Micellar Organocatalysis Using Smart Polymer Supports: Influence of Thermoresponsive Self-Assembly on Catalytic Activity.

Xiaoqian Yu,Dirk Kuckling,Artjom Herberg

doi:10.3390/polym12102265

Abstract

Micellar catalysts with a switchable core are attractive materials in organic synthesis. However, little is known about the role of the shell forming block on the performance of the catalyst. Thermoresponsive block copolymers based on poly(N-isopropylacrylamide-co-vinyl-4,4-dimethylazlactone) attached to different permanently hydrophilic blocks, namely poly(ethylene glycol), poly(N,N-dimethylacrylamide), and poly(2,3-dihydroxypropyl acrylate), were successfully synthesized via reversible addition/fragmentation chain transfer radical polymerization (RAFT). Post-polymerization attachment of an amino-functionalized L-prolineamide using the azlactone ring-opening reaction afforded functionalized thermoresponsive block copolymers. Temperature-induced aggregation of the functionalized block copolymers was studied using dynamic light scattering. It was shown that the chemical structure of the permanently hydrophilic block significantly affected the size of the polymer self-assemblies. The functionalized block copolymers were subjected to an aldol reaction between p-nitrobenzaldehyde and cyclohexanone in water. Upon temperature-induced aggregation, an increase in conversion was observed. The enantioselectivity of the polymer-bound organocatalyst improved with an increasing hydrophilic/hydrophobic interface as a result of the different stability of the polymer aggregates.

Highlights

Increasing sustainability in chemical reactions, chemical processes, and materials is one of the major challenges in modern times
The chemical nature of the hydrophilic block was varied from PDHPA and PDMAAm to poly(ethylene glycol) (PEG)
Thermoresponsive block copolymers based on PNIPAAm attached to different permanently hydrophilic blocks, namely PEG, PDMAAm, and PDHPA, were successfully synthesized via RAFT

Summary

Introduction

Increasing sustainability in chemical reactions, chemical processes, and materials is one of the major challenges in modern times. Catalyst immobilization mostly expands catalyst usability due to increased stability, improved solubility, and facilitated recycling [2,3,4,5]. The use of stimuli-responsive polymers as catalyst supports offers the opportunity of reversibly tuning the catalyst activity by altering environmental parameters (temperature, pH, ionic strength, etc.) [6,7]. One strategy for catalyst immobilization involves the attachment of the catalyst to the polymeric carrier. Such post-polymerization modification of a polymer usually requires the presence of reactive functional groups. Functional polymers based on isocyanides [8], epoxides [9], active ester [10,11], anhydrides [12], and azlactones [13] were reported and utilized for different applications

Methods

Results

Conclusion