New fibre-and film-forming polyanhydrides

Fibre- and film-forming polymeric linear aromatic anhydrides with recurring units of the formula where the carbonyl groups are either both in the meta- or both in the para-position and R is a divalent linear polymethylene radical consisting of 2-8 methylene groups, wherein one or more non-adjacent methylene groups may or may not be replaced by an ether oxygen atom, are prepared by heating mixed anhydrides of acetic acid or an aliphatic mono-carboxylic acid with a boiling point between 140 DEG and 250 DEG C. with a dicarboxylic acid of the above unit formula, until a fibre- and film-forming polymer is obtained, the monocarboxylic acid and/or anhydride being distilled off in the process (for preparation of the mixed anhydrides see Group IV (b)). The process is preferably carried out at first at atmospheric pressure and later at reduced pressure. Three types of process are discussed: (i) the reaction mass is maintained in the molten state and is mixed either by bubbling a stream of nitrogen through the mass or by mechanical agitation; (ii) the reaction mass is in powder form and is maintained at a temperature just below the melting point and preferably under high vacuum; or (iii) the reaction may be carried out in an inert high-boiling solvent for the polymer such as a -methyl naphthylamine, diphenyl or diphenyloxide. The polymer is also soluble in tetralin, tetrachloroethane, nitro benzene, dimethyl formamide, isophorone, m-cresol and phenol. Many compounds falling within the unit formula are listed and exemplified, the simplest being when R is -CH2O-, and the most complex when R is -O.(CH2)2.O.(CH2)2.O-. The polymers may be cold-drawn or extruded, e.g. to form a photographic film base. In the second Provisional Specification, R is a divalent hydrocarbon radical wherein one or more non-adjacent C atoms may or may not be replaced by an ether-oxygen atom, this divalent radical uniting directly the two benzoyl radicals with 1-8 chain atoms. Many such are specified: they include aromatic and alicyclic radicals. Further the carbonyl groups in the formula may be mixed meta- and para-. Specification 840,847 is referred to.ALSO:The invention comprises mixed anhydrides of an aromatic dicarboxylic acid, of the general formula wherein the carbonyl groups are either both in the para- or both in the meta-position, R1 is an acyl radical derived from an aliphatic monocarboxylic acid with a boiling point between 140 DEG and 250