Unreactive end groups

The key to the resolution of the apparent contradiction becomes evident upon re-examining the initial derivation which proceeds from Fig. 68. Finite, or bounded, molecular species are implied in the expression for the probability of a specific x-mev configuration thus fx — 2x + l unreacted ends in addition to the one selected at random are prescribed. An infinite network, on the other hand, is terminated only partially by unreacted end groups the walls of the macroscopic container place the ultimate limitation on its extent. Hence the network fraction is implicitly excluded from consideration, with the result that the distribution functions given above are oblivious of it. Failure of to retain the same value throughout the range in a is a... 

Synthetic methods have limited the preparation of saturated perfluoropolyethers. The most successful perfluoropolyether synthetic chemistry has been DuPont s anionic polymerization of perfluoroepoxides, particularly hexafluoro-propylene oxide and tetrafluoroethylene oxide (39). Their synthetic procedure is a three-step scheme for saturated perfluoropolyether production involving oxidation of perfluoroolefins to perfluoroepoxides, anionic polymerization to acyl fluoride terminated perfluoropolyethers, and conversion of acyl fluoride end groups to unreactive end groups by decarboxylation reactions or chaincoupling photolytic decarboxylate reactions. 

The conversion of an w-mer into an (n + l)-mer may be imagined to occur in two steps (1) fission of an M—M bond located in the middle of the chain (2) insertion of a monomer molecule into the broken linkage coupled with the formation of two new M—M bonds. Because the end groups are far away from the reaction center, their nature cannot influence the free energy of the process which is therefore independent of the magnitude of n and of the nature of X and Y. This remains true even if X and Y are some unreactive end groups of a dead polymer. 

Materials 27 and 28 decomposed above ca. 200 °C. Regarding the high thermal stability of the model compounds, this result was unexpected and was attributed to the presence of unreacted end-groups. None of the l,l -isomeric structures 28 exhibited liquid crystal properties before decomposition. Interestingly, oligomers 27 clearly showed nematic phases between ca. 180—200 °C. 

The probability P that a polymer consists of n monomers is equal to the probability that it has n - I reacted end groups and one unreacted end group. The former probability is pP the latter I — p. 

Basic analysis, including infrared spectroscopy, elemental analysis and X-ray photoelectron spectroscopy, can determine the composition, bonding and percentage of unreacted end groups. Solid-state and nuclear magnetic resonance (NMR) spectroscopy is an increasingly important technique. 

The latter reaction occurs readily and in order to control the degree of polymerization attained upon complete hydrolysis (as indicated in the general equation above), it is usual to include monofunctional chloro-silanes, e.g. chlorotrimethylsilane would lead to PDMS with unreactive end-groups... 

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