Learning Objective- After studying this PowerPoint presentation, you will be able to-  Learn about Copolymers.  Know about natural and synthetic rubbers.  Understand the vulcanization of rubber.  Learn the synthesis of Bakelite  Learn about Biodegradable Polymers. 1

2 Polymers Chain-Growth Polymers—Ionic Polymerization Copolymers Copolymers are polymers prepared by joining two or more monomers (X and Y) together.

3 Polymers Chain-Growth Polymers—Ionic Polymerization Copolymers The structure of a copolymer depends on the relative reactivity of X and Y, as well as the conditions used for polymerization. Several copolymers are commercially important: Saran food wrap is made from vinyl chloride and vinylidene chloride. Automobile tires are made from 1,3-butadiene and styrene.

4 Polymers Natural and Synthetic Rubbers Natural rubber is a terpene composed of repeating isoprene units, in which all the double bonds have the Z configuration. Since natural rubber is a hydrocarbon, it is water insoluble, making it useful for water proofing. The Z double bonds cause bends and kinks in the polymer chain, making it a soft material.

5 Polymers Natural and Synthetic Rubbers The polymerization of isoprene under radical conditions forms a stereoisomer of natural rubber called gutta-percha, in which all the double bonds have the E configuration. Gutta-percha is also naturally occurring, but is less common than its Z stereoisomer. Polymerization of isoprene with a Ziegler-Natta catalyst forms natural rubber with all the double bonds having the desired Z configuration.

6 Polymers Natural and Synthetic Rubbers Natural rubber is too soft to be used in most applications. When natural rubber is stretched, the chains become elongated and slide past each other until the material pulls apart. In 1939, Charles Goodyear discovered that mixing hot rubber with sulfur produced a stronger more elastic material. This process is called vulcanization. Vulcanization results in cross-linking of the hydrocarbon chains by disulfide bonds. When the polymer is stretched, the chains no longer can slide past each other, and tearing does not occur. Vulcanized rubber is an elastomer, a polymer that stretches when stressed but then returns to its original shape when the stress is alleviated.

7 Polymers Natural and Synthetic Rubbers Vulcanised Rubber

8 Polymers Natural and Synthetic Rubbers The degree of cross-linking affects the rubber’s properties. Harder rubber used for automobile tires has more cross- linking than the softer rubber used for rubber bands. Other synthetic rubbers can be prepared by the polymerization of different 1,3-dienes using Ziegler-Natta catalysts. For example, polymerization of 1,3-butadiene affords (Z)-poly(1,3-butadiene), and polymerization of 2- chloro-1,3-butadiene yields neoprene, a polymer used in wet suits and tires.

9 Polymers Polymer Structure and Properties The large size of polymer molecules gives them some unique physical properties compared with small organic molecules. Linear and branched polymers do not form crystalline solids because their long chains prevent efficient packing in a crystal lattice. Most polymers have crystalline regions and amorphous regions.

10 Polymers Polymer Structure and Properties The synthesis of Bakelite from phenol and formaldehyde

11 Polymers Another solution to the accumulation of waste polymers in landfills is to design biodegradable polymers. A biodegradable polymer is a polymer that can be degraded by microorganisms—bacteria, fungi, or algae— naturally present in the environment. Several biodegradable polyesters have now been developed [e.g., polyhydroxyalkanoates (PHAs), which are polymers of 3-hydroxybutyric acid or 3-hydroxyvaleric acid]. Biodegradable Polymers

12 Polymers The two most common PHAs are polyhydroxybutyrate (PHB) and a copolymer of polyhydroxybutyrate and polyhydroxyvalerate (PHBV). Biodegradable Polymers PHAs can be used as films, fibers, and coatings for hot beverage cups made of paper. Bacteria in the soil readily degrade PHAs, and in the presence of oxygen, the final degradation products are CO 2 and H 2 O.

13 An additional advantage of the PHAs is the polymers can be produced by fermentation. Certain bacteria produce PHAs for energy storage when they are grown in glucose solution in the absence of certain nutrients. The polymer forms as discrete granules within the bacterial cell. These are removed by extraction to give a white powder that can be melted and modified into a variety of different products. Biodegradable polyamides have also been prepared from amino acids (e.g., aspartic acid can be converted to polyaspartate, abbreviated TPA). It is a commonly used alternative to poly(acrylic acid), which is used to line pumps and boilers of wastewater treatment facilities. Polymers Biodegradable Polymers