Filip Du Prez

Chemistry of Crosslinking Processes for Self-Healing Polymers

Recent developments in material design have seen an exponential increase of polymers and polymer composites that can repair themselves in response to damage. In this review, a distinction is made between extrinsic materials, where the self-healing property is obtained by adding healing agents to the material to be repaired, and intrinsic materials, where self-healing is achieved by the material itself through its chemical nature. An overview of the crosslinking chemistries used in self-healing materials will be given, discussing the advantages and drawbacks of each system. The review is not only aiming to enable researchers to compare their ongoing research with the state-of-the-art but also to serve as a guide for the newcomers, which allows for a selection of the most promising self-healing chemistries.

Additive-Free Clicking for Polymer Functionalization and Coupling by Tetrazine–Norbornene Chemistry

Herein we report the use of a tetrazine-norbornene inverse electron demand Diels-Alder conjugation applied to polymer end-functionalization and polymer-polymer coupling. The reaction was found to be applicable to polymer-polymer coupling, as judged by SEC, DOSY NMR, and LCxSEC analyses, giving diblock copolymers by merely mixing the constituent homopolymers together under ambient conditions, using no catalyst, additive, or external stimulus.

New thermo-responsive polymer materials based on poly(2-ethyl-2-oxazoline) segments

Abstract First, the solution behavior of poly(2-ethyl-2-oxazoline) (PEtOx) in water has been investigated. The dependence of the cloud points on the molecular weight and concentration indicates a typical Flory–Huggins (Type I) demixing behavior with a lower critical solution temperature (LCST). Secondly, the synthesis and properties of temperature-responsive hydrogels and segmented polymer networks, based on PEtOx bis-macromonomers, are reported. PEtOx hydrogels have been prepared by UV-induced radical polymerization of the corresponding α,ω-bis-acrylates. The networks exhibited a continuous shrinkage with increasing temperature. Series of segmented networks with LCST-behavior have been obtained by free radical copolymerization of PEtOx bis-macromonomer with the comonomers 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl acrylate (HPA) or methyl methacrylate (MMA). The LCST-behavior of the materials is controlled by varying the philicity of the comonomer and the fraction of PEtOx in the networks. PHEMA–PEtOx and PHPA–PEtOx hydrogels exhibited a large and reversible swelling–deswelling process, whereas the volume changes in PMMA–PEtOx swollen networks were small and occurred in a broad temperature interval.

One-pot multistep reactions based on thiolactones: extending the realm of thiol-ene chemistry in polymer synthesis.

The in situ generation of thiols by nucleophilic ring-opening of a thiolactone with amines, followed by a UV-initiated radical thiol-ene reaction in a one-pot fashion, has been evaluated as an accelerated and versatile protocol for the synthesis of several types of polymeric architectures. After elaboration of a model amine-thiol-ene conjugation reaction, a number of routes based on readily available thiolactone-containing structures have been developed to successfully assemble functional, linear polymers and networks via a mild and facile radical photopolymerization process.

Multifunctionalized Sequence‐Defined Oligomers from a Single Building Block

Two decades of progress in the field of living and controlled polymerizations, combined with the elaboration of efficient conjugation reactions, greatly contributed to the elegant preparation of functionalized macromolecular architectures. However, these state-of-the-art methodologies, while providing a high degree of structural and topological control, are inadequate tools for controlling the polymer microstructure. Crucial parameters like primary structure (i.e. monomer sequence) and tacticity largely remain unmastered by current man-made approaches. Expectations for the next generation synthetic polymers include their performance as single chains, ability to fold and self-regulate, and to sense specific molecules and/or catalyze reactions. These precisely functionalized linear polymers should exhibit sharply defined and tailored structure-activity relationships, analogous to Nature’s delicately engineered macromolecules. Therefore, progress towards reliable sequence-controlled polymerization, enabling preprogrammed distribution of multiple functional groups along the backbone, is drawing attention in a growing number of research groups worldwide. Pioneering efforts to control the primary structure of functionalized polymers have been based on several approaches, such as different reactivity ratios of vinyl monomers, spatial prearrangement of monomers on a (macromolecular) template or, as recently demonstrated, the action of a small-molecule machine. Other attempts use (automated) sequential addition of building blocks on a solid or liquid support, leading to sequence control as a result of iterative coupling steps, thereby omitting the need for pre-organization. These protocols, established for peptide and oligonucleotide synthesis, present considerable drawbacks for sequence-controlled polymerization: they generally require the use of protecting groups and the restricted number of readily available building blocks (‘monomer alphabet’) equipped with the appropriate functional handle can further hamper the preparation of tailor-made functionalized sequences. The development of new chemical protocols for chain elongation, often on a solid support, resulting in sequence-defined (macro)molecular structures with unique backbones and side chain functionalities, or fragments thereof that could be combined to obtain sequence controlled polymers, is therefore highly desirable. We here report on a new coupling strategy for the controlled generation of sequence-defined multi-functionalized oligomers on solid support in a protecting group-free approach, inspired by the ‘submonomer’ synthetic protocol for the preparation of functionalized peptoids, via thiolactone-based chemistry. While the generated oligomers are small in size, reconstitution approaches could further allow the synthesis of larger chains, featuring designed and repetitive display of carefully selected and well-positioned functional entities.

