels–Alder (HDA) reactions at ambient temperature, without the need for a catalyst, as demonstrated by ESI-MS and UV-Vis experiments. To apply the bonding/debonding on demand system to materials science, a cyano-dithioester di-linker was synthesized and employed as a di-functional dienophile in a HDA-based polymerization reaction with a bis-cyclopentadiene polymer. The reversible bonding of the polymer systems were demonstrated by on-line UV-Vis spectroscopy, on-line NMR spectroscopy, and on-line high temperature DLS, as well as via GPC in situ trapping experiments and high-level ab initio molecular orbital calculations.

Temperature-dependent size exclusion chromatography for the in situ investigation of dynamic bonding/debonding reactions

AbstractPolymers capable of dynamic bonding/debonding reactions are of great interest in modern day research. Potential applications can be found in the fields of self-healing materials or printable networks. Since temperature is often used as a stimulus for triggering reversible bonding reactions, an analysis operating at elevated temperatures is very useful for the in situ investigation of the reaction mechanism, as unwanted side effects can be minimized when performing the analyses at the same temperature at which the reactions occur. A temperature-dependent size exclusion chromatographic system (TD SEC) has been optimized for investigating the kinetics of retro Diels−Alder-based depolymerization of Diels−Alder polymers. The changing molecular weight distribution of the analyzed polymers during depolymerization gives valuable quantitative information on the kinetics of the reactions. Adequate data interpretation methods were developed for the correct evaluation of the chromatograms. The results are confirmed by high-temperature dynamic light scattering, thermogravimetric analysis, and time-resolved nuclear magnetic resonance spectroscopy at high temperatures. In addition, the SEC system and column material stability under application conditions were assessed using thermoanalysis methods, infrared spectroscopy, nitrogen physisorption, and scanning electron microscopy. The findings demonstrate that the system is stable and, thus, we can reliably characterize such dynamically bonding/debonding systems with TD SEC. Figure3D illustration of chromatograms of a polymer after different times of a depolymerization reaction