Polymer Technology

Polymer = poly + meros (Greek: many parts)

Monomer = mono + mero (one part)

high pressure, heat & catalystC C

R2

R1 R3

R4

n C C

R2

R1 R3

R4n

n = 5,000; 10,000; etcpolymer

Classification of Polymers

(A) Natural polymers and artificial polymers

Examples of natural polymers:-proteins, polysaccharides (e.g. starch, cellulose, cotton), nucleic acids (DNA and RNA), rubber.

Assignment 1.

Qn.1 With the help of drawings, describe the structures of cotton, silk, chitin and wool.

Examples of artificial polymers:-nylon, polyester, polyethylene, etc.

(B) Fibres, elastomers and plastics Fibres are long, thin, and threadlike, with strength along the fibre (thread) Examples: cotton, wool, silk, etc. Elastomers exhibit the property of elasticity Examples: rubber Plastics can be extruded as sheets, pipes, painted on surfaces or molded (casted) into particular shapes of interest. Examples of plastics: polyethene, polyvinyl chloride (PVC), poly(tetrafluoroethylene) or PTFE ‘teflon’, ‘saran’, ‘styrene’, etc. (C) Addition (chain-growth) polymers and condensation (step-growth) polymers Addition polymers result from the rapid addition of one molecule at a time to a growing chain, usually with a reactive intermediate (cation, radical, or anion) at the growing end of the chain. They are sometimes referred to as chain-growth polymers.

Condensation polymers result from condensation (bond formation with loss of small molecules) between the monomers. The resultant polymers are also called step-growth polymers.

(D) Homopolymers and copolymers Homopolymers are made up of identical monomer units Copolymers are made by polymerisation of two or more different monomer units together. Example is Saran®, made from vinyl chloride and vinylidene chloride

Acrylonitrile, butadiene, and styrene and are polymerized to give ‘ABS plastic’ a strong, tough, and resilient material used for making articles that must resist heavy impacts, e.g. bumpers and crash helmets, Sub-divisions of copolymers Random copolymer: the different monomer units are distributed along the molecule randomly. M1M2M1M1M2M2M1M2M1M2M1M1M2M1M1

Alternating copolymer: M1M2M1M2M1M2M1M2M1M2M1M2M1M2 Block copolymer: sections made up of one monomer alternate with sections of another. M1M1M1M1M1M1M1M2M2M2M2M2M2......... Graft copolymer: a branch of one kind is grafted to a chain of another kind M1M1M1M1M1M1M1M1M1M1.........

M2M2M2M2M2M2 (E) Isotactic, syndiotactic and atactic polymers Polymers can be classified based on stereochemistry (orientation) of monomeric units in space. Isotactic polymer: polymer in which a particular functional group is directed in a consistent oriention

Note the orientation of the methyl groups. Syndiotactic polymer: polymer with a regularly alternating orientation of a given functional group (or substituent)

H H3C H H3C H H3CCH3 H CH3 H CH3 H

H H H H HH H H H H

Atactic polymer: polymers with random orientations of particular groups.

H H3C H H H3C HCH3 H CH3 CH3 H CH3

H H H H HH H H H H (F) Thermoplastic polymers Linear and Branched Polymers may be more or less crystalline and include some materials used as fibres (e.g. nylon); polyalkenes (PVC, polyethylene, polystyrene, etc). On heating these polymers soften and become mouldable. They are, thus, called thermoplastics. (G) Thermosetting Space-network polymers (or resins) are highly cross-linked to form a rigid but irregular three-dimensional structure, e.g. phenol-formaldehyde, urea-formaldehyde resins. When heated, these polymers do not soften; instead the polymer may harden due to formation of more bonds. Such polymers are called thermosetting polymers.

Addition Polymers

Many alkenes undergo chain-growth polymerisation when treated with a suitable initiator. There are three approaches of carrying out chain-growth polymerisation: (a) free-radical polymerisation; (b) cationic polymerisation (c) anionic polymerisation.

