Ring-opening polymerization
In polymer chemistry, ring-opening polymerization (ROP) is a form of chain-growth polymerization, in which the terminus of a polymer chain attacks cyclic monomers to form a longer polymer (see figure). The reactive center can be radical, anionic or cationic. Some cyclic monomers such as norbornene or cyclooctadiene can be polymerized to high molecular weight polymers by using metal catalysts. ROP is a versatile method for the synthesis of biopolymers. Ring-opening of cyclic monomers is often driven by the relief of bond-angle strain. Thus, as is the case for other types of polymerization, the  enthalpy change in ring-opening is negative. Monomers. Cyclic monomers that are amenable to ROP include epoxides, cyclic trisiloxanes, some lactones, lactides, cyclic carbonates, and amino acid N-carboxyanhydrides. Many strained cycloalkenes, e.g norbornene, are suitable monomers via ring-opening metathesis polymerization. History. Ring-opening polymerization has been used since the beginning of the 1900s to produce polymers. Synthesis of polypeptides which has the oldest history of ROP, dates back to the work in 1906 by Leuchs. Subsequently, the ROP of anhydro sugars provided polysaccharides, including synthetic dextran, xanthan gum, welan gum, gellan gum, diutan gum, and pullulan. Mechanisms and  thermodynamics of ring-opening polymerization were established in the 1950s. The first high-molecular weight polymers (M up to 10) with a repeating unit were prepared by ROP as early as in 1976. An industrial application is the production of nylon-6. Mechanisms. Ring-opening polymerization can proceed via radical, anionic, or cationic polymerization as described below. Additionally, radical ROP is useful in producing polymers with functional groups incorporated in the backbone chain that cannot otherwise be synthesized via conventional chain-growth polymerization of vinyl monomers. For instance, radical ROP can produce polymers with ethers, esters, amides, and carbonates as  functional groups along the main chain. Anionic ring-opening polymerization (AROP). Anionic ring-opening polymerizations (AROP) involve nucleophilic reagents as initiators. Monomers with a three-member ring structure - such as epoxides, aziridines, and episulfides - undergo anionic ROP. A typical example of anionic ROP is that of ε-caprolactone, initiated by an alkoxide. Cationic ring-opening polymerization. Cationic initiators and intermediates characterize cationic ring-opening polymerization (CROP). Examples of cyclic monomers that polymerize through this mechanism include lactones, lactams, amines, and ethers. CROP proceeds through an S1 or S2 propagation, chain-growth process. The mechanism is affected by the stability of  the resulting cationic species. For example, if the atom bearing the positive charge is stabilized by electron-donating groups, polymerization will proceed by the S1 mechanism. The cationic species is a heteroatom and the chain grows by the addition of cyclic monomers thereby opening the ring system. The monomers can be activated by Bronsted acids, carbenium ions, onium ions, and metal cations. CROP can be a living polymerization and can be terminated by nucleophilic reagents such as phenoxy anions, phosphines, or polyanions. When the amount of monomers becomes depleted, termination can occur intra or intermolecularly. The active end can "backbite