Control of the Alternating Sequence for N‐Isopropylacrylamide (NIPAM) and Methacrylic Acid Units in a Copolymer by Cyclopolymerization and Transformation of the Cyclopendant Group

Abstract

An alternating copolymer of methacrylic acid and N-isopropyl acrylamide (NIPAM) was synthesized by selective cyclopolymerization of a special divinyl monomer and transformation of repeating cyclo-units in the resultant cyclopolymer. Crucial to the breakthrough is the monomer design in view of two types of cleavable bonds (3° ester and activated ester) in the pendant group of the monomer and the lower reactivity ratio of the two double bonds (methacrylate and electron-poor acrylate) for the polymerizable groups. The thus-obtained cyclopolymer was transformed into the alternating copolymer by transformation of the activated ester into amide by isopropyl amine and cleavage of the 3° ester by trifluoroacetic acid. The resultant copolymer showed thermal and pH response in aqueous solution that was different from the 1:1 random copolymer as well as the homopolymers.

Unprecedented Sequence Control and Sequence‐Driven Properties in a Series of AB‐Alternating Copolymers Consisting Solely of Acrylamide Units

“…Furthermore, Figure C shows the 13 C NMR spectrum of the obtained copolymer in comparison with the random copolymer, and the spectra exhibit almost identical shapes and peak‐splitting patterns. Peculiar peaks specifically due to the alternating sequence were not observed, though 13 C NMR spectroscopy of the alternating copolymer was different from random ones, and it clarified the sequence in previous works of sequence control …”

“…Thus, we tried to control the polymerization with various conditions of reversible deactivation radical polymerization. First, atom transfer radical polymerization (ATRP) conditions were applied to the cyclopolymerization of 1 under dilute conditions ([ 1 ] 0 =100 m m ; [initiator] 0 =2.0 m m ), which are similar to those of the cyclopolymerization of the methacrylate–acrylate divinyl monomer in our previous study . However, the conversion of the vinyl groups in 1 reached a plateau less than 50 % during the early stages of the reaction, affording low‐molecular‐weight polymers using any iron and copper catalyst in conjunction with various ligands and additives (Supporting Information, Table S1, Entries 2–9).…”

“…We have recently developed a method to control the alternating sequence in copolymers by selective radical cyclopolymerization and the cleavage of the cyclo‐spacer in the resulting repeat units . The spacer of the divinyl monomer can either be cleaved or transformed, and the design determines the pendant groups in the resulting alternating sequence.…”

“…We obtained alternating copolymer of methacrylic acid and N ‐isopropyl acrylamide by selective cyclopolymerization and side‐chain cleavage of the designed divinylmonomer. The alternating copolymer showed sequence‐specific properties: the thermal and pH responses in aqueous solution were different from those of the corresponding random copolymers …”

Precisely Synthesized Sequence‐Controlled Amino Acid–Derived Vinyl Polymers: New Insights into Thermo‐Responsive Polymer Design

“…A similar cooperative dehydration mechanism was reported in an aqueous solution of poly( N ‐isopropylacrylamide) (PNIPAM) based on the consecutive sequence of the NIPAM unit . Ouchi et al also reported that the thermal response of the alternating copolymers of methacrylic acid and NIPAM was less sensitive than homo PNIPAM due to the absence of a consecutive sequence of the same NIPAM unit . Indeed, polymers belonging in Group II had low sensitivity since the inserted G ‐block inhibited cooperative dehydration even though the maximum A ‐block length was only slightly reduced from DP = 15 (Group I) to 10 (Group II).…”