Termination hindered

Solution Using the preceding equation for the terminal hindered settling velocity for laminar flow. 

While amines and some annular hydrocarbons are suitable chain terminators, hindered phenols such as di-l-butyl-p-cresol (alias butylated hydroxytoluene or BHT) are most popular because they avoid discolorization and they eliminate two free radicals per BHT molecule (Fig. 54.3). The resonance-stabilized aryloxy radical is protected by the bulky electron-releasing l-butyl groups in the 2 and 6 positions, so the hindered phenol can combine with a second peroxy radical but cannot combine readily with molecular oxygen or with another aryloxy radical nor abstract H atoms from the polymer to initiate a new firee-radical chain reaction. 

Terminal alkynes are only reduced in the presence of proton donors, e.g. ammonium sulfate, because the acetylide anion does not take up further electrons. If, however, an internal C—C triple bond is to be hydrogenated without any reduction of terminal, it is advisable to add sodium amide to the alkyne solution Hrst. On catalytic hydrogenation the less hindered triple bonds are reduced first (N.A. Dobson, 1955, 1961). 

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

The rate process for termination is hindered through the Trommsdorff effect. 

Monofunctional, cyclohexylamine is used as a polyamide polymerization chain terminator to control polymer molecular weight. 3,3,5-Trimethylcyclohexylamines ate usehil fuel additives, corrosion inhibitors, and biocides (50). Dicyclohexylamine has direct uses as a solvent for cephalosporin antibiotic production, as a corrosion inhibitor, and as a fuel oil additive, in addition to serving as an organic intermediate. Cycloahphatic tertiary amines are used as urethane catalysts (72). Dimethylcyclohexylarnine (DMCHA) is marketed by Air Products as POLYCAT 8 for pour-in-place rigid insulating foam. Methyldicyclohexylamine is POLYCAT 12 used for flexible slabstock and molded foam. DM CHA is also sold as a fuel oil additive, which acts as an antioxidant. StericaHy hindered secondary cycloahphatic amines, specifically dicyclohexylamine, effectively catalyze polycarbonate polymerization (73). 

According to this mechanism, hindered-amiae derivatives terminate propagatiag reactioas (eqs. 5 and 6) by trappiag both the alkyl and peroxy radicals. In effect, NO competes with O2, and NOR competes with RH. Siace the nitroxyl radicals are not consumed ia the overall reactioas, they are effective at low coaceatratioas. 

Ut = Terminal settling velocity, ft/sec Fs = CoiTection factor for hindered settling p, po = Density of water or oil, Ib/ff ... 

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

Five-membered ring systems can be obtained from hetero-l,3-dienes on reaction with oxiranes and thiiranes. To avoid competition from a possible 1,4-addition, the nucleophilic attack of the terminal heteroatom of the diene has to be sterically or electronically hindered by incorporation of the heteroatom into... 

Formation of five-membered ring systems (1,2-addition) can compete with formation of the seven-membered heterocycles (1,4-addition). If the first step of the reaction sequence, namely the nucleophilic attack of the terminal heteroatoin of the diene, is hindered by steric or electronic effects, the five-membered ring product is formed exclusively. 

Finally the aminoquinoline bearing a primary amine at the terminal carbon atom of the side chain is itself an effective antimalarial drug. Ring opening of 2-methyltetrahydrofuran by bromine gives the dibromide, 99. The primary halide is sufficiently less hindered so that reaction with potassium phthalimide affords exclusively the product of displacement of that halogen (100). Alkylation of the aminoquinoline with lOO affords the secondary amine, 101. Removal of the phthalimide group by means of hydrazine yields primaquine (102). ... 

A polymer-bound hindered amine light stabilizer [P-HALS] has been synthesized by terminating the living anionic polymerization of isoprene with 4(2,3-epoxy pro-poxy)-1,2,2,6,6-pentamethylpiperidine followed by hydrogenation of the resulting polymer to E-P copolymer using Zeigler type catalyst [40] ... 

V,s = settling velocity for hindered uniform spherical particle, ft/s or m/s (terminal) c = volume fraction solids K = constant given by equation above Nrc = Reynolds number, Dp V,pf/ j. 

Use of LTMP as base [52] in situ with Me3SiCl allows straightforward access to a variety of synthetically useful a, 3-epoxysilanes 232 at near ambient temperature directly from (enantiopure) terminal epoxides 231 (Scheme 5.55) [81]. This reaction relies on the fact that the hindered lithium amide LTMP is compatible with Me3SiCl under the reaction conditions and that the electrophile trapping of the nonstabilized lithiated epoxide intermediate must be very rapid, since the latter are usually thermally very labile. 

High-valent ruthenium oxides (e. g., Ru04) are powerful oxidants and react readily with olefins, mostly resulting in cleavage of the double bond [132]. If reactions are performed with very short reaction times (0.5 min.) at 0 °C it is possible to control the reactivity better and thereby to obtain ds-diols. On the other hand, the use of less reactive, low-valent ruthenium complexes in combination with various terminal oxidants for the preparation of epoxides from simple olefins has been described [133]. In the more successful earlier cases, ruthenium porphyrins were used as catalysts, especially in combination with N-oxides as terminal oxidants [134, 135, 136]. Two examples are shown in Scheme 6.20, terminal olefins being oxidized in the presence of catalytic amounts of Ru-porphyrins 25 and 26 with the sterically hindered 2,6-dichloropyridine N-oxide (2,6-DCPNO) as oxidant. The use... 

Sodium malonate attacks almost exclusively the sterically hindered site, whereas /i-oxo esters, 1,3-diketones, and the C-mcthylated analogs of dimethyl malonate lead exclusively to terminal attack. 

Values of termination constants for sterically hindered monomers may be several orders of magnitude lower than those for S and (methacrylates). Such monomers include various ra-substituted methacrylates, itaconates, fumarates, and N-substituted itaconimides and maleimides. Values of kt for these monomers have been reported to lie in the range IQ-1Q5 NT s 1 depending on the particular... 

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