Perspective

Ring-opening polymerization (ROP) is a type of chain-growth polymerization in which the terminal end group of a polymer chain serves as a reactive center to which additional cyclic monomers can be added by ring-opening and addition of the broken bond. It is possible for the reactive center to be radical, anionic, or cationic. Using metal catalysts, some cyclic monomers, such as norbornene or cyclooctadiene, can be polymerized to high molecular weight polymers. Ring-opening polymerization is similar to chain-growth polymerization in that the terminal ends of a backbone chain act as an initiator for cyclic monomeric units to act on, resulting in the opening of the ring system and the formation of a longer polymeric chain. The relief of bondangle strain frequently drives ring-opening of cyclic monomers. As a result, the enthalpy change in ring-opening is negative, as it is in other types of polymerization. Di-functional monomers with two different reactive groups, such as one amine or alcohol and one carboxylic acid that have undergone a cyclization reaction are typical monomers that can be polymerized via ROP. Epoxides, cyclic trisiloxanes, some lactones, lactides, cyclic carbonates, and amino acid N-carboxyanhydrides are examples of cyclic monomers amenable to ROP. Many strained cycloalkanes, such as norbornene, are suitable monomers for polymerization via ring-opening metathesis.

As described below, ring-opening polymerization can take place via radical, anionic, or cationic polymerization. Furthermore, radical ROP is useful in producing polymers with functional groups incorporated in the backbone chain that cannot be produced using conventional chain-growth polymerization of vinyl monomers. For example, radical ROP can generate polymers containing ethers, esters, and amides. Nucleophilic reagents are used as initiators in anionic ring-opening polymerizations (AROP). Anionic ROP occurs in monomers with a three-member ring structure, such as epoxides, aziridines, and episulfides. Most AROP monomers have polar bonds such as ester, amide, carbonate, urethane, epoxide, and phosphate, which polymerize to the corresponding polyester, polyamide, polycarbonate, polyurethane, polyepoxide, and polyphosphate. Catalytic ring-opening polymerization is distinguished by cationic initiators and intermediates (CROP). Lactones, lactams, amines, and ethers are examples of cyclic monomers that polymerize via this mechanism. CROP propagates via an SN1 or SN2 propagation and chain-growth process. The stability of the resulting cationic species has an effect on the mechanism. CROP is a living polymerization that can be stopped by nucleophilic reagents like phenoxy anions, phosphines, or polyanions. The ring size, more specifically the ring strain caused by bond angle distortion (angular strain), repulsion between eclipsed hydrogen atoms (conformational strain), and non-bonding interactions between substituents attached to different parts of the ring, determine whether or not a cyclic monomer undergoes ROP (transannular strain). When the monomer pool is depleted, termination can occur intramolecularly or intermolecularly. The active end of the chain can "backbite" the chain, resulting in the formation of a macrocycle. Alkyl chain transfer is another option, in which the active end is quenched by transferring an alkyl chain to another polymer. Ring-opening metathesis polymerization (ROMP) works by converting cycloalkenes or bicycloalkenes into unsaturated polymers. It necessitates the use of organometallic catalysts. Ring-opening polymerization can also be accomplished through free radical polymerization. Because the resulting carbon-oxygen double bond after ringopening is much more stable than a carbon-carbon double bond, introducing oxygen into the ring usually promotes free radical ring-opening polymerization. Only highly strained cyclic monomers have been shown to undergo free radical ring-opening polymerization, whereas unstrained five- or sixmembered carbocyclic systems normally undergo free radical ring-closing polymerization. Ring-opening polymerization has been used to make polymers since the early 1900s.

The oldest history of ROP is the synthesis of polypeptides, which dates back to Leuchs' work in 1906. [8] Following that, anhydro sugar ROP produced polysaccharides such as synthetic dextran, xanthan gum, welan gum, gellan gum, diutan gum, and pullulan. Free-radical polymerization is an important synthesis route for producing a wide range of polymers and materials composites.Because free-radical chemical interactions are relatively nonspecific, this is one of the most versatile forms of polymerization available, allowing for simple reactions of polymeric free-radical chain ends and other chemicals or substrates. A polymer spends the majority of its time during polymerization increasing its chain length, or propagating. After forming the radical initiator, it attacks a monomer. A sigma bond secures one electron pair between the two carbons in an ethene monomer. The other is held more loosely in a pi bond. One electron from the pi bond is used by the free radical to form a more stable bond.