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Radical Ring-Opening Polymerization
Radical ring-opening polymerization can produce polymers of the same or lower density than the monomers. This is important for applications that require constant volume after polymerization, such as tooth fillings, coatings, and the molding of electrical and electronic components. Additionally, radical ROP is useful in producing polymers with functional groups incorporated in the backbone chain that cannot otherwise be synthesized via conventional chain polymerization of vinyl monomers. For instance, radical ROP can produce polymers with ethers, esters, amides, and carbonates as functional groups along the main chain.

Free radical polymerization techniques have been recently developed to control radical ROPs, thereby controlling the molecular weight of the synthesized polymer chains. Reversible Addition Fragmentation Transfer (RAFT) has been applied to radical ROP of a cyclopropane monomer. For instance, the RAFT polymerization of the cyclic monomer to synthesize polymers with anthracene along the backbone chain has been demonstrated.

A recent hot topic among scientists has been the study of radical ROP to undergo copolymerization for the production of copolymers with ketoester linkages in the main chain. The goal is to synthesize a final copolymer that is both hydrolyzable and photodegradable.

Mechanism
In free radical ROP, the cyclic structure will undergo homolytic dissociation in radical ROP, rather than undergoing heterolytic dissociation (as is the case for any ionic ROP). There are two typical mechanistic schemes in radical ROP.

Scheme 1: The terminal vinyl group accepts a radical. The radical will be transformed into a carbon radical stabilized by functional groups (i.e. halogen, aromatic, or ester groups). This will lead to the generation of an internal olefin.

Example: Radical ROP of vinyl cyclopropane

Scheme 2: Free-radical ROP of a ketene acetal. In this case, the exo-methylene group is the radical acceptor. The ring-opening reaction will form an ester bond, and the radical produced is stabilized by a phenyl group.

Examples
Examples of monomers that undergo radical ROP include vinyl substituted cyclic monomers, methylene substituted cyclic monomers, bicyclobutanes, spiro monomers (which undergo double ring-opening). Degradable polyester can be synthesized via radical ring-opening homo- and copolymerization.

Anionic Ring-Opening Polymerization
In anionic ROP (AROP), the growing chain end undergoes nucleophilic attack by a heterocyclic monomer.

Initiation
Common nucleophilic reagents used for the initiation AROP usually will include organometals (e.g. alkyl lithium, alkyl magnesium bromide, alkyl aluminum, etc.), metal amides, alkoxides, phosphines, amines, alcohols and water. The monomers that undergo AROP will contain polarized bonds (ester carbonate, amide, urethane, and phosphate), which respectively leads to the production of the corresponding polyester, polycarbonate, polyamide, polyurethane and polyphosphate.

Propagation
There are two formal ways that the monomer ring in unsymmetrically substituted rings can open in AROP. In general, nucleophilic attack on the least substituted carbon atom is preferred.

Transfer and Termination
Termination occurs via transfer reactions to any additional monomer that is available Active centers of AROP monomers are nucleophilic and acts as bases to initiate new chains. This results in low molecular-weight polymer chains. This can be combated by adding crown ethers as complexing agents for counter-ions.