Molecular weight hydroxyl number

Sample identifi- cation Approximate molecular weight Hydroxyl number (mg KOH/g polyol) Ethylene oxide tipping moles ethylene oxide/ mole glycerol (by weight addition) Reactivity, i.e., percentage of original phenyl isocynate addition consumed in ... 

Identification. The retention times on the HPLC system and the Rf-values on silica gel TLC of compounds 1 to and 1 2 to 2A agreed with those of the known carotenoids from Anacystis nidulans and Chromatium vinosum, respectively. Spectroscopic properties, molecular weights and number of hydroxyl groups, all of which are primary and/or secondary, are shown in TABLE 1. Assignments of H-NMR spectra of 2 "to 6 and 8 to 11 were made by comparison with the data of typical carotenoids. The CD spectra indicated that the absolute configuration of the hydroxyl groups at C-2 and C-3 were 2R and 3R, respectively. The position of the aldehyde group in IjO was analyzed with the EI-MS spectrum [3,4] ... 

The excess of unchanged acetic anhydride is then hydrolysed by the addition of water, and the total free acetic acid estimated by titration with standard NaOH solution. Simultaneously a control experiment is performed identical with the above except that the alcohol is omitted. The difference in the volumes of NaOH solution required in the two experiments is equivalent to the difference in the amount of acetic add formed, i.e., to the acetic acid used in the actual acetylation. If the molecular weight of the alcohol is known, the number of hydroxyl groups can then be calculated. 

I. DETERMINATION OF NUMBER OF HYDROXYL GROUPS IN PHENOL. CjHjOH. Molecular Weight, 94. 

DETERMINATION OF THE NUMBER OF HYDROXYL GROUPS IN (ii) GLYCOL. Molecular Weight, 62. 

The mixture is kept for 3 hours at 105°C after the oxide addition is complete. By this time, the pressure should become constant. The mixture is then cooled to 50°C and discharged into a nitrogen-filled botde. The catalyst is removed by absorbent (magnesium siUcate) treatment followed by filtration or solvent extraction with hexane. In the laboratory, solvent extraction is convenient and effective, since polyethers with a molecular weight above about 700 are insoluble in water. Equal volumes of polyether, water, and hexane are combined and shaken in a separatory funnel. The top layer (polyether and hexane) is stripped free of hexane and residual water. The hydroxyl number, water, unsaturation value, and residual catalyst are determined by standard titration methods. 

Hydroxyl Number. The molecular weight of polyether polyols for urethanes is usually expressed as its hydroxyl number or percent hydroxyl. When KOH (56,100 meg/mol) is the base, the hydroxyl number is defined as 56,100/equivalent weight (eq wt). Writing the equation as eq wt = 56,100/OH No. allows one to calculate the equivalents of polyol used in a urethane formulation, and then the amount of isocyanate required. The molecular weight can be calculated from these equations if the fiinctionahty, is known mol wt = / eq wt. 

Polyethers prepared from propylene oxide are soluble in most organic solvents. The products with the highest hydroxyl number (lowest molecular weight) are soluble in water, not in nonpolar solvents such as hexane. The solubihty of 3000 molecular weight triols is high enough in solvents such as toluene, hexane, and methylene chloride that the triols can be purified by a solvent extraction process. 

Chain Transfer. A number of materials act as tme transfer agents in THF polymerization notable examples are dialkyl ethers and orthoformates. In low concentrations, water behaves as a transfer agent, and hydroxyl end groups are produced. The oxygen of dialkyl ethers are rather poor nucleophiles compared to THF and are therefore not very effective as transfer agents. On the other hand, orthoformates are effective transfer agents and can be used to produce alkoxy-ended PTHFs of any desired molecular weight (169). 

Hydroxyl number and molecular weight are normally determined by end-group analysis, by titration with acetic, phthaUc, or pyromellitic anhydride (264). Eor lower molecular weights (higher hydroxyl numbers), E- and C-nmr methods have been developed (265). Molecular weight deterrninations based on coUigative properties, eg, vapor-phase osmometry, or on molecular size, eg, size exclusion chromatography, are less useful because they do not measure the hydroxyl content. 

Composition. Shellac is primarily a mixture of aUphatic polyhydroxy acids in the form of lactones and esters. It has an acid number of ca 70, a saponification number of ca 230, a hydroxyl number of ca 260, and an iodine number of ca 15. Its average molecular weight is ca 1000. Shellac is a complex mixture, but some of its constituents have been identified. Aleuritic acid, an optically inactive 9,10,16-trihydroxypalmitic acid, has been isolated by saponification. Related carboxyflc acids such as 16-hydroxy- and 9,10-dihydroxypalmitic acids, also have been identified after saponification. These acids may not be primary products of hydrolysis, but may have been produced by the treatment. Studies show that shellac contains carboxyflc acids with long methylene chains, unsaturated esters, probably an aliphatic aldehyde, a saturated aliphatic ester, a primary alcohol, and isolated or unconjugated double bonds. 

The mean value of k for a large number of liquids is 2 12 liquids containing hydroxyl groups, or those which have an enolic modification, liquid chlorine, fused salts, and metals, have much smaller values of k, and this is attributed to polymerisation, since if the actual molecular weight is larger than that assumed in calculating V , the latter value will also be smaller. Ramsay and Aston (1894) assumed that it will be proportionally smaller, but the values of the polymerisation coefficients thereby deduced obviously depend on the formula assumed for the polymerised molecule, and are therefore arbitrary. 

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