Eco-Conscious Approach to Thermoresponsive Star-Comb and Mikto-Arm Polymers via Enzymatically Assisted Atom Transfer Radical Polymerization Followed by Ring-Opening Polymerization

Abstract

This study explores the synthesis, characterization, and application of a heterofunctional initiator derived from 2-hydroxypropyl cyclodextrin (HP-β-CD), having eight bromoester groups and thirteen hydroxyl groups allowing the synthesis of mikto-arm star-shaped polymers. The bromoesterification of HP-β-CD was achieved using α-bromoisobutyryl bromide as the acylation reagent, modifying the cyclodextrin (CD) molecule as confirmed by electrospray ionization mass spectrometry (ESI-MS), nuclear magnetic resonance (NMR), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy analysis, and differential scanning calorimetry (DSC) thermograms. The initiator's effectiveness was further demonstrated by obtaining star-comb and mikto-arm polymers via an enzymatically assisted atom transfer radical polymerization (ATRP) method and subsequent ring-opening polymerization (ROP). The ATR polymerization quality and control depended on the type of monomer and was optimized by the way of introducing the initiator into the reaction mixture. In the case of ATRP, high conversion rates for poly(ethylene oxide) methyl ether methacrylate (OEOMA), with molecular weights (Mn) of 500 g/mol and 300 g/mol, were achieved. The molecular weight distribution of the obtained polymers remained in the range of 1.23-1.75. The obtained star-comb polymers were characterized by different arm lengths. Unreacted hydroxyl groups in the core of exemplary star-comb polymers were utilized in the ROP of ε-caprolactone (CL) to obtain a hydrophilic mikto-arm polymer. Cloud point temperature (T_CP) values of the synthesized polymers increased with arm length, indicating the polymers' reduced hydrophobicity and enhanced solvation by water. Atomic force microscopy (AFM) analysis revealed the ability of the star-comb polymers to create fractals. The study elucidates advancements in the synthesis and utilization of hydrophilic sugar-based initiators for enzymatically assisted ATRP in an aqueous solution for obtaining complex star-comb polymers in a controlled manner.

Keywords: ATRP; enzymes; star-comb polymers; stimuli-responsive polymers.

Figure 1

Scheme of the synthesis of heterofunctional initiator based on CD. The asterisk (*) indicates the point of connection of the 2-hydroxypropyl group instead of R to the main molecule.

Figure 2

¹H NMR (600 MHz, DMSO-d₆) spectra of HP-β-CD (A), and 8-Br-HP-β-CD (B).

Figure 3

¹H-¹H homonuclear gCOSY 2D NMR spectrum of 8-Br-HP-β-CD.

Figure 4

Reaction scheme for the synthesis of star-comb polymers and mikto-arm star-comb polymers.

Figure 5

¹H NMR (600 MHz, D₂O) spectra of exemplary 8-arm star-comb polymer SCP7.

Figure 6

SEC traces of eight-arm POEOMA star-comb polymers: (A) POEOMA500 polymers; (B) POEOMA300 polymers.

Figure 7

¹H NMR (600 MHz, CDCl₃) spectra of MSCP1.

Figure 8

T_CP as a function of DP per arm of obtained polymers: (A) All polymers, (B) a subset of the top graph showing only star-comb polymers.

Figure 9

Absorbance as a function of the temperature of obtained polymers.

Figure 10

AFM images of obtained star-comb-polymer SCP2 at a concentration of 0.01 g/mL. (A–C) images of a single structure, (D,E) images of various structures, (F) 3D image of a single structure.

Figure 11

AFM images of obtained star-comb-polymer SCP3 at a concentration of 0.001 g/mL (A–F) and at a concentration of 0.01 g/mL (G–I).

Figure 12

AFM images of obtained MSCP1: At a concentration of 0.001 g/mL (A), at a concentration of 0.01 g/mL (B), and at a concentration of 0.001 g/mL from a solution that was set aside for a week (C,D).