Precision synthesis of advanced polymeric materials requires efficient, robust, and facile chemical reactions. Paradoxically, the synthesis of increasingly intricate macromolecular structures generally benefits from exploitation of the simplest reactions available. This idea, combined with requirements of high efficiency, orthogonality, and simplified purification procedures, has led to the rapid adoption of “click chemistry” strategies in the field of macromolecular engineering. This Perspective provides context as to why these newly developed or recently reinvigorated reactions have been so readily embraced for the preparation of polymers with advanced macromolecular topologies, increased functionality, and unique properties. By highlighting important examples that rely on click chemistry techniques, including copper(I)-catalyzed and strain-promoted azide−alkyne cycloadditions, Diels−Alder cycloadditions, and thiol−ene reactions, among others, we hope to provide a succinct overview of the current state of the art and future impact these strategies will have on polymer chemistry and macromolecular engineering.

Reversible addition-fragmentation chain transfer (RAFT) polymerization in the presence of a compound capable of both reversible chain transfer through a thiocarbonylthio moiety and propagation via a vinyl group led to highly branched copolymers by a method analogous to self-condensing vinyl copolymerization. An acryloyl trithiocarbonate prepared by copper-catalyzed azide-alkyne cycloaddition was copolymerized with N-isopropylacrylamide (NIPAM) in ratios selected to tune the distribution and length of branches in the resulting thermoresponsive polymers. The degree of branching increased with chain transfer agent (CTA) concentration, as proven by NMR spectroscopy, size exclusion chromatography, and viscometry. Retention of the thiocarbonylthio compound during the polymerization was evidenced by successful chain extension of a branched N-isopropylacrylamide (PNIPAM) macroCTA by RAFT polymerization of N,N-dimethylacrylamide. The branched polymers led to reduced lower critical solution temperatures as compared to linear PNIPAM, an effect attributed primarily to an increased contribution of hydrophobic end groups. End group cleavage by radical-induced reduction resulted in an increased transition temperature more similar to that expected for linear PNIPAM.

The combination of atom transfer radical polymerization (ATRP) and click chemistry was employed to prepare well-defined ω-(meth)acryloyl macromonomers in an efficient manner. Poly(n-butyl acrylate) (PBA), polystyrene (PS), and PS-b-PBA were prepared by ATRP and subsequently derivatized to contain azido end groups. The reaction of the azido-terminated polymers with alkyne-containing acrylate and methacrylate monomers resulted in near-quantitative chain end functionalization. Macromonomers of various molecular weights [PBA:  M n = 2.2−6.4 × 103 g/mol (DPn = 16−49); PS:  M n = 3.2−5.9 × 103 g/mol (DPn = 29−55)] and architectures were prepared by this method. The end group transformations required to incorporate the polymerizable functionality were accomplished either as a stepwise series of discrete reactions or as an in situ process, wherein azidation was immediately followed by azide−alkyne coupling in situ. In both cases, the degree of end group functionalization generally exceeded 90%. To demonstrate polymerizability, examples of ω-methacryloyloxy−PBA and ω-acryloyloxy−PS macromonomers were homopolymerized by conventional radical polymerization in toluene. The macromonomers and polymacromonomers were characterized by a combination of gel permeation chromatography using refractive index, light scattering, and viscosity detection, as well as 1H NMR spectroscopy and 1H−H NMR correlation spectroscopy (COSY). This versatile method of incorporating polymerizable end groups from commercially available reagents should be applicable to a variety of (co)polymers accessible by ATRP.

Branched polymers result in a more compact structure in comparison to linear polymers of identical molecular weight, due to their high segment density which affects the crystalline, mechanical, and viscoelastic properties of the polymer. Star polymers constitute the simplest form of branched macromolecules where all of the chains-or arm segments-of one macromolecule are linked to a centre defined as the core. Over recent years, modular ligation reactions-some of which adhere to click criteria-have enabled the synthesis of a variety of star polymers via efficient polymer-polymer conjugations. While the modified Huisgen [3 + 2] dipolar copper catalyzed azide and alkyne cycloaddition (CuAAC) has been widely employed for macromolecular star synthesis, Diels-Alder and hetero Diels-Alder reactions offer alternative pathways which allow for similarly efficient macromolecular conjugations. Moreover, combinations of these protocols afford the synthesis of more complex star polymer structures which previously had not been achievable.