Ring latent initiators

Endo and coworkers, investigated nonsalt type latent initiators, like phenols. Phenols initiate polymerizations of epoxides with difficulty, even at elevated temperatures, probably due to insufficient acidity and relatively higher nucleophilicity of the dissociated phenoxide ion that acts as a polymerization termination. This, however, can be overcome by the substituent on the benzene ring. Hino and Endo report that some substituted phenols, like difluorophenol do act as nonsalt type photoinitiators. 204 They propose the following mechanism to explain the mechanism of the reaction ... 

The idea is to add a latent Friedel-Crafts catalyst to the polymerization feed that will be inert during the polymerization. Then at >200 °C it becomes activated and converts the residual styrene monomer to phenethyl carbocations. These carbocations can either initiate cationic polymerization of the styrene or can alkylate the benzene rings of PS to become attached. With this approach PS itself is the scavenger for residual styrene monomer in PS (Scheme 4.3). 

In addition to the hnear array of pyridyl rings, it was also possible to construct cyclic arrays as in cyclosexipyridine 262 [94], This was accomplished by sequential Stille couplings initiated by reacting 258 with 259 to afford 260. The remaining bromide of 260 was then coupled with a vinyl stannane derivative that produced 261. Unmasking of the latent carbonyl functionality preceded elaboration to 262. 

Recent efforts to report and to repair mechanical damage with mechanochemical reactions form the subject of the current chapter. We start with a brief discussion of the use of the spiropyran unit as a mechanophore for reporting strain. Spiropyran mechanochemistry inspired the development of another stress probe, the highly sensitive mechanoluminescent dioxetane, whose application as scission reporter in several types of polymeric materials is discussed. The chapter continues with a description of recent efforts to develop productive mechanochemistry, where initial scission leads to the formation of new bonds. Bond formation is either induced by the scission of covalent bonds, e.g. by the opening of rings, or bonds are formed under the action of a latent catalyst when it is activated by mechanochemical dissociation of a Lewis acid-base pair. These examples of productive mechanochemistry offer exciting possibilities to develop new modes of self-healing in... 

Rather than introducing a functional group first and protecting it in a subsequent step, it is better to introduce the functional group initially in a latent (already protected) form [22,23]. The example in Scheme 7.8 illustrates how a fitran ring serves as a latent ester group to be unveiled later in the synthetic sequence [24]. Likewise an oxazole ring serves in an excellent manner as a latent carboxyl function [25,26]. 

The mechanism by which these compoimds produce active centers for polymerization is described in detail below however, at this point it should be noted that in their latent state, the compoimds possess a cationic center (situated on the iodine or sulfur atoms), and possess a large anionic counterion. The aromatic rings can generally be functionalized (ts iically in the ortho and para positions) to enhance solubility in the organic monomers and to alter the absorbance characteristics of the photoinitiator. A number of excellent reviews are available which provide an account of the development of these initiators as well as a comprehensive list of compounds that have been synthesized (36,46,47). 

GC alone can be a valuable monomer for the synthesis of hyperbranched poly(hydroxyether)s (Scheme 25). In case of polymerization, GC, containing a l,3-dioxolan-2-one ring and hydroxyl group in a single molecule, is considered a latent cyclic AB2-type monomer. The anionic ROP of the GC, which proceeds with CO2 liberation, leads to a branched polyether. l,l,l-Tris(hydroxymethyl)propane or other multihydroxyl molecules are usually used as a initiator-starter and central core of the polyether. The hyperbranched polyglycerol structure is obtained by slow addition of the cyclic carbonate monomer at above 150 °C. Such polymers are characterized by a flexible polyether core and a multihydroxyl outer sphere. They are suitable for preparation of acrylic resins for dental applications or additives for polyurethane foams. Hyperbranched poly (hydroxyether)s from biscyclic carbonate with phenol group (2, Scheme 24) were also reported. 

Onium salt cationic photoinitiators present many unique and interesting opportunities for basic studies of cationic ring-opening polymerizations. Since they are latent photochemical sources of strong Bronsted adds, they can be dissolved in the subject monomers and then precisely tri ered on demand by the application of light. Mixing problems and the use of complex stopped-flow devices and other apparatuses required to overcome them are thus avoided. Only the rate of initiation is different in a photoinitiated cationic polymerization as compared to a conventional thermally initiated polymerization. The rate of initiation for an onium salt-photoinitiated cationic potymerization (eqn [68]) is... 

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