Lipase B from Candida antartica : applications to industrial polyester synthesis.
In the early 1990s, enzymatic polymerization studies on an unactivated adipic acid (A)
/ butane-1,4-diol (B) system in anhydrous organic media suggested that in solvent-free
conditions a step-growth mechanism operates involving the effective sequential addition of an
'AB' unit. Subsequent optimisation work led to development of an efficient solvent-free
process using lipase B from Candida antarctica as catalyst at temperatures of around 60°C,
producing polyesters with unique properties compared to their counterparts synthesised using
conventional high temperature processes.
The present work describes the synthesis and unambiguous characterisation by GPC of
a series of key oligomers formed during the enzymatic process, enabling a clearer
understanding of the polymer assembly route. Use of semi-quantitative methods to compare
the reactivity of these oligomeric species along the proposed enzymatic reaction pathway
further clarifies the polymerisation mechanism and explains the differences between the final
products of conventional and enzymatic polymerisations. Comparison of polyesterifications
performed in solvent-free and toluene-based media reveals a change in lipase specificity from
a simple esterification to an esterification! transesterification mode.
Addition of a variety of compounds to the enzymatic polyesterification process led to
an enhancement of lipase activity and sometimes enabled recovery of lipase from the system
with no loss in activity. Explanations for these effects are proffered with supporting
experimental evidence. A variety of studies on the effect of changing the polarity of reaction
medium have been performed, an apparent trend of decreasing lipase activity with increasing
solvent polarity having been noted.
The low temperatures employed in the enzymatic polymerisations enable access to a
range of polyesters precluded by harsh conventional conditions. Studies on the polymerisation
of novel polymers possessing unsaturation, labile groups such as epoxides and chirality are
detailed, giving an indication of one of the potential advantages of an enzymatic system.
In addition, studies on the more conventional uses of enzymes in organic synthesis are
included, where an apparent switch in lipase stereoselectivity occurs, in line with the findings
of other workers in the field.