A new comonomer design for enhancing the pH-triggered LCST shift of thermosensitive polymers

Abstract

A Thermosensitive polymers exhibiting a lower critical solution temperature (LCST) can be made responsive to pH change by introducing acid or base comonomer units, and the LCST can be switched between higher and lower temperatures as a result of the polarity change of the comonomer units upon their pH--induced protonation or deprotonation. In the present study, we describe a new comonomer design that aims at increasing the magnitude of the pH--triggered LCST shift.Random copolymers of N--isopropylacrylamide and 4--((2--carboxyallyl)oxy)benzoic acid, denoted as P(NIPAM--co--CBA), were synthesized, in which each CBA comonomer unit bears an acrylic acid and a benzoic acid group of similar pKa. With respect to comonomers containing a single acid group, this particular comonomer structure makes it more hydrophobic in the protonated state (pH<pKa) due to the phenyl group and more hydrophilic in the deprotonated state (pH>pKa) due to the doubled charge, which results in larger pH--triggered LCST shift based on the comonomer effect. The demonstrated comonomer design principle, which is general and can also be applied to base comonomers, represents a useful strategy for enhancing the efficiency and sensitivity of the pH--responsiveness of LCST polymers. 15 A rational comonomer design leads to large pH-induced LCST shift.

Comonomer effect: Switching the lower critical solution temperature to upper critical solution temperature in thermo‐pH sensitive binary graft copolymers

“…Among all stimuli‐sensitive polymers, thermo‐responsive and pH‐sensitive materials are the most studied by their promising applications as drug carriers. Thermo‐sensitive polymers are intensively studied by their phase transitions in aqueous medium; as for example, poly(N,N‐dimethylaminoethyl methacrylate) (PDMAEMA) which has a LCST of 38 °C approximately; poly(N‐vinylcaprolactam) (PNVCL) and poly(N‐isopropylacrylamide) (PNIPAAm) that exhibit a similar critical temperature close to 32–34 °C, it means that they are hydrated below this temperature, but above 32 °C they are dehydrated (in aqueous medium) . On the other side, polymers that exhibit an UCST are those with hydrophobic behavior below certain critical temperature but above this, they show a better affinity to water .…”

Designing catechol‐end functionalized poly(DMAm‐co‐NIPAM) by RAFT with tunable LCSTs

“…In particular, thermoresponsive (co)polymers with end functionalities have been evaluated for tunable lower critical solution temperatures (LCST) in aqueous media by adjusting the hydrophilic‐hydrophobic ratio of their polymeric chains . One method for raising the LCST is by copolymerizing N‐ isopropylacrylamide (NIPAM) with hydrophilic monomers: N ‐(2‐hydroxypropyl) methacrylamide, 4‐((2‐carboxyallyl)oxy)benzoic acid, N ‐vinylcaprolactam, and propylacrylic acid . Nevertheless, conditions for the postpolymerization modification of chain‐end functionalities have been known to affect the intrinsic characteristics of the new smart materials …”

Multifunctional temperature‐responsive polymers as advanced biomaterials and beyond

“…Introducing PNIPAAm with a comonomer can help alter the thermoresponsive property of the synthesized copolymer in order to suit the desired application. In general, hydrophilic comonomers shift the LCST to a higher temperature, whereas hydrophobic comonomers shift the LCST downward and can be useful for many applications . Hydrophilic/hydrophobic comonomers have been reported to produce copolymers that are widely applicable as biomaterials and in environmental remediation technologies …”

“…Hydrophobic monomers have been primarily used to shift polymer LCST to lower temperatures (e.g., closer to ambient temperatures), introduce functionality to increase affinity for target compounds, and so forth . PNIPAAm‐based systems are utilized in numerous separation processes, making it advantageous to lower polymer LCST to near ambient temperatures in order to lower energy input for transition.…”