Polymer processing free-radical mechanism

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

The EE and phE mechanisms for neat polymers proposed by ourselves and others all involve the consequences of breaking bonds during fracture. Zakresvskii et al. (24) have attributed EE from the deformation of polymers to free radical formation, arising from bond scission. We (1) as well as Bondareva et al. (251 hypothesized that the EE produced by the electron bombardment of polymers is due to the formation of reactive species (e.g., free radicals) which recombine and eject a nearby trapped electron, via a non-radiative process. In addition, during the most intense part of the emissions (during fracture), there are likely shorter-lived excitations (e.g., excitons) which decay in a first order fashion with submicrosecond lifetimes. The detailed mechanisms of how bond scissions create these various states during fracture and the physics of subsequent reaction-induced electron ejection need additional insight. 

Although sulfur vulcanization has been studied since its discovery in 1839 by Goodyear, its mechanism is not well understood. Free-radical mechanisms were originally assumed but most evidence points to an ionic reaction [Bateman, 1963]. Neither radical initiators nor inhibitors affect sulfur vulcanization and radicals have not been detected by ESR spectroscopy. On the other hand, sulfur vulcanization is accelerated by organic acids and bases as well as by solvents of high dielectric constant. The ionic process can be depicted as a chain reaction involving the initial formation of a sulfonium ion (XI) by reaction of the polymer with polarized sulfur or a sulfur ion pair. The sulfonium ion reacts with a polymer molecule by hydride... 

Radical hydrosilylation takes place according to a usual free-radical mechanism with silyl radicals as chain carriers. Products are formed predominantly through the most stable radical intermediate. Even highly hindered alkenes undergo radical hydrosilylation. This process, however, is not stereoselective, and alkenes that are prone to free-radical polymerization may form polymers. 

A typical effect observed in the synthesis of linear polymers by a free-radical mechanism is the auto-acceleration process. At a particular conversion, when sufficient polymer has accumulated in the system for the viscosity to reach a certain level, the rate of the bimolecular termination reaction begins to fall because of diffusional restrictions to the encounter of two chain ends. However, the initiation and growth rates are hardly affected. 

Further examples for a PET involving amines are quinoline-dibenzoyl peroxide [114], rhodamine 6G-dibenzoyl peroxide [115], and auramine O-dibenzoyl peroxide [116] systems. The use of coloured amines makes it possible to initiate the polymerization with visible light, which is favorable for several practical applications. Again, the photopolymerization process proceeds by a free radical mechanism, and therefore, dye moieties are incorporated as endgroups into the polymer molecules. 

Both methods are very general. They apply to any polymer which undergoes radiolysis (i.e., "any" polymer) and the only limitation with respect to the monomer is that it polymerize by a free radical mechanism. Ionic grafting was also initiated by radiation (28,29) but the yields of this process are generally quite low. 

We shall consider here graft copolymerization only by free-radical processes. There are three main techniques for preparing graft copolymers via a free-radical mechanism. All of them involve the generation of active sites along the backbone of the polymer chain. These include (i) chain transfer to both saturated and unsaturated backbone or pendant groups (ii)radiative or photochemical activation and (iii) activation of pendant peroxide groups. 

Ichimura et al. [76] have described photoresists based on soluble polymers bearing methacrylated side-chain groups which are photocross-linked by a free radical mechanism using a diphenyliodonium salt as initiator, with a p-dimethylaminobenzylidine sensitizer. These investigators found spectroscopic evidence for in situ ground state charge transfer complex formation between the sensitizer and initiator, and that such complexes are involved both in the photocross-linking process as well as in the thermal instability of the photoresist films. 

It seems clearly established that volatilization is a free radical process initiated at chain ends transfer is responsible for the formation of dimer, trimer, etc. The mechanism of main chain scission is less clear. It seems nevertheless probable that weak links associated with peroxy structures in the chain are present in the polymers prepared by free radical mechanism. They would break very rapidly at the onset of heating. Further chain scission is probably due to random breaking of the chain. This random breaking is accelerated if head-to-head, branch chains or main chain unsaturation are present. Transfer occurs but is not very important. 

Polymerization of vinyl monomers by free-radical mechanisms is perhaps the most widely encountered and best understood mode of vinyl polymerization. The popularity of free-radical polymerization is due in substantial part to the many advantages that this route to polymers offers to industry. The polymerization process is noteworthy for its ease, convenience, and relative insensitivity to impurities, such as water and oxygen, that plague ionic polymerizations. Indeed, it is common to carry out free-radical polymerizations in water as a suspending medium, as in emulsion and suspension polymerization. Another advantage of free-radical polymerization is that it offers a convenient approach toward the design and synthesis of myriad specialty polymers for use in almost every area. 

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