POLYMERLETTERS VOL. 2, PP. 1015-1017 (1964)

THE ROLE OF FERRIC CHELATES IN POLYMERIZATION INITIATION

Earlier work has demonstrated the initiation of the emulsion polymer- ization of styrene by hydrogen peroxide and certain metal chelates of polyaminocarboxylic acids, e.g., ferrous ( l ) , chromous (2), and cupric (3). The similar use of the ferric chelates of ethylenediaminetetraace- tic acid (EDTA) and cyclohexanediaminetetraacetic acid (CDTA) has now been investigated, the experimental procedure being the s a m e as that outlined previously (1). The composition of the reaction mixture was: 50 g. styrene monomer, 100 g. distilled water,' 5g. emulsifying agent (the sulfated product of the condensation of 21 molecules of ethyl- ene oxide with 1 molecule of capryl alcohol), 0.2 g. hydrogen peroxide, 0.02 g. ferric iron added a s ferric ammonium sulfate and the equivalent amount of the polyaminocarboxylic acid to complex the metal ions.

The variation with pH of the zero-order rate of polymerization of sty- rene initiated by the ferric ion-EDTA hydrogen peroxide system is shown in Figure 1 and a similar curve was obtained with ferric ion-CDTA-hydro- gen peroxide, both curves having the same shape as those obtained with the ferrous chelate-hydrogen peroxide initiators. It was, therefore, of in- terest to compare the rates of polymerization using the same concentra- tions of the ferrous and ferric chelates respectively in the initiating sys- tems. The rate of polymerization was found to be greater in the former case in acid solution, e.g., a t pH 3 the rates of polymerization were 0.16% conversion min.-' and 0.02% conversion min.-' using respective- ly the ferrous and ferric chelates of EDTA at 4OoC. In alkaline solution the rates of polymerization with the ferrous and ferric chelate systems were found to be the same.

If only the first two steps in the mechanism proposed by Barb et al. ( 4 ) for the reaction of ferric ions and hydrogen peroxide are considered to be important in the presence of monomer, then the corresponding reac- tion between ferric chelate (Fey-) and hydrogen peroxide may be written:

Fey- + H,Oz -+ FeyZ- + HO; + HS

FeyZ- + HzOz - Fey- +HO' + OH- ( 2)

It is reported that the perhydroxyl radical is not an initiator of polymer- ization and this has been confirmed in the present work. Styrene monom- er could not be polymerized when the concentration of the hydrogen per- oxide was lo4 t i m e s that of the ferric chelate. Under these circum- stances the hydroxyl radicals would react with the excess hydrogen

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Fig. 1. The variation of the zero-order rate of polymerization of styrene with pH using the ferric ion-EDTA-hydrogen peroxide initiator.

peroxide producing perhydroxyl radicals; thus reaction (2) above must be the initiating one when the peroxide is not in overwhelming excess. Initiation by the ferric chelate must therefore involve reactions (1) and (2), while with the ferrous chelate only reaction (2) is important.

It was found that at low pH values the rate of decomposition of hydro- gen peroxide by the ferric chelates was almost negligible suggesting that reaction (1) is very slow. Reaction (Z), on the other hand is appre- ciably faster since under similar conditions the ferrous chelates decom- pose hydrogen peroxide much more rapidly. It is thus to be expected that polymerization in the presence of a ferric chelate will be slower than in the presence of a ferrous chelate.

The ferric ion-EDTA complex and the ferric ion-CDTA complex are hydroxylated above pH 6.5 and 8.5 respectively, and it is well estab- lished (6) that hydroxylated ferric chelates react rapidly with hydrogen peroxide. Kinetic measurements carried out in the present work show that the decomposition of hydrogen peroxide by the hydroxylated ferric chelates is rapid and that the rate of decomposition increases sharply with increasing pH, i.e., with increasing hydroxylation. This, together with the fact that in alkaline solution the polymerization rates in the presence of either ferrous or ferric chelates are the s a m e , suggests that reaction (1) is faster than reaction (2) at pH values above 6.5, reaction (2) thus being the rate controlling step of the initiation.

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References

(1) Bond, J . , and T. I. Jones, J . Polymer Sci., 41, 75 (1960). (2) Bond, J., and D. B. Hobson, J . Polymer Sci., 1, 2179 (1963). (3) Bond, J. , and D. B. Hobson, J . Polymer Sci., 1, 2185 (1963). (4) Barb, W. G., J . H. Baxendale, P. George, and K. R. Hargrave,

(5) Barb, W. G., J. H. Baxendale, P. George, and K. R. Hargrave,

(6) Wang, J . H., J . Am. Chem. SOC., II, 4715 (1955).

Trans. Faraday SOC., a 591 (1951).

Trans. Faraday SOC., 4L 462 (1951).

Joan Bond C. W. Brown G. S. Rushton

The Royal College of Advanced Technology Salford, Lancashire, England

Received June 9, 1964