12. Ring-Opening Polymerization

Ring-opening polymerization (ROP) is an important method of polymerization. It differs from radical polymerization (► Chap. 9), ionic polymerization (► Chap. 10), and step-growth polymerization (► Chap. 8). No low-molar-mass by-products are formed, except during the polymerization of Leuchs’ anhydride (◘ Figs. 12.41 and 12.42). Furthermore, the driving force derived from the transformation of C=C double bonds into C–C single bonds, which offsets the loss of entropy during polymerization, is not available. A general feature of ROP is that the monomers are rings of varying size. Depending on the size and type of the ring structure, the ability to polymerize and the corresponding driving force varies. Small rings (three-, four-, or five-membered rings) can be polymerized because of the ring strain released when they open. As an example, the enthalpy associated with the ring strain of oxirane is 116 kJ/mol. The release of enthalpy is also the driving force for the polymerization of seven- and eight-membered lactones and lactams even though the ring strain is only about 16 kJ/mol for these monomers. Unstrained six-membered rings often do not polymerize via ROP. By contrast, the ROP of disulfides, silicones, and carbonates can be ascribed to the increase in the entropy that occurs during the polymerization of these monomers. This increase in entropy is based on the increase in the degrees of freedom of rotation gained when rings are transformed into open chains.