Polymerizable species initiator

In this section, we consider the kinetics of propagation and the features of the propagating radical (Pn ) and the monomer (M) structure that render the monomer polymerizable by radical homopolymerization (Section 4.5.1). The reactivities of monomers towards initiator-derived species (Section 3.3) and in copolymerizalion (Chapter 6) arc considered elsewhere. 

Irradiation of monomer vapor does not yield substantial polymerization that is observed in plasma polymerization. In radiation polymerization, the bombardment of electrons creates the initiator for polymerization of the monomer, whereas in plasma polymerization, the bombardment of electrons produces the polymerizable species out of a nonpolymerizable organic molecule as well as of a polymerizable monomer. The polymerizable species could act as an initiator if polymerizable monomers exist, which is not affected by the glow discharge. This situation occurs only in the pulsed discharge of a polymerizable organic molecule in short duty cycle (long resting period). 

Cyclobutane has not been polymerised cationically (or by any other mechanism). Thermochemistry tells us that the reason is not thermodynamic it is attributable to the fact that the compound does not possess a point of attack for the initiating species, the ring being too big for the formation of a non-classical carbonium ion analogous to the cyclopropyl ion, so that there is no reaction path for initiation. The oxetans in which the oxygen atom provides a basic site for protonation, are readily polymerizable. Methylenecyclobutane polymerises without opening of the cyclobutane ring [72, 73]. 

Methylene- 1,3-dioxolane (15) is polymerized by peroxide initiator to produce a polyester 16 which is regarded as the product of the hypothetical ring-opening polymerization of the non-polymerizable heterocycle of y-butyrolactone (Eq. (14)).13). The opening of the cyclic species bearing a free radical 17 is the key step. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

Co-units constituted by a Mannich ba.se may also be present to a limited extent, as modifiers or residual initiators- (390 and 392), in the macromolecular chain originated by other different monomeric species (Z). In addition, when the polymerizable moieties arc located in both the R and R residues of the Mannich base, polymers of structure 393 are formed, along with branched as well as crosslinked derivatives. 

In the presence of a cationically polymerizable monomer the transitory carbenium ion may initiate polymerization. The above study provides insight into the nature of the initiating species and it emphasizes the active role played by solvents which are normally considered to be inert. The slow rates observed in c clopentane could be due to the absence of a suitable agent that can ftmction as a bridge opener for alkylaluminum dimers. Investigations of polymerizations of isobutylene using the above initiators in MeCl, MeBr, Mel and q clopentane solvents with trimethylaluminum support the conclusions of model studio (14). Thus, the efficiency of polymerization decreases in the order > MeBr Mel and cyclopentane. Essentially no polymerization was observed in Mel and cyclopentane. 

Monomers Not Polymerizable by Plasma Initiation. When styrene and a-methy1styrene were subjected to plasma treatment, the monomers became yellowish and only trace amounts of insoluble films were formed. The discoloration was intensified and extensive formation of dark films were observed if carbon tetrachloride was added as the solvent. No post-polymerization was detectable for these monomers. Generally styrene and a-methylstyrene readily undergo thermal polymerization. However, no thermal polymerization was possible for these monomers after having been subjected to plasma treatment for one minute or less. It has been demonstrated from the emission spectra of glow discharge plasma of benzene (6) and its derivatives (7 ) that most of the reaction intermediates are phenyl or benzyl radicals which subsequently form a variety of compounds such as acetylene, methylacetylene, allene, fulvene, biphenyl, poly(p-phenylenes) and so forth. It is possible that styrene and a-methylstyrene also behave similarly, so that species from the monomer plasma are poor initiators for polymerization. 

Since TEMPO is only a regulator, not an initiator, radicals must be generated from another source the required amount of TEMPO depends on the initiator efficiency. Application of alkoxyamines (i.e., unimolecular initiators) allows for stoichiometric amounts of the initiating and mediating species to be incorporated and enables the use of multifunctional initiators, growing chains in several directions [61]. Numerous advances have been made in both the synthesis of different types of unimolecular initiators (alkoxyamines) that can be used not only for the polymerization of St-based monomers, but other monomers as well [62-69]. Most recently, the use of more reactive alkoxyamines and less reactive nitroxides has expanded the range of polymerizable monomers to acrylates, dienes, and acrylamides [70-73]. An important issue is the stability of nitroxides and other stable radicals. Apparently, slow self-destruction of the PRE helps control the polymerization [39]. Specific details about use of stable free radicals for the synthesis of copolymers can be found in later sections. 

General scheme for photoinduced cationic polymerization is depicted in Scheme 11.1. A photosensitive compound, namely, photoinitiator (PI), absorbs incident light and undergoes decomposition leading to production of initiating species. Active species, namely, a radical cation (R+") in turn, react with cationic polymerizable monomers (M), and yield polymer (Scheme 11.1). 

Both the initially formed radical cation and the proton are potential initiating species for the reaction with a polymerizable monomer M (see Scheme 10.11). 

Over the past several years, there have been developed several new classes of onium salt photoinitiators capable of initiating cationic polymerization. The most significant of these are aryldiazonium salts, diaryliodonium salts, triarylsulfonium salts, and dialkylphenacyl-sulfonium salts. The mechanisms involved in the photolysis of these compounds have been elucidated and will be discussed. In general, on irradiation acidic species are generated which interact with the monomer to initiate polymerization. Using photosensitive onium salts, it is possible to carryout the polymerization of virtually all known cationically polymerizable monomers. A discussion of the various structurally related and experimental parameters will be presented and illustrated with several monomer systems. Lastly, some new developments which make possible the combined radical and cationic polymerization to generate interpenetrating networks will be described. 

For most metal alkoxides, hydrolysis with excess water yields insoluble oxide or hydroxide precipitates that are useless for further polymerization reactions [Eqs. (5.21)-(5.23)]. However, if small amounts of water are added slowly to a sufficiently dilute solution, it is possible to form polymerizable molecular species from these alkoxides also. For example, when a dilute solution of boron alkoxide in alcohol is exposed to water, soluble transient molecular species such as B(0R)2(0H) and B(OR)(OH)2, representing various degrees of hydrolysis, form initially, e.g.. 

Preparation of hyperbranched polymers using ATRP involves self-condensing vinyl polymerization (SCVP) (Frechet et al., 1995) of AB monomers, which contain two active species, viz., the double bond A group (polymerizable) and the initiating site B. Two main examples explored in detail within the context of ATRP are p-chloromethyl styrene or vinyl benzyl chloride (VBC) and 2-(2-bromopro-pionyloxy) ethyl acrylate (BPEA) (Fig. 11.30). Several other (meth)acrylates with either 2-bromopropionate or 2-bromoisobntyrate gronps have also been used. 

Additional coordination can also involve other components of the polymerizable system and can exert a substantial influence on the polymerization kinetics and the structures of the products formed. In particular, the formation of MCM complexes with an initiator can both accelerate (generate the initiating species) and retard its decomposition and leads to a set of products that can be incorporated in the polymer as terminal groups. Thus the unusual kinetics (the order of the reaction with respect to the monomer is -0.5) of radical polymerization of Ti" +-containing monomers are due to the occurrence of parallel reactions (scheme 27). ... 

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