PCL : polycaprolactone

Polycaprolactone (PCL) is a biodegradable polyester used in medical devices and disposable tableware. It is produced using the caprolactone monomer and a suitable catalyst as shown below:


The catalyst used to help bring about this polymerization reaction is usually an organic tin-based catalyst such as tin (II) ethylhexanoate:
The tin (II) ethylhexanoate catalyst is highly toxic and has to be disposed of appropriately.

Biochemists found a more environmentally friendly catalyst in the form of an enzyme produced by a yeast strain known as Candida antartica. In a standard batch process, the raw materials such as caprolactone monomers and a solvent such as toluene, are dumped into a vat, along with tiny beads that carry the enzyme, and stirred. This process is too inefficient to be used commercially, and the enzyme residue is a contaminant in the final polymer product.

Researchers at the National Institute of Standards and Technology (NIST) and the Polytechnic Institute of New York University are now studying the use of a new catalyst, a small block of aluminium with a tiny groove carved into it containing the enzyme coated beads.

In this continuous flow process, the feedstock chemical flows through the narrow channel, around the enzyme-coated beads, and is polymerized out the other end. This arrangement accelerates the rate of reaction and improves the ability to recover the enzyme and reduce contamination of the product.

Reference
Santanu Kundu, Atul S. Bhangale, William E. Wallace, Kathleen M. Flynn, Charles M. Guttman, Richard A. Gross, Kathryn L. Beers. Continuous Flow Enzyme-Catalyzed Polymerization in a Microreactor. Journal of the American Chemical Society, 2011; : 110325123921095 DOI: 10.1021/ja111346c

Further Reading
Polymers and Polymerization
Enzymes
Functional Groups
Esters
Ligands and Complex Ions
Reaction Rate
Intermolecular Forces

Study Questions

1. What is the molecular formula for each of the following:
   
   • caprolactone
   • ethylhexanoate
2. When caprolactone monomers react to form polycaprolactone, the ring structure must open up. Draw a diagram of this open-ring structure.
3. On the diagram of the open-ring caprolactone structure, identify and name the functional groups present.
4. Draw a diagram of showing how 3 caprolactone monomers join together to form part of the polycaprolactone polymer.
5. On the diagram of the polycaprolactone polymer you have drawn, identify and name the functional groups present.
6. Why do you think it is common for industrial chemists to look to naturally occurring enzymes to replace more environmentally hazardous metal-based catalysts?
   
7. Why do you think the caprolactone polymerization reaction is carried out in an organic solvent like toluene rather than in water?