Organic Chemistry II : Synthetic and Natural Organic Polymer PowerPoint Lecture Presentation by J. David Robertson University of Missouri Organic Chemistry II : Synthetic and Natural Organic Polymer Copyright © The McGraw-Hill Companies, Inc.  Permission required for reproduction or display.

A polymer is a high molar mass molecular compound made up of many repeating chemical units. Naturally occurring polymers Proteins Nucleic acids Cellulose Rubber Synthetic polymers Nylon Dacron Lucite 25.1

POLYMERS Substances that consists of very large molecules called macromolecules that are made up of many repeating subunits called monomers. Differences between monomers can affect properties of polymer such as solubility, flexibility or strength. Natural polymers Protein, cellulose, natural rubber. Synthetic polymers Polyethylene, poly (vinyl chloride) polystyrene, etc….. Homopolymer Copolymer

The simple repeating unit of a polymer is the monomer. Homopolymer is a polymer made up of only one type of monomer. ( CF2 CF2 )n Teflon ( CH2 CH2 )n Polyethylene ( CH2 CH )n Cl PVC 25.2

Copolymer is a polymer made up of two or more monomers. ( CH CH2 CH2 CH CH CH2 )n Styrene-butadiene rubber 25.2

Stereoisomers of Polymers: R groups on same side of chain: Isotactic R groups alternate from side to side: Syndiotactic R groups disposed at random: Atactic

Comparisons of stereoisomer Atactic Syndiotactic Isotactic Soft and rubbery, amorphous and relatively weak. Low density and low tensile strength but high degree of flexibility. Have better impact strength than isotactic. Has the highest melting point, greatest crstallinity, superior mechanical properties. * Isotactic polymer can be synthesize selectively using Ziegler-Natta catalysts. The catalysts are based on titanium tetrachloride, TiCl4 and triethylaluminum, Al(C2H5)3.

The tacticity of a polymer affects the packing between molecules (crystallinity) and thus affects its physical properties such as the melting temperature, mechanical strength and elasticity. Structure regularity: Isotactic > Syndiotactic > Atactic

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POLIMERIZATION The reactions by which monomers are joined together. Synthetic polymers are created by means of addition polymerization; condensation polymerization.

ADDITION POLYMERIZATION Addition polymerization involves the breaking of double or triple bonds, which are used to link monomers in to chains. The free radical mechanism explains how the polymerization is initiated. Free radicals are very reactive, short-lived atom or molecule which have one or more unpaired electrons.

ADDITION POLYMERIZATION The free radical mechanism can be divided into three stages: Initiation Propagation Termination

ADDITION POLYMERIZATION Initiation: The creation of free radicals is necessary for propagation. May involve the formation of free radicals from stable species or may involve reactions of free radicals with stable molecules to form more free radicals.

ADDITION POLYMERIZATION Propagation: The rapid reaction of free radicals and monomers, and the subsequent repetition to create the repeating chain.

ADDITION POLYMERIZATION Termination: Two radicals react in a way that prevents further propagation, that is by coupling where two radicals react to form a single molecule.

ADDITION POLYMERIZATION (additional info) Symmetrical monomers such as ethylene and tetrafluoroethylene can join together in only one way. Mono-substituted monomers, on the other hand, may join together in organized ways, described in the following diagram. Most mono-substituted monomers, including propylene, vinyl chloride, styrene, acrylonitrile and acrylic esters, prefer to join in a head-to-tail fashion, with some randomness occurring from time to time. This is because ‘head to tail’ polymer is more stable due to less steric hindrance (less crowding of the substitution group).

CONDENSATION POLYMERIZATION Also known as dehydration reaction in which two monomers react with the concurrent loss of water. Examples: polyester (Dacron); polyamide (Nylon 66, Kevlar); polyurethane (Spandex).

Usage of Some Plastics Type of plastic Usage PET, polyethyleneterephthalate Soft drink bottles, photographic films, etc… PVC, polyvinylchloride ‘Synthetic leather’ upholstery, water pipes, bottles for cooking oils, garden hoses, laboratory tubing, etc… PS, polystyrene Styrofoam, hot-drink cups, fast food containers, etc… PP, polypropylene Drinking straws, bottle caps, bread and cheese wrap, etc… HDPE, high density polyethylene Containers for food, liquid detergents, shampoo, etc… LDPE, low density polyethylene Films for food wrapping, plastic bags, flexible containers such as squeeze bottles for condensed milk, etc…

HDPE vs LDPE Linear PE vs Branched PE Low density High density Linear PE (HDPE) is much stronger than branched PE (LDPE), but LDPE is cheaper and easier to make.

