Polymer Revolution Chemical Storylines
PR1 The start of the revolution Lots of polymers are produced in nature Synthetic polymers have only been in widespread use since the 1950s In 19th century, plastics were made by modifying natural polymers The first polymer to be made in significant quantities as Bakelite It was first made by accident in 1872 but the manufacturing process wasn’t developed and patented until 1910 Important polymers which we now know were developed in the 1930s to 1960s… e.g. poly(ethene), poly(vinylchloride), poly(styrene), etc… …and none were manufactured inn large quantities until the 1950s Many of these were discovered by accident
What is a Polymer? A polymer is a long molecule made up of lots of small molecules (called monomers) There are two general types of polymer; If a single monomer (‘A’) is used… …it makes an A-A polymer (---A-A-A-A-A-A-A---) examples include poly(ethene), poly(styrene), pvc If two monomers (‘A’ and ‘B’) are used… …it makes an A-B POLYMER (---A-B-A-B-A-B---) examples include nylon and polyester PR1.1 PR1.2 CI12.2 “Alkenes” CI5.6
We can also look at the physical properties of the polymer… Some repeatedly soften on heating and harden on cooling These are called thermosoftening plastics (or thermoplastics) Examples include poly(ethene) and pvc Others won’t soften or change shape on heating These are called thermosetting plastics (or thermosets) They form bonds called cross links between the polymer chains Examples include bakelite Some will spring back into their original shape when stretched These are called elastomers Examples include natural rubber Others won’t stretch easily. These can be made into strong, thin threads These are called fibres Examples include nylon
PR2 The Polythene Story March 1933, Fawcett and Gibson were carrying out a reaction at 2000 atmospheres pressure Their equipment leaked and they had to add extra ethene The product was a white waxy solid. It had the empirical formula CH2 When they tried to repeat the experiment it didn’t always work In July 1933 work was halted because of the danger and the lack of consistent success Work was restarted in 1935 It was quickly discovered that; The benzaldehyde was not needed The amount of oxygen was crucial; none no waxy solid too much explosion Without the original leak, the waxy solid wouldn’t have formed
A bonus of being big The idea that a polymer molecule is just a very big molecule was only proposed in 1922. It was then heavily criticised for the next decade! All polymers are mixtures of similar molecules This is because different numbers of monomers will join together in the polymerisation processes Polythene’s chemical properties are just like those of a very long alkane, its mechanical properties, however, are very different Ass 1 CI5.3 PR2.3 Check your notes
PR3 Towards High Density polymers Original polythene is now known as LDPE (low density polythene) The polymer chains are ‘messy’... they are not straight, there is a lot of branching, they can’t fit together in a regular way. We say the polymer is amorphous As a result they take up a lot of space… …hence the low density… …high degree of flexibility… ...and lower strength
Ziegler Catalysts Karl Ziegler was studying organometallic compounds. He found that strange things happened when he used organo-aluminium compounds He tracked this down to traces of nickel in his apparatus He then tried lots of different transition metal salts with the organo-aluminium compounds He found the best mixture to be TiCl4 and Al(C2H5)3 (triethylaluminium) When ethene was passed through these at atmospheric pressure it immediately produced poly(ethene) Better still, this poly(ethene) had a very high average Mr (about 3 000 000) and very little branching Another example of a discovery which came from an impurity!
Because there is far less branching, these polythene chains can line up and pack more closely together • We say the polymer is crystalline • As a result, compared to LDPE… • they take up less space; - have a higher density; • lower degree of flexibility; - greater strength; • and greater heat resistance. • This is HDPE (high density polythene) • It is used for things like piping, petrol tanks and Tupperware • It is also used for hospital equipment as it can be sterilised with heat without deforming • Ziegler won the Nobel Prize for Chemistry in 1963 • He also patented HDPE. • On his 70th birthday he donated $10 million for further research!!
Natta and stereoregularpolymerisation Giulio Natta used Ziegler’s catalyst to polymerise propene His product contained two forms of poly(propene); A crystalline form and an amorphous form… Crystalline form: All CH3 groups on same side of the chain Strong and rigid Amorphous form: CH3 groups randomly arranged on both sides of the chain Soft and flexible
Natta went on to develop new catalysts known as Ziegler-Natta catalysts • These allow chemists to produce polymers with precise properties • Now, a new group of catalysts are being introduced into industry called metalocenes • Metalocenes are even more specific than Ziegler-Natta catalysts • Ass 2 • Ass 3
PR4 Dissolving Polymers • Poly(ethenol) is a polymer which dissolves in water • This property makes it very useful for hospital laundry bags • It contains hydroxyl groups like those in alcohols • CI13.2 (“Alcohols and Ethers”) • PR4.3 • PR4.4 • To explain why it dissolves we need to understand intermolecular forces and hydrogen bonding in particular • CI5.4 (“Hydrogen bonding”) • PR4.1 • PR4.2 • Ass 5, 6
PR5 Polymers that outdo nature • Many new addition polymers can do jobs that nature can’t • In 1938 Roy Plunkett wanted to use some tetrafluoroethene gas (C2F4) • The cylinder appeared empty but when he checked inside he found a white, waxy solid. • The gas had polymerised to form poly(tetrafluoroethene) • PTFE for short and marketed as ‘Teflon’ • Teflon has unusual properties which make it very useful… • excellent anti-stick properties • very resistant to chemical attack • very good electrical insulator • Hydrophobic – vital layer in Gore-tex fabric
ETFE • A copolymer of ethene and tetrafluoroethene • It was used to build the biomes at the Eden Project in Cornwall because of its unique properties… • very high transparency • very low density • high resistance to radiation • shatterproof • stain resistant
Neoprene • Made in 1930s as a rubber substitute • Mainly poly(chloroprene) plus some 2,3-dichloro-1,3-butadiene and sulphur to introduce some cross linking • Non porous • Resistant to heat, light and chemical attack • CI5.6 • Ass 7 • Neoprene’s properties are affected by the polymerisation process • This is because of geometric isomerism (E/Z isomerism) around the carbon-carbon double bond • CI3.5 “Geometric Isomerism” • PR5.1
Natural Rubber or Gutta Percha? • Natural rubber is an addition polymer of 2-methyl-1,3-butadiene (or isoprene) • So rubber is also called poly(isoprene) • In the rubber tree (Hevea brasiliensis) the chains form with the Z (cis) arrangement… • In Malaysia a different tree (Palaquium gutta) also makes a polymer from isoprene • This form of poly(isoprene) is know as gutta-percha • However its chains form with the E (trans) arrangement • This makes it hard and non-elastic • One use is as the coating for golf balls • CI6.4 “Infra red spectroscopy” • PR 5.2