Polymerisation chain transfer

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Gaseous vinyl chloride monomer is polymerised under high pressure conditions. Since polyvinyl chloride polymer is insoluble in its own monomer, the reaction kinetics do not follow the classical emulsion polymerisation kinetics. During polymerisation, chain transfer to monomer is extensive, and molecular weight development depends upon the reaction temperature rather than the initiator concentration. Consequently, lower reaction temperatures are needed to reach higher molecular weights. A typical formulation for the suspension polymerisation of polyvinyl chloride is given in Table 5. 

In production, anhydrous formaldehyde is continuously fed to a reactor containing well-agitated inert solvent, especially a hydrocarbon, in which monomer is sparingly soluble. Initiator, especially amine, and chain-transfer agent are also fed to the reactor (5,16,17). The reaction is quite exothermic and polymerisation temperature is maintained below 75°C (typically near 40°C) by evaporation of the solvent. Polymer is not soluble in the solvent and precipitates early in the reaction. 

Cha.in-Tra.nsferAgents. The most commonly employed chain-transfer agents ia emulsion polymerisation are mercaptans, disulfides, carbon tetrabromide, and carbon tetrachloride. They are added to control the molecular weight of a polymer, by transferring a propagating radical to the chain transfer agent AX (63) ... 

Chain transfer to monomer and to other small molecules leads to lower molecular weight products, but when polymerisation occurs ia the relative absence of monomer and other transfer agents, such as solvents, chain transfer to polymer becomes more important. As a result, toward the end of batch-suspension or batch-emulsion polymerisation reactions, branched polymer chains tend to form. In suspension and emulsion processes where monomer is fed continuously, the products tend to be more branched than when polymerisations are carried out ia the presence of a plentiful supply of monomer. 

Chain transfer also occurs to the emulsifying agents, leading to their permanent iacorporation iato the product. Chain transfer to aldehydes, which may be formed as a result of the hydrolysis of the vinyl acetate monomer, tends to lower the molecular weight and slow the polymerisation rate because of the lower activity of the radical that is formed. Thus, the presence of acetaldehyde condensates as a poly(vinyl alcohol) impurity strongly retards polymerisation (91). Some of the initiators such as lauryl peroxide are also chain-transfer agents and lower the molecular weight of the product. 

Investigation has shown that chain transfer to polymer occurs predominantly on the acetate methyl group in preference to the chain backbone one estimate of the magnitude of the predominance is 40-fold (92,93). The number of branches per molecule of poly(vinyl acetate) polymerised at 60°C is ca 3, at 80% conversion. It rises rapidly thereafter and is ca 15 at 95% conversion and 1-2 x lO" number-average degrees of polymerisation. 

It has been shown that intramolecular chain transfer to polymer occurs during the polymerisation of vinyl acetate, lea ding to short-chain branching (81,235—238). The number of short-chain branches has been estimated by nmr to be in the range of 0.12—1.7 mol % (81). The number of short-chain branches increases significantly at low monomer concentration. 

Figure 2.19. Effect of chain transfer solvents on the degree of polymerisation of polystyrene. (After...

Polymerisation could proceed from the radical in the normal way or alternatively chain transfer may occur by a second back-biting stage either to the butyl group (Figure 10.2(a)) or to the main chain (Figure 10.2(h)). 

By polymerising styrene in solution many problems associated with heat transfer and the physical movement of viscous masses are reduced, these advantages being offset by problems of solvent recovery and the possibility of chain transfer reactions. In 1955 Distrene Ltd started a plant at Barry in South Wales for the production of styrene by such a solution polymerisation process and some details have been made available. The essential details of this process are indicated by Figure 16.7. 

Mention may finally be made of graft polymers derived from natural rubber which have been the subject of intensive investigation but which have not achieved commercial significance. It has been found that natural rubber is an efficient chain transfer agent for free-radical polymerisation and that grafting appears to occur by the mechanism shown in Figure 30.8. 

Metalloporphyrins as catalysts of chain transfer in radical polymerisation and stereoselective oxidation. L. Karmilova, G. V. Ponomarev, B. R. Smirnov and I. M. Bel yovskii, Russ. Chem. Rev. (Engl. Transl), 1984, 53,132 (44). 

Chain polymerisation necessarily involves the three steps of initiation, propagation, and termination, but the reactivity of the free radicals is such that other processes can also occur during polymerisation. The major one is known as chain transfer and occurs when the reactivity of the free radical is transferred to another species which in principle is capable of continuing the chain reaction. This chain transfer reaction thus stops the polymer molecule from growing further without at the same time quenching the radical centre. 

Typical chain transfer reactions involve the abstraction of an atom from a neutral saturated molecule, which may be solvent or a chain transfer agent added to the polymerisation mixture specifically to control the final size and distribution of molar masses in the polymer product. The chain transfer reaction may be represented as in Reaction 2.7. 

Other chain transfer processes may occur. For example, the radical may abstract an atom from along the backbone of a previously formed polymer molecule, and thus initiate the growth of a branch to the main chain. There can also be chain transfer to monomer, which in the nature of the polymerisation process must be a relatively rare phenomenon. However, it can occur infrequently and give rise to a restriction in the size of the polymer molecules without ceasing the overall radical chain reaction. 

First step radical polymerisation of butadiene in the presence of a chain transfer agent to control its molecular mass. 

Figure 1 Reaction scheme for the free-radical polymerisation (I is the initiator, R the fragment of initiator, M the monomer and AH the chain transfer agent).

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