Understanding cyclic by-products and ether linkage formation pathways in the transesterification synthesis of aliphatic polycarbonates

An overall picture of the reaction pathways towards the side products and final polymers during the transesterification synthesis of biodegradable aliphatic carbonates (APCs) were delineated for the first time. Catalysed by sodium ethoxide, the normal reaction between α,ω-diols (carbon number n   =   2–6) and diethyl carbonate generates APC oligomers with propagating terminal alkoxide anions. These anions can backbite at the nearest carbonate carbons or the nearest carbonate α-CH 2 carbons in the same APC chains, eliminating corresponding cyclic carbonates or cyclic ethers. Also, each alkoxide anion can randomly attack a carbonate α-CH 2 carbon in another APC chain, generating ether linkages in the final polymers whose percentages are about 100%, 1.35%, 0.08% and 0% for n   =   2, 3, 4 and 5, respectively. The content of cyclic carbonates shows a similar decreasing trend; ethylene carbonate (n   =   2) even predominates over the final polymer while trimethylene carbonate (n   =   3) and 1,4-butylene carbonate (n   =   4) are only minor by-products. Moreover, the latter two cyclic carbonates are not stable enough and can further decarboxylate, forming relatively high amount of allyl alcohol and tetrahydrofuran, respectively. However, trimethylene oxide (n   =   3) and tetrahydropyran (n   =   5) are minor cyclic ethers. The thorough understanding of the chemical process is essential to improve the APC synthesis.