Cationic polymerisation: initiation processes with alkenyl by A. Gandini, H. Cheradame
By A. Gandini, H. Cheradame
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Extra resources for Cationic polymerisation: initiation processes with alkenyl monomers
Also, the minimum time necessary for a spectrum to be taken or for one given signal to be scanned is of the order of a minute. These constraints rule out the possibility of identifying short-lived transients or any compound present in low concentrations formed during a cationic polymerisation. Thus, while carbenium ions have been characterised by this technique under suitable conditions, their concentration in a polymerising solution is always too low to permit detection by NMR spectroscopy. On the other hand, one can easily follow the disappearance of the monomer and/or the formation of the polymer and of any other product (such as an ester) formed in appreciable quantities, an operation which is extremely convenient, not only from the point of view of the kinetics of polymerisation, but also because it can lead to mechanistic information concerning the nature and the structure of various products.
In other words, the ester molecules can propagate a chain independently of the ionic species and gives rise to the lower DP portion of the polymer. This interpretation seems to be corroborated by the fact that when Lewis acids are used as catalysts and no active ester molecules can be formed, polymodality is never observed. These considerations will be dealt with in greater details within the context of the conclusions to Chap. III. A word of caution must be finally given concerning the assumed general validity of the postulate that the looser the ionic association, the higher their propagating potential.
It has been shown that in certain systems these ester molecules can propagate the growth of a polymer chain, while in others they are inactive. Another source of covalent species in cationic polymerisation is the collapse (recombination) of the ionic pair or the X - displacement from the anion to the carbocation discussed in the previous section. The existence of active ester molecules (pseudocationic polymerisation) was recognised in 19649), but these new ideas were not easily accepted and a widespread controversy over their validity continued until recently.