The Notes for
Polymer and Coatings Science-
Chapter Three

Synthesis of ethylene glycol- 
H   H              O              H   H
|   |   [O]       / \     H+      |   |
C = C -------> - C - C - -----> - C - C -  
|   |  silver            water    |   |
H   H catalyst                    OH  OH
Note that ethylene glycol is very toxic, and that household pets and possible small children are attracted to the pretty green color, and the sweet pleasant taste, leading to fatality.

 Synthesis of propylene glycol- propylene feedstock:
Propylene glycol is the most widely used glycol for the manufacture of linear unsaturated polyesters.

H   H                     O                      CH3
|   |      [O]           / \                     |
C = C   + ----->    C - C - C + H O  ---->  HO - C - CH - OH
|   |     catalyst               2               |     2
H   CH3                                          H
Propylene glycol is less toxic.

These glycols are high boiling materials, boiling at temperatures near 200 deg C. (I will look up the specific.)

Other diols may be selected to impart special properties to the polymer. Diethylene glycol leads to greater flexibility, at a cost of additional water sensitivity. 
     H   H       H   H
     |   |       |   |
HO - C - C - O - C - C - OH
     |   |       |   |
     H   H       H   H
Neopentylene glycol (2,2- dimethyl propane 1,3-diol) improves resistance to thermal degradation. 
     H   CH3 H
     |   |   |
HO - C - C - C - OH
     |   |   |
     H   CH3 H
Another diol, 2,2- bis (4-hydroxyphenyl) propane, is shown below:



 Synthesis of maleic anhydride- benzene feedstocks:



 Synthesis of terephthalic acid- xylene feedstocks:



Phthalic anhydride gives polyesters which are compatible with polystyrene.

Synthesis of phthallic anhydride- naphthalene feedstock


Synthesis of chlorendic acid- maleic anhydride feedstocks
Chlorendic acid (HET) imparts flame resistance to materials, and can be prepared by the Diels-Alder reaction of hexachloropentadiene with maleic anhydride.





 Subdivisions of polyesters - polyesters can be subdivided into the following categories: Linear Unsaturated polyesters are linear polyesters which contain aliphatic unsaturation which provides sites for crosslinking.

Linear unsaturated polyesters are prepared commercially by the reaction of a saturated diol with a mixture of unsaturated dibasic acid and a modifying dibasic acid (or the corresponding anhydride.)

Propylene glycol can be prepared from the hydration of propylene oxide: 
    H     O         H O        OH  H   OH
    |    / \         2         |   |   |        the product is isolated by
H - C - C - C - H  ----->  H - C - C - C - H    distillation under reduced 
    |   |   |                  |   |   |        pressure.
    H   H   H                  H   H   H
Propylene glycol is the preferred diol because it forms polyesters which are compatible with styrene and which show little tendency to crystallize, and also, it is readily available at low cost.

Linear unsaturated polyesters are prepared batch wise by heating a mixture of the appropriate acid and hydroxy components in a reactor that is jacketed for heating and cooling, and fitted for distillation.

Typical formulation 
propylene glycol      100 parts
maleic anhydride       72 parts
phthalic anhydride     54 parts
The reaction between a hydroxy-compound and an anhydride proceeds in two steps:
  1. esterification of the anhydride to form an ester and a free acid group 
           O                                  O
      //                                  ||
R1 - C                               R1 - C - O - C - C - OH
      \
||     O  +  HO - C - C - OH  ---->  || 
      /                            
R2 - C                               R2 - C - OH
      \\                                  ||
       O                                  O
  2. esterification of the remaining free acid groups 
                O
            ||
       R1 - C - O - C - C - OH
      
---->  ||

       R2 - C - O - C - C - OH
            ||
            O
A segment of the polyester obtained from propylene glycol, maleic anhydride and phthalic anhydride.

"a gif belongs here--located on 109"

Appreciable cis-trans isomerization generally occurs during the polyesterification of unsaturated dibasic acids and anhydrides. This is marked mostly with maleic anhydride, which becomes incorporated into the polymer chains mostly as fumarate groups.

