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Organic Chemistry

Chemistry Matters—Degradable Polymers

Organic ChemistryChemistry Matters—Degradable Polymers

31 • Chemistry Matters

31 • Chemistry Matters

The high chemical stability of many polymers is both a blessing and a curse. Heat resistance, wear resistance, and long life are valuable characteristics of clothing fibers, bicycle helmets, underground pipes, food wrappers, and many other items. Yet when these items outlive their usefulness, disposal becomes a problem.

A photo of a bulldozer on top of a large pile of trash.
Figure 31.8 What happens to the plastics that end up here? (credit: modification of work “King of the trash hill” by Alan Levine/Flickr, CC BY 2.0)

Recycling of unwanted polymers is the best solution, and six types of plastics in common use are frequently stamped with identifying codes assigned by the Society of the Plastics Industry Table 31.2. After being sorted by type, the items to be recycled are shredded into small chips, washed, dried, and melted for reuse. Soft-drink bottles, for instance, are made from recycled poly(ethylene terephthalate), trash bags are made from recycled low-density polyethylene, and garden furniture is made from recycled polypropylene and mixed plastics.

Table 31.2 Recyclable Plastics
Polymer Recycling code Use
Poly(ethylene terephthalate) 1—PET Soft-drink bottles
High-density polyethylene 2—HDPE Bottles
Poly(vinyl chloride) 3—V Floor mats
Low-density polyethylene 4—LDPE Grocery bags
Polypropylene 5—PP Furniture
Polystyrene 6—PS Molded articles
Mixed plastics 7 Benches, plastic lumber

Frequently, however, plastics are simply thrown away rather than recycled, and much work has therefore been carried out on developing biodegradable polymers, which can be broken down by soil microorganisms. Among the most common biodegradable polymers are polyglycolic acid (PGA), polylactic acid (PLA), and polyhydroxybutyrate (PHB). All are polyesters and are therefore susceptible to hydrolysis of their ester links. Copolymers of PGA with PLA have found a particularly wide range of uses. A 90/10 copolymer of polyglycolic acid with polylactic acid is used to make absorbable sutures that are degraded and absorbed by the body within 90 days after surgery.

Glycolic acid reacts with heat to form poly-glycolic acid. Lactic acid reacts with heat to form poly-lactic acid. 3-hydroxybutyric acid reacts with heat to form poly(hydroxybutyrate). Products are inside parentheses.

Biodegradation is important, but even better than the breakdown of a small group of specialized polymers would be the development of a generalized method of breakdown that could be used on any polymer, including even hydrocarbon polymers such as polyethylene and polypropylene. Such a method has recently been reported by chemists at the University of Delaware, using hydrogen at 200 °C and 2 atm pressure in the presence of an organozirconium catalyst to turn polyethylene into simple hydrocarbons. The method uses a large amount of hydrogen at present, but it is an impressive start.

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