Fiber-forming polymers are prepared from a mixture of the hexamethylene diamine salts of terephthalic acid, isophthalic acid, and a small amount of at least one aliphatic dibasic acid (e.g. adipic acid) having from 5 to 12 carbon atoms. The polymers can be prepared in a batch autoclave and extruded therefrom without cleaning the autoclave between successive runs.
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