It is completely plausible that unmodified materials of natural origin, such as the native macromolecules cellulose or starch, are biodegradable. If these materials are modified then degradation may, depending on the degree of modification, be more difficult or even impossible. In the same manner synthesized macromolecules, whether from renewable or petrochemical sources, could be inert or completey biodegradable, depending on their chemical structure.
A newly designed automatically controlled stirred reactor suitable for kinetic measurements of reactions with half lives 2 2 s has been applied to follow the anionic polymerization of methyl methacrylate in THF with Na' as a counter ion in the presence of an excess of NaB(C6H5),.As initiators were used: benzylsodium reacted with a-methylstyrene (I), fluorenylsodium (II), and 9-methylfluorenylsodium (111). With I the initiation is fast as compared with the polymerization reaction which is first order in monomer concentration. Within the range of -50°C to -100°C an almost unperturbed "living" polymerization is observed. The Arrhenius plot of the rate constants results in astraightlinewithactivationenergyE,=4,4kcal.mol-' (= 18 kJ.mol-')andfrequencyexponent A=7,0. I1 and 111 are slow initiators, I1 giving rise to side reactions because of the "acidic" proton in 9-position after initiation, 111 exhibiting a rate constant of initiation k, = 11 .mol-' .s-' at -72°C. The termination reaction is becoming increasingly important with increasing temperature and seems to be a unimolecular reaction with Ea,, = 11,5 kcal . mol' (=48 kJ . mol-I ) and A, = 10. Since the basic feature of the reactor is the possibility of drawing samples, polymers from each state of the reaction were available to be investigated also with respect to their tacticity. The monomer addition was shown to follow Bernoullian statistics. A structure of the "living" end being in harmony with the results observed is discussed. *) Performed at the "Institut fur Makromolekulare Chemie, Universitat Freiburg". The courtesy of Prof. Cantow is greatly acknowledged.
SUMMARY:The influence of n,s-dibutylmagnesium on the kinetics of styrene polymerization initiated by sbutyllithium was investigated in cyclohexane at 50 8C. The presence of n,s-dibutylmagnesium, in molar ratios ranging from 0 to 20 with respect to polystyryllithium, leads to a drastic and continuous reduction of the reactivity of the propagating species. The living character of the polymerization, especially the control of the molar mass is preserved over the entire range studied. The experimental molar masses are in agreement with the formation of one polystyrene chain per lithium and 0.5 to 0.8 chains per magnesium atom, indicating that both alkyllithium and dialkylmagnesium species are involved in the polymerization process through the formation of "ate" complexes. The UV-visible study of the PSLi/n,s-dibutylmagnesium systems shows that several types of "ate" complexes with different stoichiometry are formed depending on the proportion of the two metal derivatives.
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