By Michael Szwarc
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Additional info for Living Polymers and Mechanisms of Anionic Polymerization (Advances in Polymer Science = Fortschritte Der Hochpolymeren, Vol. 49)
M - n)!. Since each permutation is equally probable, one finds the mole fraction of polymers terminated by - M M ÷, - M E M ÷, - M E M M ÷, and - M M E M ÷ to be* f~+ = (m-n+l)l(rn+l)~ fr,mra+ = nl(m + 1-n/m = l-r, 1) = n / m = r, fr~eMM* = fra~EM + = n ( m -- n + 1 ) / m ( m + 1) ~ (n/m)(1 -- n / m ) = r(1 -- r) . SincefuEr¢~÷ = fMMEM+and K~r = K} = KT, the pseudo-equilibrium concentrations of the monomers in contact with the living polymers having composition r is given therefore by: [trioxepane]st = KTr(1 - r) [dioxolane]~t = KDr [CH20],t = KF(1 - r ) .
An approach yielding the required data was described by Asami and Szwarc97). e. -asDa~D were prepared. Their potassium salt mixed with THF solution of the salt of the ordinary protic dimers slowly reacts yielding mixed dimers, K+,-asDa-,K +. Such a reaction is accounted for by the following mechanism: K+,-aa-,K + ~ 2a:',K + ; a - , K + + a:-sD, K + ~ K+,-asDa-sD,K + ~ 2a'sD, K + K+,-aa-sD, K + , and its initial rate in a 50 : 50 mixture gives ½ the dissociation rate of the dimers. The progress of exchange was followed by withdrawing at desired times aliquots of the mixture, protonating the dimers, and determining with the aid of a mass-spectrometer the composition of the resulting hydrocarbons.
The product was assumed to be a block polymer, possessing a virtually styrenefree block of polymethyl-methacrylate arising from the anionic polymerization, and a block of an about 50: 50 random co-polymer expected to be formed by a radical copolymerization. This suggestion was disproved by Overberger who repeated the experiments 42 Initiation of Anionic Polymerization and demonstrated by the NMR technique that at low degrees of conversion the resulting polymer was composed of a short block of homo-polystyrene followed by a block of homo-polymethyt-methacrylate94).