Polymerization by Free Radical Mechanism

The general kinetics for this mechanism [26] involve the usual three primary steps of any chain reaction, i.e., initiation, propagation, and termination, as shown below. Initiation generally occurs by the formation of free radicals through the homolytic dissociation of weak bonds (e.g., in peroxides or azo compounds) or by irradiation. Termination reactions for vinyl polymers can occur either by combination (coupling), by disproportionation, or by a combination of both reactions (discussed next). 

This sequence of steps then leads to the following simple kinetic treatment [26]  

Assuming a steady-state condition where the rate of formation of radicals is equal to their rate of disappearance, i.e., Ri = R  

Equation (15) thus illustrates the dependency of the overall rate of polymerization on the concentrations of initiator and monomer. The half-power dependence of the rate on the initiator concentration appears to be a universal feature of the free radical mechanism and has been used as a diagnostic test for the operation of this mechanism. 

Another important aspect of free radical polymerization is the dependency of the number-average degree of polymerization on initiator and monomer concentrations as shown in Eq. (16). Comparison with Eq. (15) shows that increasing the rate of initiation, by increasing the initiator concentration, increases the rate of polymerization but decreases the degree of polymerization, X , which corresponds to the number-average number of units per chain. 

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Up to the present time, use has been made almost entirely of dimethacrylates which are polymerized by free radical mechanisms to yield crosslinked products (81. Polymerization is initiated by redox systems, such as benzoyl peroxlde/aromatic amine, and by... 

It is readily apparent from the data of Table I that the polymerization rate increases with increasing temperature. This is a usual result in the free radical polymerization of many monomers, but it is a little surprising for DPHD, owing to its similarity with a-me-thylstyrene, a monomer which does not polymerize by free radical mechanism at temperatures higher than 61°C... 

Direct emulsification can be used to produce latexes from polymers that cannot be polymerized by free-radical mechanisms and from natural polymers or their derivatives. Three methods or a combination of methods can be applied in direct emulsification... 

Since Rp/[I] is constant, data are consistent with rate of polymerization by free-radical mechanism. 

CHAIN POLYMERIZATION BY FREE RADICAL MECHANISM 2.4.1 General Kinetics... 

There are some reports in the literature of ring-opening polymerizations by free-radical mechanism. One is a polymerization of substituted vinyl cyclopropanes.The substituents are radical stabilizing structures that help free-radical ring-opening polymerizations of the cyclopropane rings. This can be illustrated as follows ... 

Due to the allylic nature of a-methyl styrene it cannot be polymerized by free-radical mechanism. It polymerizes readily, however, by ionic mechanism. Resins based on copolymers of a-methyl styrene are available commercially. Other styrene derivatives that can be obtained comm ially... 

Acrylic and methacrylic esters polymerize by free-radical mechanism to atactic polymers. The sizes of the alcohol portions of the esters determine the Tg values of the resultant polymers. They also determine the solubility of the resultant polymers in hydrocarbon solvents and in oils. 

Polymerizations by free-radical mechanism are typical free-radical reactions. That is to say, there is an initiation, when the radicals are formed, a propagation, when the products are developed, and a termination, when the free-radical chain reactions end. In the polymerizations, the propagations are usually chain reactions. A series of very rapid repetitive steps follow each single act of initiation, leading to the addition of thousands of monomers. 

Compounds possessing allylic structures polymerize by free-radical mechanism only to low molecularweight oligomers. In some cases the products consist mostly of dimers and trimers. The DP for poly(allyl acetate), for instance, is only about 14. This is due to the fact that allylic monomer radicals are resonance-stabilized to such an extent that no extensive chain propagations occur. Instead, there is a large amount of chain transferring. Such chain transferring essentially terminates the reactions [151]. The resonance stabilization can be illustrated on an allyl alcohol radical ... 

Some other heterocyclic monomers, like acetals, also polymerize by free-radical mechanism [229]. Particularly interesting is an almost quantitative ring-opening polymerization of a seven-membered acetal, 2-methylene-l,3-dioxepane [230] ... 

Explain ring-opening polymerizations by free-radical mechanism, giving two examples. 

Commercially, acrylamide is formed from acrylonitrile by reaction with water. Similarly, the preferred conunercial route to methacrylamide is through methacrylonitrile. Acrylamide polymerizes by free-radical mechanism [270]. Water is the common solvent for acrylamide and methacrylamide polymerizations because the polymers precipitate out from organic solvents. 

The acid chlorides of both acrylic and methacrylic acids polymerize by free-radical mechanism in dry aromatic and aliphatic solvents. Molecular weights of the products, however, are low, usually under 10,000 [273, 274]. Polyacrylic and polymethacrylic acids are used industrially as thickeners in cosmetics, as flocculating agents, and when copolymerized with divinyl benzene in ion-exchange resins. 

Polyfluorostyrenes are described in many publications. A (3-fluorostyrene can be formed by cationic mechanism [289]. The material softens at 240-260°C. An a,p,(3-trifluorostyrene can be polymerized by free-radical mechanism to yield an amorphous polymer that softens at 240°C [290], Ring-substituted styrenes apparently polymerize similarly to styrene. Isotactic poly(o-fluorostyrene) melts at 265°C. It forms by polymerization with Ziegler-Natta catalysts [291]. The meta analog, however, polymerized under the same conditions yields an amorphous material [291]. 

Commercially, poly(vinyl acetate) is formed in bulk, solution, emulsion, and suspension polymerizations by free-radical mechanism. In such polymerizations, chain transferring to the polymer may be as high as 30%. The transfer can be to a polymer backbone through abstraction of a tertiary hydrogen ... 

C.H. Bamford, W.G. Barb, A.D. Jenkins, and P.F. Onyon, The Kinetics of Vinyl Polymerization by Free-Radical Mechanism, Butterworth, London, 1954... 

Studies on the radiation-induced polymerization were carried out mainly on the example of vinyl monomers, which polymerize by free radical mechanism, leading both reactions in the mass of monomer as well... 

Free-radical polymerization of vinyl monomers takes place through intermediates having an unpaired electron known as free radicals. Many vinyl monomers are readily polymerized by free-radical mechanisms because free-radical polymerization is relatively less sensitive to impurities compared to ionic polymerizations. Free radicals can be generated in a number of ways, including organic or inorganic initiators and even without added initiators (e.g., thermal and photoinitiation). There are over 50 different organic peroxides and azo initiators in over 100 different formulations produced commercially. Initiators are selected based on several factors polymerization rate, reaction temperature, solubility, and polymer properties. 

When ethylene is polymerized by free-radical mechanism, high pressure and high temperature are required. Organic initiators and oxygen are used as free-radical generators. The general kinetic scheme for free radical ethylene copolymerization is represented as follows ... 

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