Electron donors, living polymerization

On the basis of these studies we decided to carry out a series of AMI and IMA experiments (2) with the TMPCl/EtAlCl2/DtBP combination. Figures 1 and 2 show the results. The M versus Wp (g of poly(P-PIN) formed) plots and the N (number of moles of poly(P-PIN) formed) versus Wp plots (insets) indicate increasing deviation from the theoretical values (calculated for Ieff = 100%). According to these results chain transfer proceeds in these polymerizations, i.e., the systems are nonliving. Further experimentation would be necessary to develop satisfactory living conditions, in particular to investigate the effect of solvent polarity, temperature and electron donors on the mechanism. 

A few papers have examined the range of Lewis bases that are effective for isobutene controlled/living polymerization [91,149,156]. One proposal is that effective nucleophiles (electron donors) should have relatively high donor numbers (DN > 26) [149]. Another screening shows that triethylamine is exceedingly effective for isobutene [156]. Pratap et al., also reported recently the use of cyclic amides (lactams l-methyl-2-pyr-rolidone) [158] and cyclic amines [152] for the dicumyl acetate/BCL initiating system. 

It should be noted, however, that Scheme 8.2 depicts a highly simplified mechanism for living carhocationic polymerizations and it is in most cases not possible to find a counteranion with intermediate reactivity that spontaneously establishes an equilibrium between cationic and covalent species. Instead, the counteranion is generally a halide that preferably forms a covalent species with the carbenium ion. The addition of a Lewis acid as coinitiator is required to activate the covalently bound halide, resulting in the cationic carbenium ion. Alternatively, a nucleophile or electron donor can be added to the cationic polymerization, to reversibly form a stable cationic addition product with the carbenium ion. Both these deactivation mechanisms are depicted in Scheme 8.3. To achieve a living cationic polymerization it is of critical importance to have fast deactivation equilibria. In addition, the position of the equilibria should be carefully optimized for each monomer by variation of, for example, temperature, solvent, initiator, as well as the addition of halide activators or nucleophilic deactivators. 

The living carbocationic polymerization of styrene was reported by Kennedy using 2-chloro-2,4,4-trimethylpentane (TMPCl) with TiCl4 as initiating system in the presence of various electron donors, such as At,At-dimethylacetamide... 

The fiber-optic TR-FTIR technique has also been used to investigate the role of additives such as proton traps and electron pair donors (EDs) in carbocationic polymerisation. The role of additives is not clear and is actively debated. " In the sequential block copolymerization of IB and Sty it was shown that that in order to obtain efficient crossover from the living IB, the use of additives (both electron pair donors such as N,N-dimethylacetamide (DMA) and proton traps like 2,6-di-tert-butylpyridine (DrBP), or diphenylethylene is necessary. TR-FTlR monitoring revealed that when DMA was added from the beginning of the IB polymerization phase, the band characteristic of the C=C stretch in IB at 1655 cm appeared as a split signal, as shown in Figure 12a. 

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