Transcript C8 & C9 - uaschemistry

Chemistry Presentation
C8 – Condensation polymers
C9 – Mechanisms in the organic
chemicals industry
Seunghwan Lee
C.8.1
• Distinguish between addition and
condensation polymers in terms of their
structures.
Addition polymers
Polyethene
Polychloroethene
Polytetrafluoroethene
Condensation polymers
C.8.2
• Describe how condensation polymers are
formed from their monomers.
Phenol-methanal plastics
• Phenol-methanal plastics are made by adding
acid or alkali to a mixture of phenol and
methanal. The methanal is first substituted in
the 2- or 4- position in the phenol and then
the product undergoes a condensation
reaction with another molecule of phenol
with the elimination of water
Polyurethane
• Polyurethanes are produced from the reaction
of polyhydric alcohols (e.g. diols or triols) with
compounds containing more than one
isocyanate functional group, -NCO.
Polyethylene terephthalate (PET)
• Benzene-1,4-dicarboxylic acid + ethane-1,2diol. PET is famous for beverage container,
too.
C.8.3
• Describe and explain how the properties of
polymers depend on their structural features.
Cross-linking sturcture of a phenolmethanal plastic
• This structure makes the bonds stronger and the melting
point higher. In fact, it has such a high melting point that it
decomposes before melting. It is also very unreactive.
• Due to its high electrical resistance it was used as the
casing for early radios and is now used as one of the
ingredients in worktops and printed circuit-board
insulation.
Kevlar
• Kevlar is a polyamide made by condensing 1,4diaminobenzene with benzene-1,4-dicarbonyl
chloride.
• It forms a strong three-dimensional structure
due to hydrogen bonding between the long
rigid chains.
Kevlar
C.8.4
• Describe ways of modifying the properties of
polymers.
Examples
• Air can be blown into polyurethanes to make
polyurethane foams for use as cushions and
thermal insulation.
• The fibers of polyesters can be blended with
other manufactured or natural fibers for
making clothes which are dye-fast and more
comfortable than pure polyesters.
Examples (cont.)
• When ethyne is polymerized the cis- form of
poly(ethyne) is found to be an electrical
conductor due to the delocalization of the π
electrons.
Doping
• The conductivity can be dramatically
increased by adding chemicals such as iodine
which can remove electrons or alkali metals
which can add electrons. This process is
known as doping.
C.8.5
• Discuss the advantages and disadvantages of
polymer use.
Advantages
• Strength: Polymers, such as Kevlar, are so strong
that they are commonly used to make bullet
proof armor.
• Density: Compared to metals, polymers are much
lighter yet strong.
• Durability: Polymers hardly corrodes or oxidize.
• Insulation: Some polymers are good insulators.
• Conductivity: Some polymers are good
conductors.
• Lack of reactivity: Polymers do not tend to react
with other elements.
Disadvantages
• Disposal: The biggest problem of polymer use is
its lack of biodegradability. Since polymers cannot
be degraded, they stay long underground,
emitting toxic materials.
• Disposal (2): Polymers like polyurethanes releases
toxic hydrogen cyanide gas when they are
burned.
• Use of natural resources: Polymers are typically
made of petroleum and other natural resources.
C.9.1
• Describe the free-radical mechanism involved
in the manufacture of low-density polyethene.
Low density poly(ethene), LDPE
The manufacture of low density poly(ethene)
is carried out at very high pressures (1000 –
3000 atm) at a temperature of about 500 K.
An initiator, such as an organic peroxide or a
trace of oxygen, is added. Under these
conditions free radicals are formed.
Low density poly(ethene), LDPE
• Termination takes place when two radicals
combine. The average polymer molecule contains
between about 4 x 103 and 4 x 104 carbon atoms
with many short branches. The branches affect
both the degree of crystallinity and the density of
the material. LDPE generally has a density of
about 0.92 g cm-3 and is used mostly for
packaging.
High density poly(ethene), HDPE
High density poly(ethene) is manufactured by
polymerizing ethene at a low temperature (about 350
K) and pressure (1 – 50 atm) using a Ziegler-Natta
catalyst. The catalyst is a suspension of titanium(III) or
titanium(IV) chloride together with an alkyl-aluminium
compund (e.g. triethylaluminium Al(C2H5)3). The
mechanism is complex and still not thoroughly
understood. Essentially it involves the insertion of the
monomer between the catalyst and the growing
polymer chain. This is known as co-ordination
polymerization (sometimes described as anionic
polymerization). The titanium atom is attached to one
end of the growing hydrocarbon chain and uses its
empty d orbitals to form a co-ordination complex with
the π electrons of the new incoming ethene molecule.
High density poly(ethene), HDPE
• The resulting polymer consists mainly of linear
chains with very little branching. This gives ti a
higher density (0.96 g cm-3) and a more rigid
structure as the chains can fit together more
closely. It is used to make containers and
pipes.