Fluoride termination

Reaction (2.13a) has the smallest activation energy, meaning that the Si-OH bond is highly unstable in HF solutions and if present will be quickly replaced by other bonds. Reaction (2.13b), which results in hydrogen termination, is kinetically more favorable than reactions (2.13c) and (2.13d). This means that fluoride termination tends to be replaced by hydrogen termination, which, whether in the form of mono-, di-, or trihydride, is kinetically more stable. Similar to HF, the attack by F and HFz leading to hydrogen termination has also been proposed.  

A fluorine-terminated surface is not as stable as a hydrogen-terminated one and is readily removed by a water rinse. For example, the amount of fluorine, 1%, 

FIGURE 2.13. Fluorine concenti ation of the silicon surface after UV/HF cleaning versus HF concentration. After Takahagi et 

for the surface that is dominantly terminated by fluorine, dipping in concentrated HF solutions followed by a water rinse results in a hydroxyl-rich surface. 

The instability of fluorine termination can readily be explained by the theory for hydrogen termination, that is, bonding with fluorine significantly weakens the back silicon-silicon bond leading to the dissolution of the atoms bonded by fluorine. 

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During polymerization, a polymeric radical with a perfluoro(alkyl vinyl ether)-derived active center can have one of two fates it can cross-propagate to tetrafluoroethylene or it can undergo P-scission to yield an acid-fluoride-terminated polymer chain and generate a peduoroalkyl radical capable of initiating further polymerization (ie., chain transfer to monomer). These scenarios are illustrated in Scheme 3. 

The first models for the electrochemical dissolution process of silicon in HF assumed a fluoride-terminated silicon surface to be present in electrolytes containing HF [Ge6, Du3[. However, by IR spectroscopy it was found that virtually the whole surface is covered by hydride (Si-H) [Ni3[. No evidence of Si-F groups is found in IR spectra independent of HF concentration used [Ch9[. This is surprising insofar as the Si-F (6 eV) bond is much stronger than the Si-H (3.5 eV) bond, and so it cannot be assumed that Si-F is replaced by Si-H during the electrochemical dissolution. This led to the conclusion that if a silicon atom at the surface establishes a bond to a fluorine atom it is immediately removed from the surface. 

However, with certain acids, such as hydrofluoric acid, a protective layer of insoluble magnesium fluoride terminates the reaction. Likewise, the metal has little action on chromic acid. 

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. 

Likewise open-chain, cyclic five- and six-memhered a,a,a, a tetrafluoro-ethers are opened by anhydrous hydrogen fluoride at elevated temperatures to give acyl fluorides terminated with a trifluoromethyl group A six-inembered ether such... 

Polyhaloalkenes can be fluorinatcd with vanadium(V) fluoride. Terminal polyfluoroalkencs react at between — 20 and — 30 C, while internal polyfluoroalkenes are fluorinated at 40 to 150 C. Table 15 lists some examples. 

A PAES block copolymer was synthesized from a fluoride-terminated oligomer with methyl side groups and a hydroxyl-terminated oligomer by an aromatic nucleophilic substitution polycondensation reaction [86]. Afterwards the methyl side groups were brominated and converted into quaternary ammonium groups. The copolymer can be used for ultrafiltration membranes for protein separation. 

A propagating polymeric radical with a PAVE active-radical center can have one of two possible reaction pathways. First, and most obvious, it can cross-pro-pagate to monomer, continuing the polymerization reaction, or it can undergo yS-scission, resulting in an acid fluoride-terminated polymer and a perfluoroalkyl radical capable of initiating further polymerization. Essentially this is a chain-transfer-to-monomer step, the details of which are outlined in Scheme 9.1 [6, 9]. 

Baek and Harris reported aromatic HBPAs [63], synthesized by the self-polymerization of AB amide monomers containing hydroxyl and two fluoro groups. The reported polymers were amorphous in nature and were very much soluble in common organic solvents such as THF, CHCI3, DMF, DMAc, DMSO, NMP, and m-cresol. The aryl-fluoride-terminated, amorphous polymers had intrinsic viscosities of 0.34 and 0.24 dL/g (30.0 0.1°C in m-cresol) and T s of 210-269°C. 

Johnson, W. S. Frei, B. Gopalan, A. S. "Improved Asymmetric Total Synthesis of Corticoids via Biomimetic Polyene Cyclization Methodology" J. Org. Chem. 1981, 46, 1512-1513. Forfurther improvements see Johnson, W. S. Lyle, T. A. Daub, G. W. "Corticoid Synthesis via Vinylic Fluoride Terminated Biomimetic Polyene Cyclizations" J. Org. Chem. 1982, 47, 161-163. 

Bernal DP, Bankey N, Cockayne RC, Possum E (2002) Fluoride-terminated hyperbranched poly(arylene ether phosphine oxide)s via nucleophilic aromatic substitution. J Polym Sci A Polym Chem 40 1456-1467... 

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