When a b -lactone of general formula <FORM:0635347/IV (a)/1> where R1, R2, R3 and R4 are hydrogen or alkyl groups and a salt of a carboxylic acid are reacted together in a polar solvent a b -acyloxycarboxylic acid salt is formed which in presence of excess b -lactone, e.g. of one to ten mols., will further react to produce b -polyacyloxy carboxylic acid compounds. For further details of preparation of monomers and p polymers (see Group IV (b)). Esters of the monomers and polymers are plasticizers for synthetic rubber and in particular esters of acids of the general formul R-COO-(CH2CH2COO)xH and R-(COO-(CH2CH2COO)xH)2, wherein R is a hydrocarbon radicle and x is an integer of from 1 to 5, with alcohols containing from 8 to 18 carbon atoms such as 2-ethyl hexanol, lauryl and cetyl alcohols, are effective plasticisers for vinyl resins such as polyvinyl chloride. In examples (4) and (5) b -(acetoxypolypropionoxy)-propionic acids are prepared by reacting excess b -propiolactone with sodium acetate in aqueous solution, in (8) di-(b -polyethyl-carboxy) phthalates are obtained by reacting excess b -propiolactone and disodium phthalate in aqueous solution and in (11) b -acetoxy-propionic acid and (12) b -caproxypolypropionoxy acid are esterified with lauryl alcohol to give esters stated to be plasticizers for polyvinyl chloride. In the Specification as open to inspection under Sect. 91 the b -lactones are not limited to saturated aliphatic compounds but may include others (see Group IV (b)). This subject-matter does not appear in the Specification as accepted.ALSO:b -Acyloxy carboxylic acid compounds are obtained by the reaction in polar solvents between a b -lactone of general formula <FORM:0635347/IV (b)/1> where R1, R2, R3 and R4 may be hydrogen or alkyl groups and a salt of a carboxylic acid. On treatment with acid the salts are converted to the corresponding acyloxy-carboxylic acids which may be esterified either after separation or in situ if an alcohol is used as the solvent. The preferred solvent is water, others specified being alcohols, nitro-paraffins, nitriles and carboxylic acids. For the production of the mono-acyloxy-carboxylic acid it is necessary that the carboxylic acid salt be in a 1-10, preferably 2-5, molar excess of b -lactone, since if the latter is in excess, e.g. 1-10 molar, the salt of the acyloxy-carboxylic acid formed will react with further lactone to yield polyacyloxy - carboxylic acid compounds. Carboxylic acid salts used include those of saturated and unsaturated, aliphatic and cyclic, mono- and poly-carboxylic acids which may be substituted by halogen, nitro, oxy, hydroxy, thio, thiol, cyano, acyl, acyloxy, keto, amino and azo groups, although it is preferred that the carboxylic acid does not contain substituents containing reactive hydrogen atoms since otherwise the b -lactone may react with these groups to give complex products. Alkali metal and ammonium salts are preferred. The b -lactones preferred are water-soluble and b -propiolactone is preferred. Other b -lactones instanced are b -butyro-, b -isobutyro-, b -valero-, b -isovalero, b -n-capro-, a -ethyl-b -propio-, a -isopropyl - b - propio-, a - butyl - b - propio-, a - methyl - b - butyro-, and b - methyl - b - valero - lactones. Alcohols specified for ester formation are ethyl, butyl, 2-ethyl-hexyl, lauryl and cetyl alcohols. In examples: b -propiolactone is reacted in aqueous solution with (1) sodium acetate; (2) ammonium acetate; (4) and (5) sodium acetate; (6) sodium caprate; (7) sodium laurate; (8) disodium phthalate; (9) sodium b -chloro-propionate; and (10) sodium b -cyanopropionate to yield (1) and (2) b -acetoxypropionic acid; (4) and (5) b -(acetoxy-polypropionoxy)-propionic acid; (6) b -(caproxypropionoxy-propionoxy)-propionic acid; (7) b -lauryloxy propionic acid; (8) di-(b -carboxyethyl)-phthalate or corresponding polyacyloxy polymers thereof; (9) b -(b -chloropropionoxy)-propionic acid; and (10) b -(b -cyano-propionoxy)-propionic acid; in (3) b -propiolactone and sodium acetate are reacted in ethanol solution, acidified and refluxed to yield the ethyl ester of b -acetoxy-propionic acid, whilst in (11) and (12) b -acetoxy-propionic acid and b -caproxy-polypropionic acid are esterified with lauryl alcohol by heating therewith at 100 DEG C. Specification 649,028 is referred to. In the Specification as open to inspection under Sect. 91 the b -lactones specified are not restricted to saturated aliphatic lactones but include lactones of b -hydroxy-mono-carboxylic acids containing cycloalkyl, aryl and aralkyl substituents, lactones of unsaturated b -hydroxy carboxylic acids, mono- and di-b -lactones of dicarboxylic acids and b -lactones containing oxygen, nitrogen, sulphur and halogen combined in such groupings as nitro, ether, amino and hydroxy, as well as the carbonoxy structure and carbon and hydrogen atoms. This subject-matter does not appear in the Specification as accepted.