End group exchange

Rapid end group exchange in the presence of anionic catalysts... 

Rapid End Group Exchange in the Presence of Anionic Catalysts... 

A fast ester end group exchange occurs, according to F NMR [3]. 

CFIDF end group, no selective reaction would occur on time scales above 10 s. Figure B2.5.18. In contrast to IVR processes, which can be very fast, the miennolecular energy transfer processes, which may reduce intennolecular selectivity, are generally much slower, since they proceed via bimolecular energy exchange, which is limited by the collision frequency (see chapter A3.13). 

The same heterobifunctional initiator, 2-phenyl-2-[(2,2,6,6-tetramethy-piperidino)oxy] ethyl 2-bromo-2-methyl propanoate, was employed for the synthesis of PMMA-fo-PfBuA-fo-PS triblock terpolymers via the combination of ATRP and NMP [136]. Styrene was initially polymerized through the alkoxyamine function in bulk at 125 °C, leading to PS chains with bromine end groups. Subsequent addition of fBuA in the presence of CuBr/PMDETA provided the PS-fr-PfBuA diblock. Further addition of CuCl, to achieve halogen exchange and MMA yielded the desired triblock copolymer with... 

Lieber and Brintzinger reported a detailed study using mixtures of precatalysts with different stereospecificities [25], Isospecific precatalyst 10 undergoes facile chain transfer to TMA, as revealed by an increase in isopropyl end groups and a sharp reduction in molecular weight in the presence of TMA (there was no exchange with TiBA). An aspecific precatalyst 7 was affected similarly by TMA... 

The authors believe that electron exchange occurs at every intramolecular collision . Thus, their work should provide the first quantitative measurements of the frequency of intramolecular collision between end-groups attached to flexible chains. According to these authors such frequency reflects an intrinsic property of chain molecules, referred to as the dynamic flexibility, which is a measure of the rate of conformational change. It should be distinguished from the static flexibility, which depends on the multitude of... 

Note that the long-range classical electrostatic limit furnishes an excellent approximation for the electrical attraction of the end groups, since these are separated far outside the range of significant exchange interactions. 

The second factor that additionally effects polycondensations are exchange reactions which can occur between free end groups and junction points in the chain, for example, between OH end groups and ester groups of a polyester (transesterification) ... 

Early work on the GTP mechanism showed that the silyl groups on chain ends rapidly exchange in the presence of anionic catalysts [33, 34]. Without catalyst no exchange occurs [35]. No exchange occurred in the bifluoride catalyzed polymerization of MMA with dimethylphenylsilyl ketene acetal (Scheme 19a) in the presence of dimethyltolylsilyl fluoride [1]. However, in a similar experiment with trimethylsilyl acetate, TBA Ac, and dimethylphenylsilyl ketene acetal, complete exchange occurred within 5 min [36] (Scheme 19b). 

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