Radical chain reactions polymerization

In addition to oxidation, many other reactions occur as free radical chain reactions polymerization, decomposition, fluorination, chlorination, etc. All chain reactions have a few important general peculiarities [1—3]. 

Anionic copolymerizations have been investigated by applying the classical Mayo-Lewis treatment which was originally developed for free-radical chain reaction polymerization [198]. The copolymerization of two monomers (Mj and M2) can be uniquely defined by the following the four elementary kinetic steps in Scheme 7.21, assuming that the reactivity of the chain end (Mj" or ) depends only on the last unit added to the chain end, that is, there are no penultimate effects. 

The simplest and often most prevalent form of a star pol3rmer is a Y structure where there is one or fewer long chain branches per average polymer molecule. Low density polyethylenes prepared by high pressure, free-radical chain-reaction polymerizations are star polymers with exceedingly nonuniform structures because both long chain branching and quite a variety of short chain branches are present. See Fig. 2. 

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

A chain reaction polymerization of vinyl monomer, which is usually carried out by a photoinitiator to produce a primary radical (R ), which can interact with a monomer molecule (M) in a propagating process to form a polymer chain composed of a large number of monomer units (see Eq. [2] and reaction Scheme [3]. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

A widely used synthetic procedure is radical polymerization, polymerization by a radical chain reaction (Section 13.9). In a typical procedure, a monomer (such as ethene) is compressed to about 1000 atm and heated to 100°C in the presence of a small amount of an organic peroxide (a compound of formula R—O—O -R,... 

On condensation at low temperatures, on dissolution in inert solvents or on raising its partial pressure substantially above 1 mbar (100 Pa) S2O polymerizes with partial disproportionation. Since sulfur radicals have been detected in such condensates by ESR spectroscopy [10] it has been proposed that a radical-chain reaction takes place according to Scheme 5. 

Although benzoyl peroxide will initiate the polymerization (by a radical chain reaction) of either styrene or acrylonitrile, -methoxy- -nitrobenzoyl peroxide will not initiate polymerization efficiently in the latter monomer because it is too rapidly destroyed by the polar decomposition. Acrylonitrile, but not styrene, causes the polar decomposition to predominate, and the intermediates of the polar decomposition are not catalysts for the polymerization of acrylonitrile. 

Chain-propagating radical reaction, nonpolymeric, 14 276 Chain propagation, in low density polyethylene, 20 218-220 Chain-reaction polymerizations, 14 244 Chain rule of partial differentiation,... 

If ki and k.i are much larger than kj, the reaction Is controlled by kj. If however, ki and k.i are larger than or comparable to kz, the reaction rate becomes controlled by the translational diffusion determining the probability of collisions which Is typical for specific diffusion control. The latter case Is operative for fast reactions like fluorescence quenching or free-radical chain reactions. The acceleration of free-radical polymerization due to the diffusion-controlled termination by recombination of macroradicals (Trommsdorff effect) can serve as an example. 

Dimethacrylate monomers were polymerized by free radical chain reactions to yield crosslinked networks which have dental applications. These networks may resemble ones formed by stepwise polymerization reactions, in having a microstructure in which crosslinked particles are embedded in a much more lightly crosslinked matrix. Consistently, polydimethacrylates were found to have very low values of Tg by reference to changes in modulus of elasticity determined by dynamic mechanical analysis. 

A kinetic chain reaction usually consists of at least three steps (1) initiation, (2) propagation, and (3) termination. The initiator may be an anion, a cation, a free radical, or a coordination catalyst. Although coordination catalysts are the most important commercially, the ionic initiators will be discussed first in an attempt to simplify the discussion of chain-reaction polymerization. 

The polymerization proceeds via a radical chain-reaction mechanism, judging from some features of the polymerization initiation by irradiation or upon heating, no formation of oligomers, and polymer formation irrespective of the medium or atmosphere. The propagating radicals are readily detected by ESR spectroscopy during polymerization in the crystalline state (Fig. 2), because termination between the propagating radicals occurs less frequently in the solid state [50]. 

Raising the temperature of a radical chain reaction causes an increase in the overall rate of polymerization since the main effect is an increase in the rate of decomposition of the initiator and hence the number of primary radicals generated per unit time. At the same time the degree of polymerization falls since, according to Eq. 3.3, the rate of the termination reaction depends on the concentration of radicals (see Example 3-2). Higher temperatures also favor side reactions such as chain transfer and branching, and in the polymerization of dienes the reaction temperature can affect the relative proportions of the different types of CRUs in the chains. 

Most radicals are highly reactive, and there are few examples where one would produce a stable radical product in a reaction. Reference to a radical reaction in synthesis or in Nature, almost always concerns a sequence of elementary reactions that give a composite reaction. Multistep radical sequences are discussed in general terms in this section so that the elementary radical reactions presented later can be viewed in the context of real conversions. The sequences can be either radical chain reactions or radical nonchain reactions. Most synthetic apphcations involve radical chain reactions, and these comprise the bulk of organic synthetic sequences and commercial applications. Nonchain reaction sequences are largely involved in radical reactions in biology. Some synthetic radical conversions are nonchain processes, and some recent advances in commercial polymerization reactions involve nonchain sequences. 

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