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(i) an alkoxysilyl-containing thiocarboxylic acid ester of the general formula (I): 

where R$^{1 }$is a monovalent group selected from straight chain alkyl group containing from 1 to 18 carbon atoms, a branched chain alkyl group containing from 3 to 18 carbon atoms, a cycloalkyl containing from 5 to 18 carbon atoms, an alkenyl containing from 2 to 18 carbon atoms, an aryl containing from 6 to 18 carbon atoms, aralkyl containing from 7 to 18 carbon atoms or hydrogen, or is a divalent group selected from an alkyl containing from 1 to 10 carbon atoms, a cycloalkyl containing from 5 to 10 carbon atoms or phenyl; 

each R $^{2 }$is independently a divalent group selected from straight chain alkyl group containing from 1 to 10 carbon atoms, a branched chain alkyl group contain› ing from 3 to 10 carbon atoms, a cycloalkyl group containing from 5 to 10 carbon atoms, an alkenyl group containing from 2 to 10 carbon atoms, an aryl group containing from 6 to 10 carbon atoms or aralkyl group containing from 7 to 10 carbon atoms; 3 

each R is independently a monovalent group selected from straight chain alkyl group containing from 1 to 6 carbon atoms, a branched chain alkyl group con› taining from 3 to 6 carbon atoms, a cycloalkyl group containing from 5 or 6 carbon atoms, an alkenyl group containing from 2 to 6 carbon atoms, an aryl group containing 6 carbon atoms or an aralkyl group containing from 7 to 10 carbon atoms; 4 

R is independently a monovalent group selected from straight chain alkyl group containing from 1 to 6 carbon atoms, a branched chain alkyl group contain› ing from 3 to 6 carbon atoms, a cycloalkyl group containing from 5 or 6 carbon atoms, an alkenyl group containing from 2 to 6 carbon atoms, an aryl group containing 6 carbon atoms, aralkyl group containing from 7 to 10 carbon atoms, a straight chain alkyl group containing 2 to 6 carbon atoms and a hydroxyl group, an alkyl group containing at least one oxygen atom having the structure -R 2 - (OCH$_{2}$CH$_{2}$)m(OCH$_{2}$ CH(CH$_{3}$))nOR 1 or a divalent group formed from two R 4 groups being bonded together through a covalent bond, with the provision that (i) if two R 4 groups are bonded together, then a is O or 1 and (ii) the sum of m+n is from 1 to 20; 

a, m, n and z are integers wherein a is 0, 1 or 2, m is from Oto 10, n is from Oto 10 and z is 1 or 2, and 

(I) 

(ii) a thioester of the general formula (II): 

<img src='content_image/943601.jpg'>

wherein 1 

R is a monovalent group selected from straight chain alkyl group containing from 1 to 18 carbon atoms, a branched chain alkyl group containing from 3 to 18 carbon atoms, a cycloalkyl containing from 5 to 18 carbon atoms, an alkenyl containing from 2 to 18 carbon atoms, an aryl containing from 6 to 18 carbon atoms, aralkyl containing from 7 to 18 carbon atoms or hydrogen, or is a divalent group selected from an alkyl containing from 1 to 10 carbon atoms, a cycloalkyl containing from 5 to 10 carbon atoms or phenyl; 5 

each R is independently a monovalent group selected from straight chain alkyl group containing from 1 to 18 carbon atoms, a branched chain alkyl group containing from 3 to 18 carbon atoms, a cycloalkyl group containing from 5 to 18 carbon atoms, an alkenyl group containing from 2 to 18 carbon atoms, an aryl group containing from 6 to 18 carbon atoms, aralkyl group containing from 7 to 18 carbon atoms, and wherein R $^{1 }$and z is an integer wherein z is 1 or 2. 

24. The alkoxysilyl-containing thiocarboxylic acid ester composition of claim 23 wherein the composition contains from about 70 to about 99.9 weight percent alkoxysilyl› containing thiocarboxylic acid ester of formula (I) and from 0.1 to about 30 weight percent thioester of formula (II), based on the total weight of the alkoxysilyl-containing thiocarboxylic acid of formula (I) and thioester of formula (II). 

25. The alkoxysilyl-containing thiocarboxylic acid ester composition of claim 23 further comprising a mercapto› functional alkoxysilane of general formula (V): 4 

<img src='content_image/943613.jpg'>

wherein R 2 , R 3 , R 4 and a are defined above. 

26. The alkoxysilyl-containing thiocarboxylic acid ester composition of claim 25 wherein the mercapto-functional alkoxysilane of general formula (V) is from 0.1 to 50 weight percent, based on the total weight of the alkoxysilyl-con› taining thiocarboxylic acid (i) and thioester (ii). 

27. A rubber composition comprising the alkoxysilyl› containing thiocarboxylic acid ester composition of claim 23. 

28. The rubber composition of claim 27 further compris› ing a mercapto-functional alkoxysilane of formula (V). 

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