Polyethylene, and polypropylene are by far the most important from a commercial prospective. In 1950, Breckly used a metal catalyst to prepare linear "polymethane" from diazomethane.
metal catalyst
CH2N2 -------------> - ( CH2 )n -
which had a melting point of 136 degrees C.
In 1937, Faucett developed a free radical process that prepared polyethylene
from ethylene using pressure and a catalyst. The maximum molecular weight
achieved occured from using 3000 atm pressure. The notes don't say what
molecular weight was achieved.
Raw Materials for polyolefins are obtained from
petroleum and natural gas. The cost can run from 20 to 40 cents per pound. As
you may recall from a geography class, hydrocarbon materials are obtained from
crude oil, coal, parafin, which originate from dead plant life.
Fraction Distillation temperature range # of carbons in compound
gas up to 25 C1 to C4
naphtha 20 - 100 C4 to C7
gasoline 70 - 200 C6 to C12
kerosene 175 - 275 C9 to C16
gas oil 200 - 400 C15 to C25
paraffin wax 230 - 300 (50 - 75 mm Hg) C18 to C35
cubic oil 300 - 365 (50 - 75 mm Hg) C35 to C40
asphalt (pitch) residue C30 to C70
Many complex reactions occur during cracking but the main ones are dehydrogenation and chain-scission.
CnH2n+2 à CnH2n + H2
450- 530
C ---> C through C --------> parafins and olefins
n 1 4
Timeline of Olefin Polymerization-
came in
stream ("came in stream" may be jargon for "it began to
operate.")
At this time polyethylene is now one of the major commercial polymers and is used in such diverse applications as
Branched polyethylene is often made by free radical vinyl polymerization.
Linear polyethylene is made by Ziegler-Natta polymerization.
UHMWPE is made using metallocene catalysis polymerization.
High Pressure Process for making branched polyethylene
When ethylene is compressed, it gets very hot and it may polymerize before polymerization is desired. Each compressor has its own cooling system. For this process, the ethylene is a vapor as dense as a liquid, since the temperature used is above the critical temperature of 9.7 C. The pressure makes the vapor appear as a liquid, and the polymer is dissolved by this "pseudo-liquid.
All the properties depend on branching. Such as
Ziegler-Natta polymerization can be used to make LDPE, too. By copolymerizing ethylene monomer with a alkyl-branched comonomer such as one gets a copolymer which has short hydrocarbon branches. Copolymers like this are called linear low-density polyethylene, or LLDPE. BP produces LLDPE using a comonomer with the catchy name 4-methyl-1-pentene, and sells it under the trade name Innovex. LLDPE is often used to make things like plastic films.
Polyethylene oxidizes in air
or under exposure to UV light at elevated temperatures.
Anti-oxidants are added to the polymer to prevent
oxidation during the processing of PE and when the finished product is exposed
to sunlight. Examples of anti-oxidants include aromatic amines and phenols. The
extraction of a hydrogen from an anti-oxidant by a peroxy radical interrupts the
propagation:
Chemical Reactions of PE
CHLORINATION OF POLYETHYLENE- PE may be halogenated in solution using solvents
such as carbon tetrachloride, chloroform, and chlorobenzene at temperatures from
45 to 75 degrees C. There is a comment in the notes that suggests that light or
peroxides are added. Up to 30% chlorine can be added. The chlorination reduces
the crystallinity of PE.
CHLOROSULPHONATED POLYETHYLENE- PE may be chlorinated in the presence of
chlorine and a small amount of sulfur dioxide to make chlorosulphonated
polyethylene.
After the produces polyethylene is converted to the following functional groups:
Cl
|
O=S=O
|
-CH2–CH2–CH2–CH2–CH2–CH2–CH2 ..
Copolymerization of Polyethylene Polyethylene has
been copolymerized with propylene, 1-butene, vinyl acetate, ethyl acrylate and
carboxylic acid. Ethylene is often the primary monomer component for a
copolymerization, and the intent of other monomer(s) is to place a non-hydrogen
functional group on the main chain, ever so often to modify properties.
An increase in flexability may be desired because PE homopolymer has a high
crystallinity, and flexability is a "conjugate property" to crystallinity.
Copolymers of polyethylene do not suffer the problem from plasticizer migration (out of the polymer) since the plasticizer is the oxygen that is a part of the polymer.
Polypropylene is prepared from propylene (the official IUPAC name is propene) which comes from the cracking process.
Polypropylene is one of those rather versatile polymers out there. It serves double duty, both as a plastic and as a fiber. As a plastic it is used to make things like dishwasher-safe food containers. It can do this because it doesn't melt below 160 oC. Polyethylene, on the other hand, will anneal at around 100 oC, which means that polyethylene dishes will warp in the dishwasher.
