Benzoyl peroxide, ethylene polymerization

Polymerization of styrene is carried out under free radical conditions often with benzoyl peroxide as the initiator Figure 1111 illustrates a step m the growth of a poly styrene chain by a mechanism analogous to that of the polymerization of ethylene (Sec tion 6 21)... 

I Initiation The polymerization reaction is initiated when a few radicals are generated on heating a small amount of benzoyl peroxide catalyst to break the weak 0-0 bond. A benzoyloxy radical then adds to the C=C bond of ethylene to generate a carbon radical. One electron from the C=C bond pairs up with the odd electron on the benzoyloxy radical to form a C-O bond, and the other election remains on carbon. 

Radical Polymerization. Radical chain polymerization involves initiation, propagation, and termination. Consider the polymerization of ethylene. Initiation typically involves thermal homolysis of an initiator such as benzoyl peroxide... 

In 1988, Terry and coworkers attempted to homopolymerize ethylene, 1-octene, and 1-decene in supercritical C02 [87], The purpose of their work was to increase the viscosity of supercritical C02 for enhanced oil recovery applications. They utilized the free radical initiators benzoyl peroxide and fert-butyl-peroctoate and conducted polymerization for 24-48 h at 100-130 bar and 71 °C. In these experiments, the resulting polymers were not well studied, but solubility studies on the products confirmed that they were relatively insoluble in the continuous phase and thus were not effective as viscosity enhancing agents. In addition, a-olefins are known not to yield high polymer using free radical methods due to extensive chain transfer to monomer. 

In 1838 Regnault [15] reported that vinylidene chloride could be polymerized. In 1839 Simon [16] and then Blyth and Hofmann (1845) [17] reported the preparation of polystyrene. These were followed by the polymerization of vinyl chloride (1872) [18], isoprene (1879) [19], methacrylic acid (1880) [20], methylacrylate (1880) [21], butadiene (1911) [22], vinyl acetate (1917) [23], vinyl chloroacetate [23], and ethylene (1933) [24]. Klatte and Rollett [23] reported that benzoyl peroxide is a catalyst for the polymerization of vinyl acetate and vinyl chloroacetate. 

In general, there are two distinctively different classes of polymerization (a) addition or chain growth polymerization and (b) condensation or step growth polymerization. In the former, the polymers are synthesized by the addition of one unsaturated unit to another, resulting in the loss of multiple bonds. Some examples of addition polymers are (a) poly(ethylene), (b) poly(vinyl chloride), (c) poly(methyl methacrylate), and (d) poly(butadiene). The polymerization is initiated by a free radical, which is generated from one of several easily decomposed compounds. Examples of free radical initiators include (a) benzoyl peroxide, (b) di-tert-butyl peroxide, and (c) azobiisobutyronitrile. 

A further extension of the principle of polymerization through ethylenic unsaturation allows copolymers to be made with the chemically similar organic substances. For example, methylallyldichloro-silane may be hydrolyzed to the corresponding diol and mixed with monomeric methyl methacrylate, and the mixture polymerized to a homogeneous solid by heating with benzoyl peroxide as a catalyst. 

Manufacture. The monomer of polyvinyl chloride is gas at room temperature and has a boiling point of -13.9°C. The monomer is manufactured in two ways (1) Acetylene and HCl gas are allowed to pass over active carbon and to react with each other at 150 200 C (2) chlorine gas is allov/-ed to react on ethylene to produce C H CI CEDC) at 200 350 C to produce vinyl chloride and HCl gas. These are separated by distillation. The monomer is then polymerized in a pressure vessel the mixture of water and liquid monomer is stirred violently. Benzoyl peroxide is used as a catalyzer and polyvinyl alcohol as a dispersion agent for the reaction. 

Historically, ethylene polymerization was carried out at high pressure (1000-3000 atm) and high temperature (100-250 °C) in the presence of a catalyst such as benzoyl peroxide, although other catalysts and reaction conditions are now more often used. The key step is the addition of a radical to the ethylene double bond, a reaction similar in many respects to what takes place in the addition of an electrophile. In writing the mechanism, recall that a curved halfarrow, or "fishhook" A, is used to show the movement of a single electron, as opposed to the full curved arrow used to show the movement of an electron pair in a polar reaction. 

Problem 7.20 Bulk polymerization of methyl methacrylate (MMA) at 60°C with 0.9 g/L of benzoyl peroxide yielded a polymer with a weight average degree of polymerization of 8600 at low conversions. Predict the conversions of MMA at which gelation would be observed if it is copolymerized with 0.05 mol% of ethylene glycol dimethaciylate (EGDMA) at the same temperature and initiator concentration as in the homopolymerization case. 

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