Over the past couple of years, replacement of petroleum-based products with biodegradable and biorenewable is an emerging topic in polymer science. Biodegradable polyglycolic acid (PGA), the simplest aliphatic linear polyester, can traditionally be synthesized through the ring-opening polymerization of glycolide. Our previous studies revealed that PGA can alternatively be produced via one-step cationic polymerization of formaldehyde from trioxane and carbon monoxide (CO), which are potentially sustainable C1 feedstocks, under Bronsted acidic conditions. In this study, trioxane, CO, and a minor amount of fatty ester epoxides are copolymerized to improve on the physical properties of PGA, such as solubility and appearance, under the same reaction conditions for PGA homopolymer synthesis (in DCM, at 800 psi CO, with triflic acid catalyst, reaction duration of 72 h). The results have shown that the addition of minor quantities of epoxide comonomers vastly improved the solubility and decreased the melting temperature of the PGA. The melting temperatures of the obtained copolymers decreased by increasing incorporation percentages of the epoxide comonomers and decreasing polymerization temperatures. The solubility of the copolymers increased with incorporation of the epoxides in the PGA backbone.