The table below shows some examples of addition polymers

Free radical polymerisation

Initiators are usually peroxides, e.g. benzoyl peroxide acts as an initiator for the polymerisation of styrene when heated at 100 ºC.

Machanism of free-radical polymerisation

Three major steps involved; initiation, propagation and termination

Initiation step:

The initiator forms free radicals that react with the monomer to start a chain.

C OO

COO heat

benzoyl peroxidephenyl radical

H

HH

H

H

H

HH

H

H

C C

H H

HC C

H

HH

Propagation stage: other molecules of the monomer add to the chain resulting in a bigger chain.

C C

H

HH

C

HC C

H H

HC C

H

HH

C

H

H

C C

H

HH

C

H

C

H

Hn

Termination:

The reaction chain is terminated by steps that consume but do not form free-radicals: combination or disproportionation of two free-radicals.

Combination

C C

H

HH

C

H

C

H

Hn

CC

H

H H

C

H

C

H

Hn

C C

H

HH

C

H

C

H

Hn

combination

2

Or disproportionation

C C

H

HH

C

H

C

H

Hn

CC

H

H H

CH

H

C

H

Hn

C C

H

HH

C

H

C

Hn

disproportionation

+

2

Qn 1. Polystyrene formed with isotopically labelled AIBN as initiator was found to contain

two initiator fragments per molecule of the polymer.

What termination reaction is implied by these results?

Chain branching

Chain branching may result from abstraction of a hydrogen atom in the middle of a chain by the free radical at the end of another chain resulting in a free-radical in the middle point of the chain. A new chain grows from the free-radical in the middle of the chain.

C

H

H

C C

H

HH

C

H

H

C

H

H

H2C CH2 C

H

H

C C

H

HH

C

H

H

C

H

H

H2C

CH2

Effects of chain branching The polymer formed is amorphous and less orderly compared to linear polymeric units. Example of branched polymers is low density poly(ethylene) or LDPE which is soft and flimsy.

Inhibitors Inhibitors are substances added to stop and terminate the polymerization process. Many of such compounds are capable of reacting with the growing free-radical chain to generate a new free radical that is not reactive enough to add to a monomer. Examples of inhibitors include amines, quinones and phenols.

Assignment 1: (a) Write short notes about chain-transfer. (b) What are the effects of using chain-transfer agents?

Cationic Polymerisation Very similar to free-radical polymerisation except that it uses carbocation intermediates.

Strongly acidic catalysts are used as initiators. Examples include BF3 (a Lewis acid) with traces of water or methanol as co-catalysts.

Mechanism of cationic polymerisation

The counter ion ensures electrical neutrality of the final product.

One major requirement for cationic polymerization: The monomer should be capable of forming a stable carbocation. Stability of carbocation is in the order of 3º ˃ 2º ˃ 1º.

Study question

Anionic polymerization

Initiated by a strong carbonion-like reagent such as organolithium or Grignard reagent. Example is the anionic polymerisation of acrylonitrile in the presence of n-butyllithium as the catalyst.

Other bases such as K+NH2- can also be used to initiate the anionic polymerisation.

Group I metals (e.g. Li and Na) in the presence of naphthalene can also be used as catalysts. Read about the mechanism involved here. (Hint: blue solutions of group I metals in ammonia)

Anionic polymerisation can take place with epoxides as well. For example the ethylene oxide can polymerise in the presence of catalytic amount of a methoxide.

Epoxy Resins and modern glues

Most modern glues constitute epoxy adhesives. They often from copolymerisation of an epoxy with bisphenol A.

Addition of a base catalyst to the prepolymer initiates the polymerisation.

In making the glue, epichlorohydrin is used in excess such the chains remain short and a runny product results. Otherwise the polymerisation may continue to extremely long chains which may easily solidify a feature not desired for glues before being used.

Usually epoxy glues are sold in two parts: the resin (the prepolymer) and the hardener with basic or nucleophlic properties (e.g. a polyamine). The hardener initiates polymerisation of different chains as well as cross-linking them. The result is a three dimension structure that is strong and resistant to chemical attack.

