Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes

Summary

A protocol for the controlled photoredox ring-opening polymerization of O-carboxyanhydrides mediated by Ni/Zn complexes is presented.

Abstract

Here, we describe an effective protocol that combines photoredox Ni/Ir catalysis with the use of a Zn-alkoxide for efficient ring-opening polymerization, allowing for the synthesis of isotactic poly(α-hydroxy acids) with expected molecular weights (>140 kDa) and narrow molecular weight distributions (M_w/M_n < 1.1). This ring-opening polymerization is mediated by Ni and Zn complexes in the presence of an alcohol initiator and a photoredox Ir catalyst, irradiated by a blue LED (400 - 500 nm). The polymerization is performed at a low temperature (-15 °C) to avoid undesired side reactions. The complete monomer consumption can be achieved within 4 - 8 hours, providing a polymer close to the expected molecular weight with narrow molecular weight distribution. The resulted number-average molecular weight shows a linear correlation with the degree of polymerization up to 1000. The homodecoupling ¹H NMR study confirms that the obtained polymer is isotactic without epimerization. This polymerization reported herein offers a strategy for achieving rapid, controlled O-carboxyanhydrides polymerization to prepare stereoregular poly(α-hydroxy acids) and its copolymers bearing various functional side-chain groups.

Introduction

Poly(α-hydroxy acid) (PAHA) is an important class of biodegradable and biocompatible polymers with applications ranging from biomedical devices to packaging materials.^1,2 Although PAHAs can be prepared directly by polycondensation of α-hydroxy acids, the molecular weights (MWs) of the resulting PAHAs are generally low.³ Ring-opening polymerization (ROP) of lactones (e.g., lactide and glycolide) is an alternative synthetic approach that provides the better control on MWs and molecular weight distribution (Đ) than polycondensation. However, the lack of side-chain f....

Protocol

Caution: Please consult all relevant materials safety data sheets (MSDS) before use. Many chemicals used in the synthesis are acutely toxic and carcinogenic. Please use all appropriate safety practices when performing reaction including the use of engineering controls (fume hood and glovebox) and personal protective equipment (safety glasses, gloves, lab coat, full-length pants, closed-toe shoes, blue-light blocking safety goggles). Following procedures involve standard air-free handling techniques in a glove box. All solutions are transferred using pipette.

1. Preparation of Catalyst and Initiator

NOTE: The whole ....

Results

The conversion of OCA is monitored by Fourier-transform infrared spectroscopy, as shown in Figure 2. The peak at 1805 cm^-1 is assigned as the anhydride bond stretch in OCA; the peak at 1760 cm^-1 corresponds to the formation of the ester bond in the polymer. Once the monomer's peak at 1805 cm^-1 completely disappears, the polymerization is finished.

The MW and Đ

Discussion

The critical step within the protocol is maintaining the reaction temperature at -15 ± 5 °C. All catalysts solutions and OCA monomers have to be stored in a glove box freezer at -35 °C before the polymerization. The reaction vials have to be pre-cooled in the cold trap. During the reaction, because the LED light dissipates heat, it is necessary to monitor the reaction every 15 - 20 minutes. Once the temperature is raised up to -10 °C, liquid nitrogen should be added into the dewar to cool the trap. Th.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ni(COD)2	Strem	28-0010	Stored in the glove box freezer.
2,2′-bipyridine	Strem	07-0290	Stored in the glove box freezer.
Zn(HMDS)2	N/A	N/A	Synthesized following reported procedures.25 Stored in the glove box freezer.
Benzyl alcohol	Sigma-Aldrich	402834	Stored with 4Å molecular sieve
Ir[dF(CF3)ppy]2(dtbbpy)PF6	Strem	77-0425	Stored in the glove box freezer.
THF	Sigma-Aldrich	34865	Dried by alumina columns and stored with 4Å molecular sieve in the dark bottle in the glove box.
Ethanol	Sigma-Aldrich	793175
GPC with an isocratic pump	Agilent	Agilent 1260 series
Dawn Heleos II Light Scatterer	Wyatt
Optilab rEX differential refractive index detector	Wyatt
Size exclusion columns	Phenomenex
Glass Scintillation Vials - 7 ml	VWR
FTIR spectrometer	Agilent
Stir bars	VWR	58948-091
Balance
Glove box	Mbraun	Labstar Pro