The Notes for
Polymer and Coatings Science-
Chapter Two- part two

Polyolefins

The notes list five polymers:
  1. polyethylene
  2. polypropylene
  3. polyisobutene
  4. poly but-1-ene
  5. poly 4-methyl-pent-1-ene
The first two polymers on this list, polyethylene, and polypropylene are by far the most important from a commercial prospective. Historically, the first polyolefin synthesis was the 1898-preparation of "azeoalkanes". 
CH N  -->  -(CH ) -
  2 2          2 n
which had a melting point of 128 degrees C.

 In 1950, Breckly used a metal catalyst to prepare linear "polymethane" from diazomethane. 
      metal catalyst
CH N  -------------> - ( CH ) -   
  2 2                      2 m
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. 
C H     ---> C H   + H
 n 2n+2       n 2n    2
You probably saw this at the beginning of Organic I lecture when the textbook discussed the formation of alkenes. 
C    H         ---> C H  + C H
 m+n  2(m+n)+2       m 2m   n 2n+2
for the purpose of hydrogen counting, remember from algebra:

2(m+n)+2 = 2m +2n + 2

Catalytic cracking- The primary objective of catalytic cracking is the production of gasoline. The gasoline obtained by the direct distillation of crude petroleum is insufficient to meet demand (i.e., not enough is produced) and additional gasoline is obtained by the cracking of higher boiling petroleum fractions. The reaction is carried out by heating the higher fraction, such as gas oil at 450- 550 degrees C in the presence of an aluminum silica type catalyst. 
                      450- 530
C  ---> C  through C  --------> parafins and olefins
 n       1          4
Thermal cracking- Timeline of Olefin Polymerization- At this time polyethylene is now one of the major commercial polymers and is used in such diverse applications as
  1. chemical plants
  2. domestic goods
  3. electrical insulation
  4. packing films
  5. toys
  6. etc.
Cost and Commercial Production of Some polymers

Some material present on the clciris version needs to be transported to here.
High Pressure Process for making 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.:

Standard Oil Process for making polyethylene

For the Standard Oil Process ethylene is chemisorbed (I assume the past participle "chemisorbed" refers to the partial bond attractions we drew for mechanisms for Organic lecture) on the catalyst surface by interaction with the monomer. There is an interaction between the the pi orbitals of the monomer and the d orbitals of the suspended metal.

Initiation: There is a supply of unpaired electrons from the metal atoms to an adjacent adsorbed monomer molecule. Propagation occurs by bound ion radicals which are fixed to a catalyst surface.



The above graphic differs from what appears in the notes because it was discovered that the figure in the notes did not preserve the tetravalency (four bonds) of each carbon. Dashed lines are assumed to be "half-bonds."

Properties of Polyethylene: The high solubility of PE leads to many different applications.

High Pressure Polyethylene:

The electrical insulating properties of polyethylene are outstanding. Since it is a non-polar material, properties such as dielectric constant, and power factor (I've never heard of power factor) are almost independent of the frequency and temperature. The dielectric constant increases slightly with increasing density.

At elevated temperatures PE shows appreciable solubility in hot hydrocarbons and halogenated hydrocarbons, hot xylene and dichloroethane.

The temperature necessary to dissolve PE in a given solvent increases as the crystallinty of the polymer increases (60 C to 80 C for commercial materials.)

Strong oxidizing agents such as concentrated nitric acid (HNO3), hydrogen peroxide, and potassium permanganate (KMnO4) oxidize the polymer, resulting in an increase in the power factor and a deterioration of mechanical properties.

As the polyethylene density increases, the permeability of the polyethylene to the above oxidizers decreases, and the polymer is thus more chemical resistant.

Polyethylene oxidizes in air or from exposure to UV light (in a vacuum?) at elevated temperatures. The way the notes wrote this, it could be inferred that at high enough temperature, in the absence of UV light and air, polyethylene oxidizes. Well, the decomposition of any organic is in a sense, oxidation, as I understand it. But this is getting off the subject.

 Oxidation of Polyethylene-

Initiation       Polymer        ->  P.  (Polymer radical)

                                    O
                                    ||
Propagation      P. + O2        ->  R - O.

