[SH:1][CH:2]([CH3:14])[C:3]([O:5]CCCCCC(C)C)=[O:4].[SH:15]C[C:17]([O:19][CH2:20][CH2:21]CCCCCCCCCCCCCCCC)=[O:18].SC(C)C([O-])=O.SCC(O)=O.[SH-].[Na+:50].ClCC([O-])=O.[Na+]>C(#N)C>[SH:15][CH2:14][CH2:2][C:3]([OH:5])=[O:4].[SH-:1].[Na+:50].[C:17]1(=[O:18])[O:19][CH2:20][CH2:21]1Alkyl esters, as thus characterized, of mercaptoacetic (thioglycolic), β-mercapto-propionic, and mercaptosuccinic (thiomalic) acids exemplify mercaptoesters suitable for the method of this invention. The 2-ethylhexyl, iso-octyl, and n-octadecyl esters are preferable from the standpoint of the effectiveness of the heat stabilizers ultimately derived from them. Particularly preferred for that purpose are the iso-octyl and 2-ethylhexyl mercaptoacetates, iso-octyl mercaptopropionate, and the n-octadecyl mercaptoacetate and mercaptopropionate. All of these mercaptoesters are either available commercially or are easily made by conventional esterification procedures. The acids from which the esters are derived are also generally available commercially but may be made if so desired by procedures of long-standing in the chemical arts. Mercaptoacetic acid, for example, may be made by the reaction of sodium hydrosulfide with sodium chloroacetate and subsequent acidification. β-mercaptopropionic acid is obtained in about 80% yield from the reaction of sodium hydrosulfide with β-propiolactone in acetonitrile and subsequent acidification. Mercaptosuccinic acid may be made by adding hydrogen sulfide across the double bond of maleic anhydride followed by hydrolysis. Also suitable for the purposes of this invention are the alkyl esters of α-mercapto-propionic acid, which may be prepared from α-chloroproprionic acid and sodium thiosulfate according to the procedure described in U.S. Pat. No. 2,413,361, which is incorporated herein by reference. The alkyl esters may be made by conventional techniques. The respective zinc derivatives of the particularly preferred mercaptoesters are thus the preferred heat stabilizers in the compositions of this invention. Either French or American process zinc oxide is suitable for their preparation according to the method of this invention. Both are available in lead-free grades, which are preferable.
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