The extent of isomerization is governed by:

The polymerization of maleic anhydride with propylene glycol gives almost entirely poly(propylene-co-fumarate), 70 to 90% fumarate groups, 10 to 30% maleic groups, , and crosslinking occurs readily.

Crosslinking involvess the reaction of unsaturated sites in the polymer chain with a vinyl type monomer.

High temperature (100 to 200 deg C) peroxides liberate free radicals as a result of thermal decomposition. Benzoyl peroxide is an example.

"some gifs should go here"

Such peroxides are used in processes employing notch metal moulds, which short burning times are required.

Room Temperature initiator systems normally consist of a peroxy compound and an activator (accelerator.)

An example, MEK peroxide and cyclohexanone peroxide, is shown below. I would guess both are initiators, but I don't know that for a fact.

"gifs should go here."

 Accelerators may be salts of metals which exhibit more than valency (huh?)

Naphthenic acid- extracted from the gas oil and kerosene fractions of petroleum and consists of a mixture of carboxlyic acids, substituted cyclopentanes and cyclohexanes.

"a gif needs to go here"

The decomposition of a hydroperoxide (ROOH) by a metal salt such as cobalt naphthenate to give free radicals proceeds according to the following chain reaction:

  +2                   *              +3
Co   + HOOH  ---->  R-O  +  OH-  +  Co

O                     O
||         +3         ||   *      +2      +
R - OH + Co    ---->  R - O  +  Co    +  H
Either one of the above radical species can go on and initiate the crosslinking reaction. The cycle is repeated until all the peroxide is decomposed.

      ++         +++       -
H O   +  Fe    -->  Fe     +  OH  
 2 2         

           ++         +++       --       -
S O   +  Fe    -->  Fe     +  SO    +  SO .
 2 8                            4        4
Unsaturated polyesters

ball park molecular weight: 2,500 g/mol

The crosslinking of an unsaturated linear polyester by means of vinyl monomer such as styrene which undergoes a chain reaction, incorporating the unsaturated polyester as a comonomer.

A trans unsaturated polyester is more stable than a cis unsaturated polyester, and the reaction therefore goes at a faster rate for the trans polyester.

Glass fiber may be used with unsaturated polyester to form a composite, and the physical properties of the final product depend greatly on the type and quantity of glass fiber used.

Unsaturated polyesters may have good heat stability up to 200 deg C.

Unsaturated polyesters may be attacked by chlorinated hydrocarbons such as Unsaturated polyesters may be crosslinked at room temperature by:
  1. radiation (for certain applications)
  2. classic addition monomers such as styrene, methylmethacrylate
  3. redox reactions


Saturated Polyesters

Low and higher molecular weight examples are produced.

Saturated polyesters can serve as plasticizers, or for the preparation of polyurethanes.

 Polyester plasticizers: Normally prepared by a diol-carboxylic acid condensation. Often a nonohydric alcohol or a monocarboxlyic acid is included in the reactants. From this, reactive end groups eliminated and the molecular weight can be controlled.

Ingredients for polyester plasticizers

Diol Dicarboxylic Acids Monohydric Alcohols Monocarboxylic Acids

Preparation of a saturated polyester- In a typical process, the mixture of reactants is heated at 200 to 250 deg C in an inert atmosphere for several hours with continuous removal of H2O.

Polyesters of molecular weight up to 1000 may be prepared to serve as plasticizers.

If higher molecular weight is desired, an excess of diol is used and the initial product is further heated at 200 to 250 deg C under reduced pressure (1 mm Hg.)

Ester interchange involving diol-terminated chains occurs and free diol is formed. 
     O                      O                   O       O
    ||                      ||                 ||       ||
HO - C - R - OH  + HO - R - C - OH  --->   HO - C - R - C - OH + HO - R - OH
the diol distills from the mixture as the diacid is formed.

Applications of saturated polyesters- plasticizers which are less volatile, and more resistant to solvent extraction than conventional monomeric plasticizers.

The preparation of saturated polyesters involves continuously heating, up to 200 deg C, the mixture of reactants in the presence of a catalyst. Water is removed from the reaction by azeotropic distillation. Xylenes may be used for an azeotropic process if the boiling temperature of xylenes, 140 deg C, is sufficient.

