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Cationic ring-opening polymerization
Related article: Cationic polymerization Cationic ring-opening polymerization (CROP) is characterized by having a cationic initiator and intermediate. Examples of cyclic monomers that polymerize through this mechanism include lactones, lactams, amines, and ethers. CROP proceeds through an SN1 or SN2 propagation, chain-growth process. The predominance of one mechanism over the other depends on 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 SN1 mechanism. The cationic species is an heteroatom and the chain grows by the addition of cyclic monomers thereby opening the ring system.

Not all cyclic monomers containing an heteroatom undergo CROP. Ring size influences whether the cyclic monomer polymerize through this mechanism. For example, 4, 6 and 7-membered rings of cyclic esters polymerize through CROP. When considering the ring size of the monomer, the reactivity toward polymerization is dictated by the ability to release the ring strain. Therefore, cyclic monomers with small or lacking ring strain will not polymerize.

Initiation
The monomers can be activated by Bronsted acids, carbenium ions, onium ions, photoinitiators, and covalent initiators.

Propagation
The cationic species is an heteroatom and the chain grows by the addition of cyclic monomers thereby opening the ring system.

In CROP, three mechanisms are distinguished by the propagating species.
 * When the cationic species is a secondary ion, polymerization proceeds by ring expansion. This mechanism is observed when the monomer is in low concentration.
 * When it is a tertiary ion, polymerization proceeds by linear growth.
 * The monomer can likewise be activated (i.e. cationic) and the propagation step will proceed via electrophilic addition from the growing chain to the activated monomer.

Termination
CROP can be considered as a living polymerization and can be terminated by intentionally adding termination 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" the chain, forming a macrocycle. Alkyl chain transfer is also possible, where the active end is quenched by transferring an alkyl chain to another polymer.

Ring-Opening Metathesis Polymerization
Main article: Ring-opening metathesis polymerization Ring-opening metathesis polymerization (ROMP) is used for making unsaturated polymers from olefin monomers that are typically cycloalkenes or bicycloalkenes. It involves organometallic catalysts of transition metals such as W, Mo, Re, Ru, and Ti carbenes complexes. Similarly, ROMP occurs for strained cyclic monomers. The enthalpy for relieving the ring strain must be very favorable for ROMP to occur because the entropy decreases during polymerization (see Gibbs free energy). Cyclic alkenes of 5, 7, and 8 member rings, for example, undergo ROMP at room temperature, whereas the 6 member ring analog does not. Some commercially relevant unsaturated polymers are synthesized by ROMP and are illustrated in the figure to the right.

Catalysts for ROMP

 * Schrock catalyst: tungsten- and molybdenum-based homogeneous catalysts provide faster initiation and good control over polydispersity and chain tacticity, but are limited by type of functional groups, thus type of monomers available.
 * Grubbs catalyst: