02857nas a2200145   4500008004100000020002000041022002000061245006300081210006200144260001400206300001200220520236200232100001902594856009802613       2017                        eng d  a978-149876758-3  a978-149876757-600aRing-opening polymerization and metathesis polymerizations0 aRingopening polymerization and metathesis polymerizations  bCRC Press  a137-1603 aAliphatic polyesters are important industrial polymers that have applications as fibers, coatings, bulk packaging materials, and films. Their biodegradable nature makes them an environmentally friendly alternative to the nondegradable plastics. 1 Copolymers of lactide and trimethylene carbonate (TMC) are used as thermoplastic elastomers and biomaterials for applications in tissue engineering, drug delivery, as biodegradable devices for bone fracture repair and sutures. The monomer for poly(lactic acid) (PLA), d, l-lactide is a dimer of lactic acid (LA), which is produced from natural sources such as a starch or sugar via bacterial fermentation of D-glucose. 2 Hence, production of poly(lactide) is environmentally friendly. Manufacturing of PLA has become profitable over the years. Natureworks LLC, a joint venture between Cargill and Teijen Limited, set up a 300 million pounds per year PLA production plant and sells PLA under the trade name Ingeo™, which is produced in isotactic form by a carbon neutral process. Its physical properties are similar to polyolefins and polystyrene. 3 The polycondensation route for PLA is undesirable since it is difficult to produce high molecular weight polymer, making ring-opening polymerization (ROP) the method of choice. In this chapter, ROP toward sustainable polymers such as PLA and poly(e-caprolactone) (PCL) only will be discussed. ROP is a living polymerization, that is, it shows fast initiation and minimal termination and transfer reactions, however it follows step-growth kinetics. Polydispersity is usually low but can be influenced by trans-esterification reactions and is of concern while making block copolymers. The thermodynamic driving force for the polymerization is 138the relief of ring strain, which helps to overcome the high entropy values (Lactide: δS = 25.0 J mol-1 K-1; ε-CL: δS = 53.9 J mol-1 K-1). 4 ROP performed using metal catalysts that operate through cationic mechanism do not yield high molecular weight polymer desirable for practical applications. Therefore immortal ROP that follows chain-transfer pathway and involves a catalyst and a nucleophile (either part of the catalyst or externally added) that acts simultaneously as the initiator and chain-transfer agent (CTA), was developed as an efficient alternative to the classical living cationic polymerization.1 aAmbade, A., V.  uhttps://library.ncl.res.in/content/ring-opening-polymerization-and-metathesis-polymerizations