Low density Polyethylene (LDPE) Produced by free radical polymerization of ethylene gas. Polymer chain with branching. Low crystallinity, more flexible, melting point ~115oC. Uses: film and sheet-packaging, trash bag, household wraps, toys, squeeze bottle, etc… A molecule of LDPE.

High density Polyethylene (LDPE) Synthesized from ethylene using metal as the catalyst. Linear polymer molecule with no branches. High crystallinity, stiffer, harder and more opaque than LDPE. Melting point 133oC-138oC. Uses: containers and lid, food bottles, motor oil bottles, etc… A molecule of HDPE.

Sample Questions: 1. Which pair of polymer both occurs naturally? A. Starch and nylon. B. Starch and cellulose. C. Protein and nylon. D. Protein and plastic. E. Cellulose and nylon. 2. The segment below represents the polymer named A. Polybutylene. B. Polyvinyl chloride. C. Polypropylene. D. Polystyrene. E. Polyethylene.

Sample Questions: 3. A manufacturer plans to construct plastic reagent bottles, some of which will be used for 20% solutions of hydrochloric acid and sodium hydroxide. Among the following materials, which would be least suitable for this purpose? High density polyethylene. Polypropylene. Nylon 66. Polystyrene. Polyethylene oxide.

Sample Questions: 4. Which one of the following molecules COULD NOT serve as a monomer for an addition polymer? A. D. B. E. C.

Sample Questions: 5. A polymerization reaction that produces small molecules (such as water) as well as the polymer is classified as a/an _________ polymer. A addition B. natural C. condensation D. elimination E. co-polymer 6. What are Natta-Ziegler catalysts? What is their role in polymer synthesis?

Sample Questions: 7.(a) Write all the equations involve in the three steps of polymerization of ethylene, CH2 = CH2 to form polyethylene. (b) What do you think is attached at the end of the polymer chain when all of the ethylene monomer molecules have been polymerized?

Sample Questions: 8. Ethan-1,2-diol and hexan-1,6-dioic acid can produce a long chain compound of repeating units whereas ethanol and hexanoic acid are incapable of forming a long chain compound. (a) What name is given to a long chain compound formed by repeating units of small molecules? (b) What name is given to small molecules capable of forming such long chain compound? (c) What type of reaction occurs in the linking of units of ethanol and hexanoic acid? (d) What type of polymerization occurs in the linking of ethan-1,2-diol and hexan-1,6-dioic acid?

Sample Questions: 9. Draw the polymer that is form by the reaction between: HO2C-CH2-CH2-CH2-CH2-CO2H and H2N-CH2-CH2-CH2-CH2-CH2-CH2-NH2 What type of polymerization reaction is this? 10. High density polyethylene (HDPE) and low density polyethylene (LDPE) are both made from the same monomer, ethylene. However, HDPE is tough and rigid, whereas LDPE is soft and flexible. What difference in structure accounts for the difference in the properties of HDPE and LDPE?

Sample Questions: 11. The structure of the monomer styrene is shown below: Draw the structure of a portion of a chain of polystyrene that contains two (2) repeating units. 12. Below are some monomers used in making polyamides and polyesters. Which two monomers can be combined to make a polyamide?

Syndiotactic polymers Sample Questions: 13. Match the following terms with the correct definition: Ziegler-Natta catalyst , Isoprene , Homopolymer Syndiotactic polymers Definition Answer (i) One of the major components that constitute natural rubber and are used to make synthetic rubbers. (ii) A polymer structure in which the monomer units attached to the polymer backbone alternate in a-b-a-b fashion on one side of the backbone.

Ziegler-Natta catalyst , Isoprene , Homopolymer Sample Questions: Ziegler-Natta catalyst , Isoprene , Homopolymer Syndiotactic polymers Definition Answer (iii) This polymer is formed from a single monomer; an example is polyethylene, which is formed from the polymerization of ethylene. (iv) These compounds on titanium tetrachloride and the organometallic compound triethylaluminium, they are used in the production of unbranched, stereoregular polyalkene polymers.

Sample Questions: 14. The addition polymer polyvinyl chloride (PVC) has the structure: (i) Draw the structure of the monomer from which PVC is made. (ii) How many monomer units are in a PVC polymer that has a molar mass of 1.33  105 g/mol?

Sample Questions: 15. The structures below represent two different samples of polyethylene, each with the same number of monomer units. Structure I Structure II Based on the concept of density, which is the structure of high-density polyethylene (HDPE) and which is low-density polyethylene (LDPE)? (ii) List three (3) differences between LDPE and HDPE.