By Frederick R. Eirich (Auth.)
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Furthermore, there are two kinds of growing chains, depending on which type of monomer unit occupies the growing end. Thus, using the free radical mechanism as the simplest example, the four types of propagation steps can be written as follows: Μ^ + Μ Α Μ Γ (22) ΜΓ + Μ 2 ^ Μ 2 ' (23) Μ2· + M2 ^ Μ2· (24) Μ2· + Μ 1 ^ Μ 1 " (25) where M* and M refer to the growing chain and the monomer, respectively, as before, while the subscripts refer to the two kinds of monomer in the mixture. It can be seen that these four propagation reactions lead to four propagation rate constants, as shown.
4), it only expresses the instantaneous chain length for an increment of polymer, and not the cumulative value for the total polymer obtained. From Eq. (17) it follows that the number- and weight-average chain lengths xn and xw are expressed by xn - and xw = (18) 1- p 1- p 1- p since p must always be close to unity for high polymers. Hence it follows again that *w/*n = 2 (19) 33 2. POLYMERIZATION The value of xw/xn for the cumulative polymer may, of course, be much higher, depending on the changes in the value of p with increasing conversion.
DISSOLVEO SOAP \ 1 o\r^CM^/^° o FREE / * MONOMER ~IO,§CM"3 10 ????????????? 3 SWOLLEN 1 »■ 500 A RADICAL y DISSOLVED MONOMER DROPLET i OL·/ ^V^ /'POLYMER X Q QI MONOMER \_1 \^ PARTICLE Γ° , SOAP MICELLE ~Ιθ'·θΜ-5 1 Fig. 1. Loci in mechanism of emulsion polymerization. 36 MAURICE MORTON diffusing from the aqueous phase. The major portion of the polymerization reaction apparently occurs within this large number of latex particles isolated from each other and kept saturated [17] with monomer diffusing from the monomer droplets, and it is this aspect which leads to the unique characteristics of this system [18].