several novel pha polymer compositions produced using biological systems include monomers such as 3 - hydroxybutyrate , 3 - hydroxypropionate , 2 - hydroxybutyrate , 3 - hydroxyvalerate , 4 - hydroxybutyrate , 4 - hydroxyvalerate and 5 - hydroxyvalerate . these pha compositions can readily be extended to incorporate additional monomers including , for example , 3 - hydroxyhexanoate , 4 - hydroxyhexanoate , 6 - hydroxyhexanoate or other longer chain 3 - hydroxyacids containing seven or more carbons . this can be accomplished by taking natural pha producers and mutating through chemical or transposon mutagenesis to delete or inactivate genes encoding undesirable activities . alternatively , the strains can be genetically engineered to express only those enzymes required for the production of the desired polymer composition . methods for genetically engineering pha producing microbes are widely known in the art . these polymers have a variety of uses in medical , industrial and other commercial areas .