Propylene is cheaper than ethylene because of its stability
factor.
The free radical process is not used for the production of polypropylene from
propylene because of the extensive transfer of hydrogens to the propagating
centers which results in a resonance stabilized alkyl radical which has little
tendency to react with another monomer molecule.
* *
R* + CH2-CH=CH2 --> RH + [CH2-CH=CH2 <--> CH2=CH-CH2 ]
Advantages of the Ziegler
process include better control of the tacticity. The product isolated is mostly
isotactic.
Physical properties of polypropylene
Ethylene Propylene Rubber is prepared
in the presence of Zeigler Natta catalyst system. Copolymerization of propylene
and ethylene yields noncrystalline products that have rubbery behavior and are
chemically inert because of their saturation. .
C. Polyisobutene
Polyisobutylene is a synthetic rubber, or elastomer. It is special because it is that only rubber that is gas impermeable, that is, it is the only rubber which can hold air for long periods of time. (like balloon) Because polyisobutylene will hold air, it is used to make things like the inner liner of tires, and the inner liners of basketballs.
It is made from the monomer isobutylene, by cationic vinyl polymerization.
Usually, a small amount of isoprene is added to the isobutylene. The polymerization is carried out at a right frosty -100 oC. This is because the reaction is so fast we can't control it unless we freeze it.
Polyisobutylene was first developed during the early 1940s. At that time, the most widely used rubber was natural rubber, polyisoprene. Polyisoprene was an excellent elastomer, and easy to isolate from the sap of the hevea tree. Huge plantations thrived in Malaysia which grew hevea trees to supply the world's rubber needs.
When isoprene is polymerized with the isobutylene we get a polymer that looks like this:
About one or two out of every hundred repeat units is an isoprene unit, shown in blue. These have double bonds, which means the polymer can be crosslinked by vulcanization just like natural rubber.
D. Natural rubber and other polyisoprenes.
Natural rubber is a high molecular-weight polymer of isoprene, in which essentially all the isoprene have the cis-1,4 configuration.
Polyisoprene is diene polymer, which is a polymer made from a monomer containing two carbon-carbon double bonds. Like most diene polymers, it has a carbon-carbon double bond in its backbone chain. Polyisoprene can be harvested from the sap of the hevea tree, but it can also be made by Ziegler-Natta Polymerization. This is a rare example of a natural polymer that we can make almost as well as nature does.
E. Polybutadiene
Polybutadiene was one of the first types of synthetic elastomer, or rubber, to be invented. It didn't take a great a degree of imagination to come up with, as its very similar to natural rubber, polyisoprene. It is good for uses which require exposure to low temperatures. Tires treads are often made of polybutadiene copolymer. Belts, hoses, gaskets and other automobile parts are made from polyubutadiene, because it stands up to cold temperatures better than other elastomers.
Polybutadiene is a also a diene polymer, that is a polymer made from a monomer containing two carbon-carbon double bonds, specifically butadiene. It is made by Ziegler-Natta polymerization.
Poly(styrene-butadiene-styrene),
or SBS, is a hard rubber, which is used for things like the soles of shoes,
tire treads, and other places where durability is important. It is a type of
copolymer called a block copolymer. Its backbone chain is made up of three
segments. The first is a long chain of polystyrene, the middle a long chain of
polybutadiene, and the last segment is another long section of polystyrene.
Here's a picture:
Polystyrene is a tough hard plastic, and this gives SBS its durability. Polybutadiene is a rubbery material, and this gives SBS its rubber-like properties. In addition, the polystyrene chains tend to clump together. When one styrene group of one SBS molecule joins one clump, and the other polystyrene chain of the same SBS molecule joins another clump, the different clumps become tied together with rubbery polybutadiene chains. This gives the material the ability to retain its shape after being stretched.
SBS is made with some really clever chemistry using living anionic polymerization.
SBS is also a type of unusual material called a thermoplastic elastomer. These are materials which behave like elastomeric rubbers at room temperature but when heated can be processed like plastics (also include S-I-S). Most types of rubber are difficult to process because they are crosslinked. But SBS and other thermoplastic elastomers manage to be rubbery without being crosslinked, making them easy to process into nifty useful shapes.
F. Polychloroprene
Polychloroprene is usually sold under the trade name Neoprene. It's especially resistant to oil. It was the first synthetic elastomer, or rubber to be a hit commercially. It was invented by the Arnold Collins, while working under the same fellow who invented nylon, Wallace Carothers.
Polychloroprene is made from the monomer chloroprene, believe it or not, like this:
Chloroprene has two double bonds, so we call it a diene. Polychloroprene has properties similar to those of other diene polymers like polyisoprene and polybutadiene.
Homework.
