2003 Academic Award
Professor Richard A. Gross
New Options for Mild and Selective Polymerizations Using Lipases
Innovation and Benefits: Professor Gross developed a highly selective, more efficient way to make polyesters using enzymes. This technique requires less energy and toxic substances than conventional methods that typically use heavy metal catalysts and hazardous solvents. This innovation also makes it possible to manufacture new types of polyesters.
Isolated lipases, harvested from living organisms, have been used as catalysts for polymer synthesis in vitro. Professor Richard A. Gross’s developments on lipase-catalyzed polymer synthesis have relied on the ability of enzymes to reduce the activation energy of polymerizations and, thus, to decrease process energy consumption. Further, the regioselectivity of lipases has been used to polymerize polyols directly. Alternative synthetic pathways for such polymerizations require protection-deprotection chemical steps. The mild reaction conditions allow polymerization of chemically and thermally sensitive molecules. Current alternative chemical routes require coupling agents (e.g., carbodiimides) that would be consumed in stoichiometric quantities relative to the reactants. Fundamental studies of these polymerizations have uncovered remarkable capabilities of lipases for polymerization chemistry. Selected examples include: (1) lipases catalyze transesterification reactions between high-molecular-weight chains in melt conditions; (2) lipases will use non-natural nucleophiles such as carbohydrates and monohydroxyl polybutadiene (Mn 19,000) in place of water; (3) the catalysis of ring-opening polymerization occurs in a controlled manner without termination reactions and with predictable molecular weights; and (4) the selectivity of lipase-catalyzed stepcondensation polymerizations leads to nonstatistical molecular weight distributions (polydispersities well below 2.0). These accomplishments are elaborated below.
A series of polyol-containing polyesters was synthesized via a one-pot lipase-catalyzed condensation polymerization. By using various mixtures of polyols (e.g., glycerol, sorbitol) with other diacid and diol building blocks, the polyols are partially or completely solubilized, resulting in highly reactive condensation polymerizations. By this method, organic solvents and activated acids (e.g., divinyl esters) are not needed. The polymerization reactions give high-molecular-weight products (Mw up to 200,000) with narrow polydispersities (as low as 1.3). Further, the condensation reactions with glycerol and sorbitol building blocks proceed with high regioselectivity. Although the polyols used have three or more hydroxyl groups, only two of these hydroxyl groups are highly reactive in the polymerization. Thus, instead of obtaining highly cross-linked products, the regioselectivity provided by the lipase leads to lightly branched polymers where the degree of branching varies with the reaction time and monomer stoichiometry. By using lipase as the catalyst, highly versatile polymerizations result that can simultaneously polymerize lactones, hydroxyacids, cyclic carbonates, cyclic anhydrides, amino alcohols, and hydroxylthiols. The method developed offers simplicity, mild reaction conditions, and the ability to incorporate carbohydrates, such as sugars, into polyesters without protection-deprotection steps.
Professor Gross’s laboratory discovered that certain lipases catalyze transesterification reactions between high-molecular-weight chains that contain intrachain esters or have functional end-groups. Thus, lipases, such as Lipase B from Candida antarctica, catalyze intrachain exchange reactions between polymer chains as well as transesterification reactions between a monomer and a polymer. For polymers that have melting points below 100 °C, the reactions can be conducted in bulk. Transacylation reactions occur because the lipase has the ability to accommodate large-molecular-weight substrates and to catalyze the breaking of ester bonds within chains. Immobilized Candida antarctica Lipase B (Novozyme-435) catalyzed transesterification reactions between aliphatic polyesters that had Mn values in excess of 40,000 grams per mole. In addition to catalyzing metal-free transesterifications at remarkably low temperatures, lipases endow transesterification reactions with remarkable selectivity. This feature allows the preparation of block copolymers that have selected block lengths.
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