Chain linking

Cross-linking

Study question

Assignment

Write short notes about ‘living polymers’ and explain with an example how block copolymers may be made by use of ‘living polymers’. In what ways can ‘living polymers’ be ‘killed’?

Coordination Polymerisation, the Ziegler-Natta Polymerization

When Ziegler-Natta catalyst is used, an isotactic polymer can be produced.

Two major advantages of using Ziegler-Natta catalysts:

1) Polymerization is stereoselective. Either isotactic or syndiotactic forms may be produced by selecting a proper Ziegler-Natta catalyst.

2) Very little hydrogen abstraction occurs and the resulting polymer linear and with almost no branching.

3) With Ziegler-Natta catalyst high density poly(ethylene) can be produced with almost no chain branching and much greater strength than low-density poly(ethylene). A typical Ziegler-Natta catalyst can be formed by adding a solution of TiCl4 to a solution of (CH3CH2)3Al (triethyl aluminium). The mixture is heated to ‘age’ for about one hour.

General mechanism

Exchange of alkyl group and chloride ion between the metals

Formation of Ti-π complex with the first molecule of the monomer (e.g. propene) and then carbo-titanation of the π bond.

Insertion of the next monomer, which is a repeat of the previous step. This continues with the result of an isotactic polymer, [in this case poly(propelene)].

Condesation Polymerization

Polyamides: Nylon

Nylon is the common name of polyamides

Examples: Nylon 6,6 is made by reaction a six-carbon diacid (adipic acid) with a six-carbon diamine (e.g 1,6-hexanediamine)

Nylon can also be made from a single monomer having an amino group at one end and an acid at the other end (polymerisation is similar to formation of proteins from α-amino acids).

Nylon 6 is made from 6-aminohexanoic acid a process that starts with the cyclic compound called ε-caprolactam

Assignment

a) List the properties of (i) Nylon 6,6 and Nylon 6 b) Give the different uses of (i) Nylon 6,6 and (ii) Nylon 6 c) Draw the molecular structure of;

(i) Nomex® made from meta-phthalic acid and meta-diaminobenzene (ii) Kevlar® made from terephthalic acid (para-phthalic acid) and para-

diaminobenzene

Polyesters

Example: Dacron®

Assignment

Kodel® is made by transesterification of dimethyl terephthalate with 1,4-di-(hydroxymethyl)cyclohexane. Draw the structure of Kodel®.

Glyptal® makes a strong, solid polymer matrix for electronic/electrical parts. It is made from made from terephthalic acid and glycerol. Draw the structure of Glyptal®.

Polycarbonates

Carbonic acid (a diacid) and its derivatives can polymerise with suitable diols to form polyesters called poly(carbonate ester).

Example: Lexan® polycarbonate is a strong, clear, and colourless material that is used for bulletproof windows and crash helmets

Polyurethanes

Polyurethanes are made from reactions of diols with diisocyanates (e.g. toluene diisocyanates). The reaction process is exothermic. Volatile liquids such as butane are often added to the reaction mixture and the heat evolved by the polymerisation vaporizes the volatile liquid produces bubbles, hence converting the viscous polymer to a frothy mass of polyurethane foam. Note: the trade names of Crest foam, Royal foam, Eurofoam

Assignment:

a) Explain why the addition of a small amount of glycerol the polymerisation of polyurethanes gives a stiffer foam product.

b) Draw the structure of the polyurethane formed by the reaction of toluene diisocyanate with bisphenol A

Study questions:

Qn.1 Give the structure of the monomer from which each of the following polymers is likely to be made:

a) Orlon (fibers, fabrics, carpets), -[CH2CH(CN)CH2CH(CN)]- b) Saran (packaging film, seat covers), -[CH2CHClCH2CCl2]- c) Teflon (Chemically resistant articles), -[CF2CF2CF2CF2]-

Qn.2 Ozonolysis of natural rubber yields the compound below

Predict the structure of rubber.