Polyethylene terephthalate (PET) can be prepared with molecular weights of 10,000 to 100,000. Note that these molecular weights are considered high. This contrasts with addition reactions, where molecular weights have to exceed 1,000,000 to be considered "high."

 Synthesis of ethylene glycol- ethylene oxide feedstock 
      O                         H   H
     / \                        |   |
H - C - C - H  + H O  --->  H - C - C - H
    |   |         2             |   |
    H   H                       OH  OH

ethylene oxide              ethylene glycol
Synthesis of dimethyl terephthalate- xylene feedstock 
                                               CH
                                               | 3
                         OH                    O
                         |                     |
    CH                   C=O                   C=O
    | 3                  |                     |
                 
 benzene    ---->     benzene     ---->     benzene

    |                    |                     |
    CH                   C=O                   C=O
      3                  |                     |
                         OH                    O
                                               |
                                               CH
                                                 3

xylene                                      dimethyl terephthalate
In an alternate process, the starting material is once again, p-xylene, but there are four steps instead of two:
  1. one methyl of the xylene is oxidized to carboxylic acid
  2. that carboxylic acid is methylated
  3. the other methyl is oxidized to carboxylic acid
  4. the carboxylic acid is methylated
Transesterification (also called ester exchange, ester interchange, or alcoholysis) may be used to convert an ester to another ester: 
  O                               O
  ||                              ||
- C - O - R1  + R2 - OH   --->  - C - O - R2 + R1 - OH
Synthesis of Poly(ethylene terephthalate) (PET)- from ethylene glycol and terephthalic acid feedstocks 
      H   H             O             O                     H   H       O             O
      |   |            ||             ||                    |   |      ||             ||      
2 H - C - C - H  + HO - C - benzene - C - OH  --->  -[- O - C - C - O - C - benzene - C -]-  + water
      |   |                                                 |   |
      OH  OH                                                OH  OH
In an alternate scheme, the diacid above is replaced with the corresponding dimethyl ester: 
  O                    O
  ||                   ||
- C - OH   becomes   - C - O - CH
                                 3
and methanol is given off instead of water.

The notes are unclear as to intent on page 112. The reactions show a stoichiometric imbalance of 2 moles diol to 1 mole diacid. I believe excess diol is used because the diol is more volatile than the diacid, and some is expected to be lost by evaporation during the reaction. However, 2:1 molar stoichiometry is excessive. Perhaps this is a good time to discuss the need for stoichiometric balance . The highest gains in molecular weight occur at the very end of the reaction. If you are reacting A and B, and there is more B than A, then in the end, your polymer chains will be capped with B units. An example is shown below.
High temperature of PET synthesis The bulk synthesis of PET is carried out at 270 deg C to 285 deg C, with continuous evacuations (suction removal of gas) to pressure below 1 mm Hg. The removal of methanol or water increases the molecular weight of the polymer. If the methanol or the water were left in the same system, they would cause a reverse reaction which would cause depolymerization.

The polymerization of PET can be carried out in the presence of a catalyst such as Sb, Ba, Ca, Cd, Co, Pb, Mn, Mg, Ti, and Zn. (I don't think you need them all--just pick one, maybe.) The polymerization is continued until the molecular weight is 20,000, then the molten polymer is extruded as a film. PET is used for 2 liter soda bottles.

 Properties of PET- colorless, rigid, easily crystallized.

For fiber production, the molten polymer is extruded through spinnerets into air at room temperature.

The filaments produced cool rapidly and the material is amorphous and weak.

The filaments are then heated above the glass transition temperature, and molecular orientation and crystallinity are induced (via the application of a tensile stress--stretching, to use the venacular.)

The temperature is raised to 200 C and the tension applied at that temperature leads to maximum possible crystallinity allowed by thermodynamic law. This insures the best possible dimensional stability.
To summarize the above: 1) extrusion, 2) orientation, 3) stabilization

Amorphous PET is transparent and has good impact strength (this helps when you drop your soda bottle.) Crystalline PET is probably opaque, or at least translucent. Crystalline PET has high strength, good creep resistance, and a high surface hardness.




Last Update- July 8, 1995- wld