Organic Chemistry II : Synthetic and Natural

Organic Polymer

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PowerPoint Lecture Presentation byJ. David RobertsonUniversity of Missouri

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

•Lucite25.1

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

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25.2

Copolymer is a polymer made up of two or more monomers.

Styrene-butadiene rubber

( CH CH2 CH2 CH CH CH2 )n

R groups on same side of chain:

Isotactic

R groups alternate from side to side:

Syndiotactic

R groups disposed at random:

Atactic

Stereoisomers of Polymers:

Comparisons of stereoisomerAtactic 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

a) addition polymerization;

b)condensation polymerization.

The free radical mechanism explains how the polymerization is initiated.

ADDITION POLYMERIZATION

Addition polymerization involves the breaking of double or triple bonds, which are used to link monomers in to chains.

Free radicals are very reactive, short-lived atom or molecule which have one or more unpaired electrons.

The free radical mechanism can be divided into three stages:

a) Initiation

b)Propagation

c) Termination

ADDITION POLYMERIZATION

Initiation:

ADDITION POLYMERIZATION

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.

The creation of free radicals is necessary for propagation.

Propagation:

ADDITION POLYMERIZATION

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

Termination:

ADDITION POLYMERIZATION

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).

Examples: polyester (Dacron); polyamide (Nylon 66, Kevlar); polyurethane (Spandex).

CONDENSATION POLYMERIZATION

Also known as dehydration reaction in which two monomers react with the concurrent loss of water.

Usage of Some PlasticsType 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

High density Low density

Linear PE (HDPE) is much stronger than branched PE (LDPE), but LDPE is cheaper and easier to make.

Low density Polyethylene (LDPE)

Uses: film and sheet-packaging, trash bag, household wraps, toys, squeeze bottle, etc…

A molecule of LDPE.

Produced by free radical polymerization of ethylene gas.

Polymer chain with branching.

Low crystallinity, more flexible, melting point ~115oC.

High density Polyethylene (LDPE)

Uses: containers and lid, food bottles, motor oil bottles, etc…

A molecule of HDPE.

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.

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?

Sample Questions:

A. High density polyethylene.

B. Polypropylene.

C. Nylon 66.

D. Polystyrene.

E. Polyethylene oxide.

4. Which one of the following molecules COULD NOT serve as a monomer for an addition polymer?

Sample Questions:

A.

B.

C.

D.

E.

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?

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?

Sample Questions:

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?

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.

Sample Questions:

12. Below are some monomers used in making polyamides and polyesters. Which two monomers can be combined to make a polyamide?

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.

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:

Sample Questions:

(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?

15. The structures below represent two different samples of polyethylene, each with the same number of

monomer units.

Structure I Structure II

(i) 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.

Sample Questions: