PR2 – The polythene Story

An Accidental Discovery.

·        Polythene discovered by accident in 1933 by Gibson + Fawcett working for ICI on reaction.

·        They left the mixture over the weekend, but it leaked and more ethene had to be added.

·        Produced a white waxy solid with empirical formula CH₂.

·        Couldn’t reproduce work consistently – sometimes it exploded – so work was halted.

·        Work restated in 1935 – managed to control process by adding cold ethene at correct rate.

·        Role of oxygen was identified by Perrin in 1935.

·        Too much O₂ – loss of control – explosion.

·        No oxygen – no polymerisation.

·        They also discovered that the benzaldehyde is not actually needed!!

Properties of Poly(ethene)

v     Tough and durable

v     Excellent electrical insulator.

v     Not adversely affected by environment.

v     Almost no tendency to absorb electrical signals.

v     Easily melted and moulded.

Uses.

¨      Insulating telephone cables (e.g. between UK and Isle of White)

¨      In development of Radar.

¨      Washing up bowls, carrier bags etc….

Overuse and exploitation of early plastics led to ‘cheap’ image!!

Now do assignment 1.

A bonus of being big.

·        A polymer is just a big molecule. Large molecules are chemically similar to smaller ones, therefore the properties of polymers can be predicted from those of similar smaller molecules.

·        Poly(ethene) is like a giant alkane therefore it;

Ø      Burns well

Ø      Is unreactive.

·        Polymers are not pure compounds but mixtures of similar molecules because different numbers of monomers combine during the chain building process.

·        Because of this poly(ethene) softens and melts over a range of temperatures.

NB Poly(ethene)’s mechanical properties are, by contrast, completely unlike those of similar smaller molecules.

PR3 – Towards High Density Polymers.

          Fawcett + Gibson’s poly(ethene) was low density (ldpe) – it was ‘messy’ at a molecular level with lots of branching (due to back-biting) so the chains did not fit well together – hence lower density and therefore lower strength.

Karl Zeigler – (Zeigler catalysts)

·        He was studying ORGANOMETALLIC compounds – cmpds which contain covalent metal-carbon bonds.

·        He was using an Al organometallic cmpd and got some strange results – found later to be due to Ni impurities in the equipment – so he tried some other transition metals.

·        In 1953 he tried Ti cmpds;

C₂H₄ (atm press) + TiCl₄ + (C₂H₅)₃Al ®  Poly(ethene)

                                                                     ¯

                                                          Av. Rmm 3,000,000

                                         v. little branching

                                         greater close packing

                                         more crystalline

                                         \higher density

                                         \Greater strength

·        He had produced high density poly(ethene) (hdpe) which is;

Ø      Stronger (used for water tanks, pipes, car parts etc…)

Ø      Not as easily deformed.

Ø      Can be reshaped during heating.

Ø      Can be heat sterilised therefore health applications i.e. can withstand higher temps.

·        Zeigler patented process and became very rich – He put millions back into further research!!

Natta and Stereoregular Polymerisation.

·        Used Zeigler’s catalyst to polymerise propene and got two forms;

ISOTACTIC Poly(propene)            ATACTIC Poly(propene)

Regular structure                                random structure

Crystalline                                          amorphous

Tough (like hdpe)                                chains loosely held

                                                          Soft and flexible

Uses;

Isotactic – sheets and films for packaging, containers, fibres for carpets.

Atactic – roofing materials, sealants and weatherproof coatings.

·        Natta also developed new catalysts – which allowed polymer chain to ‘grow’ outward from catalyst surface (see fig 10 pg. 98 for mechanism). Called these Zeigler-Natta catalysts – and earned them a joint Nobel prize for chemistry (1963)

Into the new Millennium.

·        A new generation of catalysts for addition polymerisation are being introduced – metallocenes.

·        Have structure like a sandwich with filling of Zr, or other transition metal.

·        Bread ‘slices’ are flat organic molecules with arene ring systems.

·        Allow control of M_r of polymer as well as structure.

·        Poly(ethene) and poly(propene) produced using metallocenes give thin films which are;

·        Stronger

·        More tear resistant

·        More impermeable to air and moisture

than conventional forms of these polymers. They can be used to protect substances susceptible to air and moisture e.g. food!

SL PR4 – The Teflon Man.

·         Roy Plunkett of Du Pont (USA)

·         In 1983 wanted to use tetrafluoroethene (a gas)

·         Cylinder seemed to be empty.

·         Opened it up and found it had polymerised to give a white waxy solid (poly(tetrafluoroethene) or PTFE)

            Tetrafluoroethene                             Poly(tetrafluoroethene)

Properties;

·         Low friction (anti-stick)

·         Resistant to chemical attack.

·         Good electrical insulator.

·         Hydrophobic (water hating)

Uses

·         Roof of Millennium dome.

·         Non-stick pans (brand name Teflon)

·        Weather proof clothing – lets water vapour out but doesn’t let liquid water in – GORE-TEX – read green box on pg. 101.

SL PR5 – Dissolving Polymers.

·         Hospital laundry poses risk of infection if handled.

·         Hence it is put in bags made of dissolving plastics.

·         In washing machine bag dissolves to release linen.

·         Plastic used is poly(ethenol)

·         Monomer?

CH₂=CH-OH       Doesn’t exist!!!!

·         Made by following reaction;

R – O – COCH₃ is an example of an ESTER we will meet these later

·         Reaction controlled by adjusting time or temp.

·         Solubility of plastic depends on % of OH groups present (see table 1 on pg. 102)

·         Different solubilities give plastic different uses.

·         Why is polymer soluble?

Now do assignment 4.