LCST Behavior of Symmetrical PNiPAm-b-PEtOx-b-PNiPAm Triblock Copolymers

Abstract

Poly­(N-isopropylacrylamide) (PNiPAm) and poly­(2-ethyl-2-oxazoline) (PEtOx) represent two polymer types that are well-known for their lower critical solution temperature (LCST) behavior in aqueous media. To synthesize triblock copolymers containing both polymers, a crossover of two different polymerization methods was applied using a bifunctional initiator for the living cationic ring-opening polymerization (CROP) of EtOx. Quantitative end-functionalization with a trithiocarbonate resulted in a bifunctional PEtOx macro chain transfer agent (CTA). A series of well-defined PNiPAm-b-PEtOx-b-PNiPAm triblock copolymers were obtained by subsequent reversible addition–fragmentation chain transfer (RAFT) polymerization of NiPAm. The influence of the PNiPAm to PEtOx ratio on the thermoresponsive properties was intensively investigated via turbidimetry, dynamic light scattering, cryo transmission electron microscopy, and 1H NMR studies, revealing hydrogen bonds between both copolymer segments that strongly lower the phase separation temperature of aqueous solutions.

Polymers Exhibiting Lower Critical Solution Temperatures as a Route to Thermoreversible Gelators for Healthcare

“…There are reports in the literature of the synthesis of ABA tri‐block copolymers where the A block is PNIPAM and the B block has been PEG, [ 75,122,126–129 ] poly(vinyl pyrrolidone) (PVP), [ 130 ] or poly( N , N ‐dimethylacrylamide) (PDMA). [ 131,132 ] All of these block copolymers exist as free polymers in solution below the LCST of PNIPAM.…”

Synthesis of PNIPAM–PEG Double Hydrophilic Polymers Using Oleic Acid Macro Peroxide Initiator

“…Sahn et al obtained a series of well-defined poly(N-isopropylacrylamide)-b-poly(2-ethyl-2-oxazoline-b-poly(N-isopropylacrylamide) triblock copolymers via RAFT polymerization of N-isopropylacrylamide using a new bifunctional cationic ring opening polymerization initiator with a larger biphenyl spacer. The influence of the ratio of the polymer blocks on the thermo-responsive properties is provided by the physicochemical and optical methods [33].…”

Bifunctional Initiators as Tools to Track Chain Transfer during the CROP of 2‐Oxazolines

“…As a consequence, bimodal molar mass distributions would already be detected by simple SEC upon first occurrence of a chain transfer. To investigate the feasibility of this approach, we selected a range of bifunctional bromo‐type initiators for the CROP of 2‐ethyl‐2‐oxazoline (EtOx) known from literature to serve as models for the well‐known CROP initiator benzyl bromide (BnBr) . It is known that elevated polymerization temperatures and solvents such as acetonitrile or N,N ‐dimethylacetamide provoke chain transfer reactions, while they can be reduced at lower temperatures or in less polar solvents such as, e.g., chloroform .…”