Free radical ring-opening polymerization, examples

Very recently, Bailey and Endo have enlarged the scope of the free-radical ring-opening polymerization.13 15> Several examples of their studies are described below. All the polymerizations are represented by the following general pattern of fundamental reaction. It is seen that the free-radical ring-opening by a P-scission mechanism is coupled with the addition of a free radical to the carbon-carbon double bond 13). 

The above examples of free-radical ring-opening polymerization, which have been explored by Bailey and Endo, produce polymers containing ketonic carbonyl and/or ester groups in the main chain. In addition, these cyclic monomers can be copolymerized with vinyl monomers by free-radical mechanism. Thus, the variety of the polymers produced by radical polymerization has been enlarged. 

B. (1996) First example of free radical ring opening polymerization with some characteristics of a living polymerization. Chem. Mater., 8, 604. 

In this review the polymerization of formaldehyde, h her aliphatic aldehydes and haloaldehydes will be discussed with particular emphasis on the kinetics of the polymerization. As will be apparent the kinetics of aldehyde polymerization have not been studied as extensively as the kinetics of more conventional polymerizations, for example, the free radical bond opening polymerizations of styrene, vinyl chloride or methylmethacrylate or the ring opening polymerizations of tetrahydro-furan or ethylene oxide. One reason is that polyoxymethylene is the only polyaldehyde produced commercially and much of our knowledge on formaldehyde polymerization is proprietary information. Another is that the polymerization systems are very complex and the polymers precipitate during polymerization. 

Even within the small numbers of studies conducted to date, we are already seeing potentially dramatic effects. Free radical polymerization proceeds at a much faster rate and there is already evidence that both the rate of propagation and the rate of termination are effected. Whole polymerization types - such as ring-opening polymerization to esters and amides, and condensation polymerization of any type (polyamides, polyesters, for example) - have yet to be attempted in ionic liquids. This field is in its infancy and we look forward to the coming years with great anticipation. 

Ring-opening polymerization of 2-methylene-l,3-dioxepane (Fig. 6) represents the single example of a free radical polymerization route to PCL (51). Initiation with AIBN at SO C afforded PCL with a of 42,000 in 59% yield. While this monomer is not commercially available, the advantage of this method is that it may be used to obtain otherwise inaccessible copolymers. As an example, copolymerization with vinyl monomers has afforded copolymers of e-caprolactone with styrene, 4-vinylanisole, methyl methacrylate, and vinyl acetate. 

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. 

Several examples of ring-opening polymerization proceeding via free-radical mechanism have been reported, e.g. the free-radical polymerization of vinylcyclo-propane 7 and its derivatives having alkoxycarbonyl substituents 98,9) ... 

Many polymerization techniques have been combined with CRP through site transformation of the active species. These include non-living techniques like condensation (or step) and conventional free radical processes or living methods like anionic, cationic, and ring-opening polymerizations, as well as others. Early examples were undertaken perhaps just to show that two different techniques could be combined, while later examples show how elegant the combinations have become and provide a foundation for controlled synthesis of materials from any type of monomers. These types of reactions are detailed below. 

Explain ring-opening polymerizations by a free-radical mechanism, giving two examples. 

Unlike other ring-opening polymerizations, most C-C single bond polymerizations have been carried out by free radical initiation, even though examples of cationic, anionic, and coordination polymerizations have been presented. The bicyclobutane monomers, such as bicyclobutane-1-carbonitrile (la X = CN), are as reactive in free radical polymerization as vinyl monomers ( ). 

This method is the only viable route to prepare nanocomposites for most thermoset polymers [15]. Linking interactions between the monomer, the surfactant, and the clay siuface, exfoliated nanocomposites have been successfully synthesized via the ring-opening polymerization, for example epoxy, nylon-6 and polycarbonate. The functional group in the organic cation can catalyze the intralayer polymerization and facilitate layer separation. Many thermoplastic nanocomposites have been synthesized by free radical polymerization. Efforts have been made to anchor initiators in the interlayer region to improve the intralayer polymerization rate for exfoliated nanocomposites [15, 74]. 

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