Free radical polymerization chain initiation

Problem 6.8 Consider the following scheme of reactions for free-radical chain polymerization initiated by thermal homolysis of initiator with cage effect [6] ... 

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

Recendy, photopolymer systems have aroused increased interest because of their manifold applications in several high technologies [1-3]. Among such systems, those derived from photoinduced polymerization play an important role. The fundamental principles of these systems are based on the production of species X by photoreactions, which then initiates thermal reactions of low-molecular products leading to polymer or network formation see Eq. (1). In general, these thermal reactions are associated with low activation energies (about 60 kJ mol 1 for free radical chain polymerization). Therefore, such processes can also occur suffidentiy fast at room temperature. 

Although the presence of water is generally not an issue in free-radical chain polymerization (indeed water may be a suitable medium for polymerization as in Protocols 5-7) unlike, for example, chain-growth polymerization initiated by anionic species, it is always advisable to use solvents of the highest purity and this will generally include some element of predrying. In general, solvents should be distilled, particularly as a number of suitable solvents for polymerization reactions contain stabilizers which usually serve to mop up free radicals and therefore inhibit the polymerization... 

Thble 6.5 G-Values (100-eV yields of initiating radicals) in Radiolytic Free-Radical Chain Polymerization... 

The second step of initiation [Eq. (8.83)], being slower than the first [Eq. (8.82)], is rate-determining for initiation (unlike in the case of free-radical chain polymerization) and so though the amide ion produced upon chain transfer to ammonia can initiate polymerization it is but only at a rate controlled by the rate constant, ki, for initiation. Therefore, this chain transfer reaction may be considered as a true kinetic-chain termination step and the application of steady-state condition gives Eq. (8.90). 

Free-Radical Chain Polymerization. In contrast to the typically slow stepwise polymerizations, chain reaction polymerizations are usually rapid with the initiated species rapidly propagating until termination. A kinetic chain reaction usually consists of at least three steps, namely, initiation, propagation, and termination. The initiator may be an anion, cation, free radical, or coordination catalyst. 

Because most synthetic plastics, elastomers, and fibers are prepared by free-radical chain polymerizations, this method will be discussed here. Initiation can occur through decomposition of an initiator such as azobisisobutyronitrile (AIBN), light, heat, sonics, or other technique to form active free radicals. Here initiation will be considered as occurring by decomposition of an initiator, I, and is described as follows. 

Though resembling free-radical chain polymerization in terms of initiation, propagation, transfer, and termination steps, ionic polymerizations have signif-... 

Chain reactions also occur in the liquid phase, and many synthetic polymers are produced by free-radical chain polymerizations. The initiation step is the decomposition of an added initiator, an unstable molecule such as a peroxide or persulfate ... 

The synthesis of macromolecules by the free radical chain polymerization of low molar mass compounds, denoted as monomers, commences with the generation of free radicals, which is conveniently performed through photoreactions of initiator molecules. The subsequent processes, i.e. propagation, including chain transfer, and termination, are thermal (dark) reactions, which are not affected by light The simplified overall mechanism is described in Scheme 10.1. 

The putative dimerization products 15 and 16 lack one or two types of this latter radical-stabilizing effect ( 8-10 kj moP per radical), which is directly reflected in the higher reaction barriers leading to their formation. Thus, it can be concluded that the Giich reaction is a free radical chain polymerization of the p-quinodimethane monomers 11, initiated by the dimer diradicals 14. If unsuccessful as initiators, the diradicals 14 can undergo an intramolecular radical recombination, leading to the [2.2]paracyclophane side products. 

The most common restoratives comprise a mineral filler mixed with a vinyl monomer (often called a resin), which undergo photoinitiated free radical Chain polymerization. Typical components inclnde the adduct of bis-phenol A and glycidy methacrylate (bis-GMA) or nrethane dimethacrylate (UDMA), camphorquinone initiator (activated by a visible bine light sonrce of 480 nm) and a filler such as zirconia or borosilicate glass, which can comprise 70% by mass of the system and provides the strength . The whole is referred to as a composite . 

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. 

Reactive acrylic adhesives generally consist of a solution of a toughening rubber (chlorosulphonated polyethylene) in a partly polymerized mixture of monomers this is mainly methylmethacrylate but ethane diol dimethacrylate is added as a cross-linking agent. The remaining monomer is polymerized in a free radical chain polymerization redox initiation involves an organic peroxide and a tertiary amine. Acrylic cements consist of a partly polymerized acrylic monomer containing an initiator. Cure is established by the thermal or UV decomposition of the initiator (see Radiation-cured adhesives). 

Explain why only a catalytic amount of the radical initiator is required in a free-radical-chain polymerization reaction. 

The ceiling temperature constraint in the homopolymerization of alphamethyl styrene (AMS) can be circumvented by copolymerization with acrylonitrile (AN) to prepare multicomponent random microstructures that offer higher heat resistance than SAN. The feasibility of a thermal initiation of free radical chain polymerization is evaluated by an experimental study of the terpolymerization kinetics of AMS-AN-Sty. Process considerations such as polyrates, molecular weight of polymer formed, sensitivity of molecular weight, molecular weight distribution, and kinetics to temperature were measured. 

The vast majority of azopolymers developed for optical storage are polyacrylates and polymethacrylates, which are generally prepared by free radical chain polymerization in solution using conventional experimental conditions. For example, azobisisobutyronitrile (AIBN) is used as a thermal initiator in dry organic solvents such as A(A-dimethylformamide (DMF), tetrahydrofuran (THF) or dioxane as the most common. Occasionally, the polymerization process of azobenzene (meth)acrylates can be limited by the radical transfer reaction promoted by the azo group, which seems to be associated with the formation of hydrazyl radicals (Nuyken and Weidner, 1986 Hallensleben andWeichart,1989). 

Figure 2 Simplified mechanism of free radical chain polymerization. On heating or by UV-irradiation, the initiator generates free radicals 1 whose reaction with methyl methacrylate 2 creates the active species 3. Further reaction with monomer molecules leads to poly(methyl methacrylate) PMMA 4. Applied to 2,3-epoxypropyl methacrylate 5 this process gives the linear aliphatic polymer 6 with pendent epoxy groups.

Free-Radical Polymerization The free-radical chain polymerization of appropriate monomers can be initiated with the aid of Vis/UV hght. Here, light serves only as an initiating tool, and does not interfere with the propagation and termination stages of the chain process. The initiation affords the presence of an appropriate light-absorbing initiator, as shown in Scheme 3.1. 

Termination of free-radical chain polymerization may also take place by disproportionation. The description for chain termination by disproportionation is given in Eq. (9). The kinetic chain length (v) is the number of monomer molecules consumed by each primary radical and is equal to the rate of propagation divided by the rate of initiation for termination by disproportionation. The kinetic equation for the termination by disproportionation is... 

Inhibitors slow or stop polymerization by reacting with the initiator or the growing polymer chain. The free radical formed from an inhibitor must be sufficiently unreactive that it does not function as a chain-transfer agent and begin another growing chain. Benzoquinone is a typical free-radical chain inhibitor. The resonance-stabilized free radical usually dimerizes or disproportionates to produce inert products and end the chain process. 

Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

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