Elucidation of Substantial Differences in Ring-Opening Polymerization Outcomes from Subtle Variation of Glucose Carbonate-Based Monomer Substitution Patterns and Substituent Types

Abstract

The substituents present upon five-membered bicyclic glucose carbonate monomers were found to greatly affect the reactivities and regioselectivities during ring-opening polymerization (ROP), which contrast in significant and interesting ways from previous studies on similar systems, while also leading to predictable effects on the thermal properties of the resulting polycarbonates. Polymerization behaviors were probed for a series of five five-membered bicyclic 2,3-glucose-carbonate monomers having 4,6-ether, -carbonate, or -sulfonyl urethane protecting groups, under catalysis with three different organobase catalysts. Irrespective of the organobase catalyst employed, regioregular polycarbonates were obtained via ROP of monomers with ether substituents, while the backbone connectivities of polymers derived from monomers with carbonate protecting groups suffered transcarbonylation reactions, resulting in irregular backbone connectivities and broad molar mass distributions. The sulfonyl urethane-protected monomers were unable to undergo organobase-catalyzed ROP, possibly due to the acidity of the proton in urethane functionality. The thermal behaviors of polycarbonates with ether and carbonate pendant groups were investigated in terms of thermal stability and glass transition temperature (T_g). A two-stage thermal decomposition was observed when tert-butyloxycarbonyl (BOC) groups were employed as protecting side chains, while all other polycarbonates presented high thermal stabilities with a single-stage thermal degradation. T_g was greatly affected by side-chain bulkiness, with values ranging from 39 to 139 °C. These fundamental findings of glucose-based polycarbonates may facilitate the development of next-generation sustainable highly functional materials.

Figure 1

Past and current 4,6- versus 2,3-glucose bicyclic carbonate monomer structures having ether versus carbonate substituents in the 2,3- versus 4,6-positions, respectively, with designation of their unusual regiochemical outcomes following ROPs. *The purple highlight was the six-membered carbonate ring, and the pink highlight was the five membered carbonate ring.

Scheme 1. Synthesis of Bicyclic Glucose Carbonate Monomers (a) with Ether Substituents 1 and 2, (b) with Carbonate Substituents 3 and 4, and (c) with Sulfonyl Urethane Substituents 5

Scheme 2. Synthesis of Polycarbonates via Organobase-Catalyzed ROPs

Figure 2

Plot of M_n and Đ as a function of monomer conversion (%) for the polymerization of (a) 1 or (d) 3 using TBD as the catalyst. The ratio of [monomer ]₀/[ initiator ]₀/[TBD]₀ was 50:1:1. (b) Kinetic plots of monomer conversion (ln([M]₀/[M])) as a function of time using data obtained by SEC (RI detector). SEC traces (THF as the eluent, 1 mL/min) of the ROP of (c) 1 or (e) 3 as a function of polymerization time, with normalization of the intensity of the polymer peaks.

Figure 3

(a) ¹H NMR spectra (500 MHz) of PM(EE²)GC, 13 (top), and PM(EC²)GC, 15 (bottom), and (b) the partial ¹³C NMR spectra (126 MHz) of 1, 13, 15, and 3 (from top to bottom) in CDCl₃ presenting carbonyl carbon, C₉ and C_9′ resonances. *The full spectra can be found in Figure S38. **Polymers were prepared via TBD-catalyzed ROP.

Figure 4

(a) TGA thermograms and (b) DSC thermograms of polymers 13–16.