Ring-opening polymerization reversibility

The polymerizations of tetrahydrofuran [1693-74-9] (THF) and of oxetane [503-30-0] (OX) are classic examples of cationic ring-opening polymerizations. Under ideal conditions, the polymerization of the five-membered tetrahydrofuran ring is a reversible equiUbtium polymerization, whereas the polymerization of the strained four-membered oxetane ring is irreversible (1,2). 

Although reversible or equilibrium polymerizations would almost always be carried out in an irreversible manner, it is interesting to consider the kinetics of polymerization for the case in which the reaction was allowed to proceed in a reversible manner. (The kinetics of reversible ring-opening polymerizations are discussed in Sec. 7-2b-5). 

Some cationic ring-opening polymerizations take place without termination and are reversible. Oxirane and oxetane polymerizations are seldom reversible, but polymerizations of larger-sized rings such as tetrahydrofuran are often reversible. The description of reversible ROP is presented below [Afshar-Taromi et al., 1978 Beste and Hall, 1964 Kobayashi et al., 1974 Szwarc, 1979]. It is also applicable to other reversible polymerizations such as those of alkene and carbonyl monomers. The propagation-depropagation equilibrium can be expressed by... 

RAFT Reversible addition-fragmentation chain transfer ROP Ring-opening polymerization... 

The mass spectra [17] give a grossly simplified picture of the complex mixture which can result from a ring-opening polymerization. Knotted isomers can have almost identical mass spectra, dominated by the parent ion. With the reversibility of metathesis, the polyolefin mixture near equilibrium is expected to contain a large variety of knotted and linked species. 

Generation of free-radicals by Kolbe s reaction is well-known [Eq. (10)]. Formation of a radical-cation of monomer [Eq. (11)] has never been been proved and is only a possible conjecture from the right reverse consideration of the radical-anion formation at the cathode [Eq. (6)], although the perchlorate anion has actually been found to yield an unstable perchlorate free-radical by discharge at the anode. Nor is it certain that the monomer radical-cation is formed by direct discharge from the anode [Eq. (12)]. The ring-opening polymerization of oxides, caprolactam and isocyanides is also initiated on the electrode. A few examples of condensation polymerization have developed recently, like Eq. (7) and (12). Details of this work are described in the appropriate section. 

This type of polymerization is specific for ring-opening polymerization, and it has already been mentioned several times (in Chap. 1, Sect. 9.3 and Chap. 4, Sects. 2.2 and 2.3). Propagation of lactams mostly proceeds by a system of consecutive reversible reactions... 

Finally, the polymerization of vinyl monomers usually proceeds as a practically irreversible reaction. Exceptions are heavily substituted monomers like a-methylstyrene for which propagation is clearly reversible. In the ring-opening polymerization, the driving force for polymerization comes from ring strain, thus it varies greatly for different monomers. 

In many instances in cationic ring-opening polymerization, all the reaction steps, however, are reversible. The final composition of copolymer (in equilibrium) is governed then by thermodynamics. Thermodynamic approaches have been developed [305] and recently reviewed [306]. Such thermodynamic approach has been used to analyze the copolymerization of pairs of cyclic acetals (1,3-dioxolane with 1,3-dioxepane and... 

The formation of polyamides by ring-opening polymerization of lactams can proceed through several types of reversible transacylation reactions in which the lactam amide bond is cleaved while a polymer amide bond is formed, viz. 

Complexes X, XI and XlVa mimic the steps of the ring-opening polymerization of five-membered cyclic alkenes and account for the reversibility of the polymerization of cyclopentene. In the structure of compound XlVb the double bond shifts so that, once more a five-membered ring is formed. 

For reversible ring-opening polymerizations, the propagation-depropagation... 

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