Radical addition polymerization

Proliferous Polymerization. Eady attempts to polymerize VP anionicaHy resulted in proliferous or "popcorn" polymerization (48). This was found to be a special form of free-radical addition polymerization, and not an example of anionic polymerization, as originally thought. VP contains a relatively acidic proton alpha to the pyrroHdinone carbonyl. In the presence of strong base such as sodium hydroxide, VP forms cross-linkers in situ probably by the following mechanism ... 

The first use of ionic liquids in free radical addition polymerization was as an extension to the doping of polymers with simple electrolytes for the preparation of ion-conducting polymers. Several groups have prepared polymers suitable for doping with ambient-temperature ionic liquids, with the aim of producing polymer electrolytes of high ionic conductance. Many of the prepared polymers are related to the ionic liquids employed for example, poly(l-butyl-4-vinylpyridinium bromide) and poly(l-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide [38 1]. 

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

Suppose the chain-reaction mechanism for radical-addition polymerization of a monomer M (e.g., CH2CHC1), which involves an initiator I (e.g., benzoyl peroxide), at low concentration, is as follows (Hill, 1977, p. 124) ... 

In a certain radical-addition polymerization reaction, based on the mechanism in Example 7-4, in which an initiator, I, is used, suppose measured values of the rate, (-tm), at which monomer, M, is used up at various concentrations of monomer, cM, and initiator, 0, are as follows (Hill, 1977, p. 125) ... 

Free market, for silver, 22 648 Free moisture content, 9 97 Free on board (FOB), 25 329 Free-radical addition polymerization,... 

According to free radical addition polymerization theory, the rate of polymerization of monomer M is proportional to the square root of the initiator I concentration (Equation 3) when termination is bi mol ecular (18). 

Free-Radical Addition. In free-radical addition polymerization, the propagating species is a free radical. The free radicals, R-, are most commonly generated by the thermal decomposition of a peroxide or azo initiator, I (see Initiators, free-radical) ... 

Exercise 25-10 A cation-exchange resin can be prepared by radical-addition polymerization of phenylethene (styrene, Section 10-8) in the presence of about... 

Kinetics. Monomer can be converted into polymer by any chemical reaction which creates a new covalent bond. Most of this review will concern polymerization of vinyl monomers by free radical addition polymerization. However, some attention will be given to cationic polymerization of epoxy functional materials. No extensive review of polymerization processes and kinetics will be given here, but some of the fundamental notions will be described. For reviews, see (4a-d). 

The free radical addition polymerization process is commonly used to synthesize polymers for pharmaceutical applications. In this type of polymerization, four reactions take... 

Tihe theory of free-radical addition polymerization, described in numer-ous publications (2, 3, 4, 17, 21), makes it clear that radical chain-growth reactions of polymers are regulated by statistical laws. Because of their statistical character the products from these reactions must be heterodisperse. The ranges extend from a single unit upward, depending upon kinetic details of the reactions. 

Figure 5.9. Reactions involved in free-radical addition polymerization. Shown are (a) (i)-(iii) generation of free radicals from a variety of initiators, (b) initiation of polymer chain growth through the combination of a free radical and unsaturated monomer, (c) propagation of the polymer chain through the combination of growing radical chains, (d) chain-transfer of free radicals between the primary and neighboring chains, and (e) termination of the polymer growth through either combination (i) or disproportionation (ii) routes.

Plasma-induced species act as initiator of polymerization. Polymerization characteristics and properties of polymers formed by plasma-induced polymerization strongly resemble those of the thermal polymerization of the corresponding monomer [2-12]. Results indicate that plasma-induced polymerization is a free radical addition polymerization initiated by difunctional free radicals created by plasma. The molecular weight of polymer increases with the polymerization time, which is distinctively different from the initiator-initiated free radical addition polymerization. 

After a long reaction time, polymers with exceptionally high molecular weight can be synthesized by plasma-induced polymerization. Since only brief contact with luminous gas phase is involved, plasma-induced polymerization is not considered to be LCVD. However, it is important to recognize that the luminous gas phase can produce chemically reactive species that trigger conventional free radical addition polymerization. This mode of material formation could occur in LCVD depending on the processing conditions of LCVD, e.g., if the substrate surface is cooled to the extent that causes the condensation of monomer vapor. 

The effect of pulsed discharge on plasma polymerization may be viewed as the analogue of the rotating sector in photoinitiated polymerization. The ratio r of off time 2 to on time ti, r = l2lti, is expected to influence the polymerization rate depending on the relative time scale of t2 to the lifetime of free radicals in free radical addition polymerization of a monomer. This technique was used to estimate the average lifetime of free radicals in the polymerization. 

Free radical polymerization offers a convenient approach toward the design and synthesis of special polymers for almost every area. In a free radical addition polymerization, the growing chain end bears an unpaired electron. A free radical is usually formed by the decomposition of a relatively unstable material called initiator. The free radical is capable of reacting to open the double bond of a vinyl monomer and add to it, with an electron remaining unpaired. The energy of activation for the propagation is 2-5 kcal/mol that indicates an extremely fast reaction (for condensation reaction this is 30 to 60 kcal/mol). Thus, in a very short time (usually a few seconds or less) many more monomers add successively... 

Fig. 2.2. Free radical addition polymerization of MMA using AIBN as initiator.

Generalized methods of initiating the polymerization of these monomers have recently been reviewed in detail [9], and were also mentioned briefly earlier in this Chapter. As with vinyl monomers initiation can be efficient and rapid, with the production of a fixed number of active centres. Propagation appears to be much slower, however, and rates of polymerization are comparable to those in free radical addition polymerizations. Techniques such as dilatometry, spectrophotometry etc. are therefore convenient for kinetic investigation of this type of cationic reaction. 

In the next section, we shall encounter another example of free-radical addition-polymerization— which has played a key part in the creation of this age of plastics. 

The specificity of the reaction mechanism to the chemistry of the initiator, co-initiator and monomer as well as to the termination mechanism means that a totally general kinetic scheme as has been possible for free-radical addition polymerization is inappropriate. However, the general principles of the steady-state approximation to the reactive intermediate may still be applied (with some limitations) to obtain the rate of polymerization and the kinetic chain length for this living polymerization. Using a simplified set of reactions (Allcock and Lampe, 1981) for a system consisting of the initiator, I, and co-initiator, RX, added to the monomer, M, the following elementary reactions and their rates may be... 

Copolymers of 8 and vinylferrocene, containing two different transition metals as shown in 33, were readily prepared by radical addition polymerization. 

Polybutadiene formed by high-temperature, free-radical addition polymerization is a copolymer of these three kinds of structural units. With isoprene (2-methyIbutadlene), the number of ways the unit can enter the polymer chain is still larger for example, the... 

Hayashi has investigated in some detail the ionic photopolymerization of styrene monomers. Free ion lifetimes measured by pulse electrical conductivity measurements were found to agree with those calculated from steady-state conductance measurements. Other studies of interest on radical addition polymerization include the photodimerization of polymers containing thymine bases, diene polymerization by terbium complexes, polymerization of vinyl acetate, and preparation of light-sensitive polyacrylates. 

Radical addition polymerization of exocyclic vinyl groups to... 

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