A process for producing a polyester in a stable emulsion state, which comprises thermal polymerization of a bis(.omega.-hydroxyalkyl)terephthalate or an oligomer thereof in the form of finely divided particles dispersed in a silicone oil by using a graft polymer, as a dispersion stabilizer, constructed of both a polysiloxane and an acrylic polymer.

The monofilament for precision woven fabrics is made of a polyethylene terephthalate/polydialkyl siloxane copolymer having a silicon content from 0.05 to 2.0 percent by weight, a polyethylene terephthalate proportion of at least 85% by weight, a diameter from 0.01 to 0.1 mm, an elongation at break of less than 30%, a strength of at least 45 cN/tex, a modulus of elasticity at 5% extension greater than 500 cN/tex and a fibrillation characterizing parameter from 0.0 to 0.5.

An aromatic polyester-polyorganosiloxane block copolymer comprising a polyester segment consisting essentially of: (a) an aromatic polyester of an aromatic dicarboxylic acid, or mixtures thereof, and a dihydric phenol, a lower aliphatic diol, an alicyclic diol, or mixtures thereof, (b) an aromatic polyester of an aromatic hydroxycarboxylic acid or mixtures thereof, or (c) a copolymer of (a) and (b), and a polyorganosiloxane segment having a recurring unit represented by the formula: ##STR1## in which R.sub.1 represents a lower alkylene group or a lower alkylene ether group; R.sub.2 and R.sub.3 independently represent a methyl group or a phenyl group; and n represents an integer not smaller than 10, in which bonding between the polyester segment and the polyorganosiloxane segment is an ester linkage formed by a condensation reaction between the terminal carboxyl group of the polyester segment and the terminal hydroxyl group of the polyorganosiloxane segment.

Silicon-containing copolyester are in the form of a block copolymers comprising, (i) between about 1 to 99% by weight of a siloxane segment which is at least one selected from the group consisting of siloxane segments expressed by the following general formulas; ##STR1## where R and R', which may be the same or different, are each hydrocarbon moieties having 1 to 10 carbon atoms, and m is an integer between 1 to 2000; and (ii) between about 99 to about 1% by weight of a polyester segment which is at least one selected from polyester segments of the following general formulas: ##STR2## where R.sub.1 is at least one radical selected from the group consisting of aromatic bivalent radicals having 6 to 12 carbon atoms and aliphatic bivalent radicals having 4 to 20 carbon atoms, R.sub.2 is at least one radical selected from the group consisting of aromatic bivalent radicals having 6 to 20 carbon atoms, and aliphatic bivalent radicals having 2 to 20 carbon atoms, and n is an integer between 1 and 1000. The block copolymers are advantageously prepared by reacting a polyester with a silicon oil having terminal reactive epoxy moieties.

Polyester resin is modified with siloxane block polymers in such a manner so as to form silicone-modified polyester copolymer domains of controlled size and distribution dispersed in the polyester matrix. These domains undergo microphase segregation and migration during the melt spinning and cold-drawing to the surface of the polyester fiber being formed so as to provide a silicone-sheathed polyester fiber.