Heterogeneous azide–alkyne click chemistry: towards metal-free end products

Heterogeneous copper catalyzed azide–alkyne click chemistry (CuAAc) has been a quickly emerging research field during the last couple of years as it shows many advantages in comparison to its homogeneous counterpart. In this Minireview, an overview of the state-of-the-art is presented for the first time. For the sake of completeness, not only successful heterogeneous supports but also systems that particularly failed will be discussed. Furthermore, the future prospects and challenges with regard to the application of heterogeneous CuAAC are highlighted.

Thiol-ene and thiol-yne chemistry in microfluidics: a straightforward method towards macroporous and nonporous functional polymer beads

Thiol-ene and thiol-yne reactions are explored as efficient pathways towards rapid production of diverse monodisperse macroporous and nonporous functional beads. In a straightforward method, polymer beads containing amine, hydroxyl and carboxyl groups have been prepared by reacting a tetrafunctional thiol with a range of mono and/or multifunctional -enes/-ynes containing the desired functional groups. The thiol-ene and thiol-yne reactions have been performed in a simple home-made microfluidic device utilizing thiol and ene/yne monomers at a 1 : 1 ratio of thiol to π-bond. The porous functional beads were prepared making use of a porogen in combination with a photoinitiator. The optical and scanning electron microscopy images demonstrated monodispersity of the beads with a spherical shape ranging in size from 210 to 600 μm. The beads were characterized in terms of glass transition temperature, surface area measurement and composition. The accessible amine and hydroxyl loading in the beads ranges from 0.23 to 0.69 mmol g−1 and 0.24 to 0.64 mmol g−1 respectively, as determined by the Fmoc method. This work demonstrates the applicability of thiol-ene and thiol-yne reactions in microfluidics as a powerful tool for the rapid design of functional beads for diverse applications.

“Sandwich” Microcontact Printing as a Mild Route Towards Monodisperse Janus Particles with Tailored Bifunctionality

A “sandwich” microcontact printing method is reported. A monolayer of porous epoxy polymer microspheres is transformed into Janus particles with distinct functionality on each face by reaction with amine functional fluorescent dyes, carbohydrates, and magnetic nanoparticles.

Polymer networks containing crystallizable poly(octadecyl vinyl ether) segments for shape-memory materials

New polymer networks were prepared by free radical initiated copolymerization of crystallizable α,ω-bismethacrylate-terminated poly(octadecyl vinyl ether) (polyODVE) with butyl acrylate (BA). The polyODVE bismacromonomers were obtained by end-capping the bifunctionally living cationic polymerization of ODVE with 2-hydroxyethyl methacrylate (HEMA). The segmented networks show a high degree of phase separation over a wide range of compositions. The shape memory properties of a material containing 20 wt.-% of polyODVE are reported.

Straightforward synthesis of functionalized cyclic polymers in high yield via RAFT and thiolactone–disulfide chemistry

An efficient synthetic pathway toward cyclic polymers based on the combination of thiolactone and disulfide chemistry has been developed. First, heterotelechelic linear polystyrene (PS) containing an α-thiolactone (TLa) and an ω-dithiobenzoate group was synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization, employing a newly designed TLa-bearing chain transfer agent (CTA). The subsequent reaction of this heterotelechelic polymer with an amine, which acts as a nucleophile for both the TLa and dithiobenzoate units, generated the α,ω-thiol-telechelic PS under ambient conditions without the need for any catalyst or other additives. The arrangement of thiols under a high dilution afforded single cyclic PS (c-PS) through an oxidative disulfide linkage. The cyclic PS (c-PS) disulfide ring formation was evidenced by SEC, MALDI-TOF MS and 1H-NMR characterization. Moreover, we demonstrated a controlled ring opening via either disulfide reduction or thiol–disulfide exchange to enable easy and clean topology transformation. Furthermore, to illustrate the broad utility of this synthetic methodology, different amines including functional ones were employed, allowing for the one-step preparation of functionalized cyclic polymers with high yields.