A method for cyclic and tadpole‐shaped polymers

Abstract

Reversible addition–fragmentation chain transfer polymerization (RAFT) was used to produce well‐defined telechelic poly(N‐isopropylacrylamide) (PNIPAM) with dibromomaleimide and dithiobenzoate groups at each end of the polymer chain by virtue of the functional RAFT agent 3,4‐dibromo‐1‐(4‐cyano‐4‐(phenylcarbonothioylthio)pentanoic)‐2,5‐dihydro‐1H‐pyrrole‐2,5‐dione. Cyclic PNIPAM was then obtained using an in situ intramolecular bromomaleimide–thiol substitution reaction after NaBH4 reduction of the dithiobenzoate end group to release a thiol group. In this ring‐closure process, another bromo substituent on the maleimide group survived, being a single reactive site for post‐functionalizing the cyclic PNIPAM by a second bromomaleimide–thiol substitution reaction. After being reacted with 2‐aminoethanethiol, the resultant amino‐functionalized cyclic PNIPAM was further demonstrated as a building block to construct a tadpole‐shaped block copolymer by coupling with linear succinimidyl ester end‐functionalized poly(ethylene oxide) based on active ester click chemistry. © 2015 Society of Chemical Industry

Cyclic Polymers by Ring‐Closure Strategies

“…[80] Thegroup of Zhang,who are highly active in the field of catalyst-free RC reactions,h ave also presented other convenient and highly efficient alternatives to the usual CuAAC reaction since 2014 by combining RAFT polymerization and the thiol-bromomaleimide substitution click reaction, [84,85] as well as ATRP and UV-induced strain-promoted azide-alkyne cycloaddition (SPAAC). [80] Thegroup of Zhang,who are highly active in the field of catalyst-free RC reactions,h ave also presented other convenient and highly efficient alternatives to the usual CuAAC reaction since 2014 by combining RAFT polymerization and the thiol-bromomaleimide substitution click reaction, [84,85] as well as ATRP and UV-induced strain-promoted azide-alkyne cycloaddition (SPAAC).…”

“…[86] Well-defined water-soluble linear poly(N,N-dimethylacrylamide) (PDMAM) was synthesized by RAFT polymerization of DMAM using afunctional tailormade RAFT agent. [85] By using ac yclopropenone-masked dibenzocyclooctynefunctionalized ATRP initiator for the polymerization of styrene followed by ap ost-modification to incorporate an azide functionality,S un et al succeeded in preparing welldefined PS chains (M n % 2750 gmol À1 )w ith protected alkyne and free azide functionalities,asrepresented in Scheme 23. [84] Purities higher than 95 %w ere readily achieved.…”

“…TheCuAACroute has already been widely utilized in the past decade as ap owerful method to tailor-make cyclic polymers by ah eterodifunctional unimolecular ring closure, [10] however, the potential toxicity of the copper catalysts and its limitations for ar ange of advanced applications has very recently created increasing interest in developing metalfree or catalyst-free click reactions.W ith the increased development of metal-free conjugation reactions in the last decade,t heir application to polymer cyclizations is timely. Among them, the Diels-Alder (DA) cycloaddition, [75][76][77][78][79][80][81] the thiol-ene or the base-catalyzed Michael reaction (conjugated addition), [82,83] the thiol-bromo click coupling, [84,85] and the strain-promoted alkyne azide cycloaddition (SPAAC) [86] are of particular interest since their efficiency is well-highlighted in the literature for polymer-polymer ligation. [87] In 2013, Lu et al explored the use of ab ase-catalyzed thiol-Michael click coupling for the preparation of macrocycles.…”

“…Interestingly,t his method could also be used to produce cyclic polymers in ao ne pot through diluting the polymerization solution, thereby avoiding the post-functionalization step and related purification of the linear precursor intermediate.I na ddition, when combining with ab atchwise operation, consisting of successive addition of the linear precursor after ap eriod of irradiation (30 min) to allow complete conversion of the previous batch, this technique could rapidly produce as ignificant amount (tens of mg) of very high purity cyclic polymers.I nt he current context of sustainable development, Josse et al further demonstrated in 2015 the possibility to conduct this particular photoenol chemistry under irradiation with natural sunlight, both in the summer and winter,a nd without the need for any special equipment such as expensive UV lamps. [80] Thegroup of Zhang,who are highly active in the field of catalyst-free RC reactions,h ave also presented other convenient and highly efficient alternatives to the usual CuAAC reaction since 2014 by combining RAFT polymerization and the thiol-bromomaleimide substitution click reaction, [84,85] as well as ATRP and UV-induced strain-promoted azide-alkyne cycloaddition (SPAAC). [86] Well-defined water-soluble linear poly(N,N-dimethylacrylamide) (PDMAM) was synthesized by RAFT polymerization of DMAM using afunctional tailormade RAFT agent.…”

“…By using thiol-bromomaleimide substitution, cyclo-PNIPAM, another water-soluble polymer,w as also prepared in high yields and purities. [85] By using ac yclopropenone-masked dibenzocyclooctynefunctionalized ATRP initiator for the polymerization of styrene followed by ap ost-modification to incorporate an azide functionality,S un et al succeeded in preparing welldefined PS chains (M n % 2750 gmol À1 )w ith protected alkyne and free azide functionalities,asrepresented in Scheme 23. [86] After UV irradiation (l max = 315 nm) for 5hin dilute solution (10 À4 m ;ca.…”