Polymerization addition/chain-reaction

Ring opening polymerization may also occur by an addition chain reaction. For example, a ring opening reaction polymerizes trioxane to a polyacetal in the presence of an acid catalyst. Formaldehyde also produces the same polymer ... 

Vinyl addition polymerizations, being chain reactions, are more complex than condensation polymerizations and they are generally more likely to include minor side reactions leading to branching or cross-linking. The structures of the pol3oneric products are consequently more difficult to establish with certainty. 

Nearly all synthetic polymers are synthesized by the polymerization or copolymerization of different "monomers." The chain growth process may involve the addition chain reactions of unsaturated small molecules, condensation reactions, or ringopening chain-coupling processes. In conventional polymer chemistry, the synthesis of a new polymer requires the use of a new monomer. This approach is often unsatisfactory for Inorganic systems, where relatively few monomers or cyclic oligomers can be Induced to polymerize, at least under conditions that have been studied to date. The main exception to this rule is the condensation-type growth that occurs with inorganic dl-hydroxy acids. 

There is an alternative, somewhat less meaningful system of classification addition polymerization, in which molecules of monomer are simply added together and con-iensation polymerization, in which monomer molecules combine with loss of some simple nolecules like water. As it happens, the two systems almost exactly coincide nearly all ases of chain-reaction polymerization involve addition polymerization nearly all cases pf step-reaction polymerization involve condensation polymerization. Indeed, some hemists use the term addition polymerization to mean polymerization via chain reactions. 

In addition polymerization, a chain reaction appends new monomer units to the growing molecule one at a time. Each new unit creates an active site for the next attachment. The polymerization of ethylene gas (C2H4) is a typical example. The process begins with a monomer of ethylene gas in which the carbon atoms are joined by covalent bonds as below ... 

Many authors divide polymerizations into chain reactions and stepwise reactions. Of course, all reactions proceed in steps, that is, one reaction step follows another. Termolecular reactions are rare but in the organic chemistry sense, the term stepwise reaction signifies that intermediate products can be isolated and subsequently again made to react. This means that, in the absence of impurities, certain reactions can be frozen. In actual fact, anionic addition polymerizations can be frozen at low temperatures and made to run again at higher temperatures. That this procedure is not possible in the presence of water or carbon dioxide is an experimental consideration and not a conceptual difficulty. If we lived in an isocyanate atmosphere, no steps could be isolated in the polyamide synthesis. Thus, such classifications are based on experimental expertise, which can never be the basis of a physical definition. 

In addition polymerization (sometimes called chain-growth polymerization), a chain reaction adds new monomer units to the growing polymer molecule one at a time through double or triple bonds in... 

The process or chemical reaction in which the molecules of a monomer are linked together to form macromolecules whose molecular weight is a multiple of that of the original substance nM [M] , where n is a degree of polymerization, DP. It is said that the polymerization leads to oligomer if 10 < DP < 50 and to polymer if DP > 50. When two or more monomers are involved, the process is called copolymerization. Most polymerization processes are classified as condensation (step) reactions or addition (chain) reactions. 

Monomers containing rings or double bonds can be polymerized by chain polymerization, which is also known as addition polymerization. (It should be contrasted with Step polymerization.) The chain reaction involves the sequential steps of initiation, propagation and termination. Initiation is the process by which active centres are formed these may be free radicals, anions or cations. The free radical chain polymerization of a vinyl monomer is illustrated below. 

Addition Polymerization Chemical chain-reaction process in which monomers bond to each other in the step-by-step growth of a polymer, without generating any by-products. 

The second category of polymerization reactions does not involve a chain reaction and is divided into two groups poly addition and poly condensation [4]. In botli reactions, tire growth of a polymer chains proceeds by reactions between molecules of all degrees of polymerization. In polycondensations a low-molecular-weight product L is eliminated, while polyadditions occur witliout elimination ... 

The mechanism of these reactions places addition polymerizations in the kinetic category of chain reactions, with either free radicals or ionic groups responsible for propagating the chain reaction. 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

The degree of polymerization is controlled by the rate of addition of the initiator. Reaction in the presence of an initiator proceeds in two steps. First, the rate-determining decomposition of initiator to free radicals. Secondly, the addition of a monomer unit to form a chain radical, the propagation step (Fig. 2) (9). Such regeneration of the radical is characteristic of chain reactions. Some of the mote common initiators and their half-life values are Hsted in Table 3 (10). 

The use of agarose as an electrophoretic method is widespread (32—35). An example of its use is in the evaluation and typing of DNA both in forensics (see Forensic chemistry) and to study heritable diseases (36). Agarose electrophoresis is combined with other analytical tools such as Southern blotting, polymerase chain reaction, and fluorescence. The advantages of agarose electrophoresis are that it requires no additives or cross-linkers for polymerization, it is not hazardous, low concentration gels are relatively sturdy, it is inexpensive, and it can be combined with many other analytical methods. 

The mechanism of ion polymerization in formaldehyde crystals proposed by Basilevskii et al. [1982] rests on Semenov s [1960] assumption that solid-phase chain reactions are possible when the arrangement of the reactants in the crystal prepares the configuration of the future chain. The monomer crystals capable of low-temperature polymerization fulfill this condition. In the initial equilibrium state the monomer molecules are located in the lattice sites and the creation of a chemical bond requires surmounting a high barrier. However, upon creation of the primary dimer cation, the active center shifts to the intersite, and the barrier for the addition of the next link... 

The thermal (or photochemical) decomposition of the azo group gives rise to a radically initiated polymerization. The reactive site F, the transformation site, however, can, depending on its chemical nature, initiate a condensation or addition type reaction. It can also start radical or ionic polymerizations. F may also terminate a polymerization or even enable the azo initiator to act as a monomer in chain polymerizations. 

Addition polymerization requires a chain reaction in which one monomer molecule adds to a second, then a third and so on to form a macromolecule. Addition polymerization monomers are mainly low molecular-weight olefinic compounds (e.g., ethylene or styrene) or conjugated diolefins (e.g., hutadiene or isoprene). 

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. 

Addition polymerizations of unsaturated monomers leading to the formation of products of high molecular weight invariable proceed by chain reaction mechanisms. Primary activation of a monomer M (or a pair of monomers) is followed by the addition of other monomers in rapid succession... 

A corresponding anionic mechanism in the presence of a strong base (or electron donor) is plausible. Other cyclic compounds may be susceptible to polymerization by similar ionic mechanisms. Inasmuch as the growth step must be extremely rapid, a chain reaction is indicated and classification with vinyl-type addition polymerizations should be appropriate in such cases. 

Transfer constants for polystyrene chain radicals at 60° and 100°C, obtained from the slopes of these plots and others like them, are given in the second and third columns of Table XIII. Almost any solvent is susceptible to attack by the propagating free radical. Even cyclohexane and benzene enter into chain transfer, although to a comparatively small extent only. The specific reaction rate at 100°C for transfer with either of these solvents is less than two ten-thousandths of the rate for the addition of the chain radical to styrene monomer. A fifteenfold dilution with benzene was required to halve the molecular weight, i.e., to double l/xn from its value (l/ rjo for pure styrene (see Fig. 16). Other hydrocarbons are more effective in lowering the degree of polymerization through chain transfer. 

Other additives can promote chain reactions by acting as initiators of the chain. These compounds are materials that can be more readily dissociated than the primary reactants. For example, benzoyl peroxide has often been used to initiate the polymerization of olefinic monomers because of the ease with which it dissociates. 

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