                 O                  O
                 ||                 || 
                 R - O. + P-H   ->  R - OH + P.    (P-H is a polymer)
The most viable hydrogens for extraction are hydrogens on the tertiary carbon. 
O
||
R - OH  undergoes further decomposition (homolytic cleavage)

O         O
||        ||
R -OH --> R. + .OH

  O         O      O
  ||        ||     ||
2 R -OH --> R.  +  R -O. + H2O
This cleavage of bonds in the main chains of the polymer lowers the molecular weight and thus decreases the mechanical properties. The presence of trace metals in the polymer results in an enhanced response to attack by oxygen.

ROOH + Co(2+) ----> RO. + OH(-) + Co(3+)

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:

AH + ROO. --> ROOH + A. A. is unreactive, usually because of aromatic resonance.

PE may be crosslinked intentionally by exposure to high energy radiation such as X-rays, -rays (??I don't know what this refers to, my guess is gamma rays) and fast electrons.

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.

From the notes it appears that this produces polyethylene with the following functional groups: 
             Cl
             |
           O=S=O
             |
-CH -CH -CH -CH -CH -CH -CH ..
   2   2   2   2   2   2   2
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) iss 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.

Miscellaneous: monomers and polymers

Copolymers of ethylene and methacrylic acid form ionomers: 
 H H     H H                         H H H H 
 | |     | |    (high pressure)      | | | |
 C=C  +  C=C         ---->        -[-C-C-C-C-]-
 | |     | |                         | | | |
 H H     H C=O                       H H H C=O
           |                               |
           O                               O
           |                               |
           CH3                             CH3
methacrylic acid constitutes 1 to 10 of every 100 monomers.

 Ionomers mave low softening points, and are more moisture sensitive. Applications of ionomers include extension coatings and skin packing films (??I don't know what this means,possibly saran wrap?)

 Polypropylene is prepared from propylene (the official IUPAC name is propene) which comes from the cracking process. 
H   H               H   H
|   |  Ziegler      |   |
C = C  ------>  -[- C - C -]-
|   |               |   |
H   CH3             H   CH3

propylene       poly(propylene)
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 ]  or  CH2 ... CH ... CH2
Advantages of the Ziegler process include better control of the tacticity. The product isolated is mostly isotactic.

Physical properties of polypropylene

Applications include the plastic bottle industry and injection molding transportation film??.

Ethylene Propylene Rubber is prepared in the presence of Zeigler Natta catalyst system. EPR rubber can be thought of as polypropylene with pendent methyl groups missing wherever ethylene monomers were incorporated into the EPR chain.

 Polyisobutylene is made from isobutylene as follows: 
H   CH3       BF   Et O         H   CH3
|   |           3    2          |   |
C = C         ---------->   -[- C - C -]-
|   |                           |   |
H   CH3                         H   CH3
The polymerization is best carried out at temperatures from -40 deg C to room temperature. This is a cationic polymerization process. It is more resistant to oxidation than EPR because there are no tertiary hydrogens. There is a cure problem, though. For curing some diene monomer such as isoprene (1 - 3%) is added. Crosslinking can be done with sulfur.

 Acrylic polymers are based on acrylic acid and its homologenes and their derivatives. The principle commercial acrylic polymers are polymerized from the following monomers: 
 H   H            H   CH3             H   H
 |   |            |   |               |   |
 C = C            C = C               C = C
 |   |            |   |               |   |
 H   C=O          H   C=O             H   C=O
     |                |                   | 
     OH               OH                  O-R     R is a nonhydrogen group

acrylic acid      methacrylic acid    esters of acrylic acid


 H   CH3          H   H               H   H
 |   |            |   |               |   |
 C = C            C = C               C = C
 |   |            |   |               |   |
 H   C=O          H   C               H   C=O
     |               |||                  |
     O-R              N                   NH2

esters of         acrylonitrile       acrylamide
methacrylic 
acid


Applications of acrylics include coatings, textiles, plastics, and plexiglass.

Synthesis of acrylic acid from ethylene oxide in the presence of the basic catalyst diethylamine. 
  H   H                         H   H               H   H         H   H
  |   |          HCN            |   |     - H2O     |   |   H2O   |   |
H-C - C-H  ---------------->  H-C - C-H  ------->   C = C  -----> C = C
   \ /       55 - 60 deg C      |   |   dehydration |   |         |   |
    O                          :O:  C               H   C         H   C=O
   : :                          |  |||                 |||            |
                                H   N                   N             OH




Last Update- July 8, 1995- wld