Polymers are made up of monomers. The monomers may be always the same or different.

monomers may be always the same or different

Polymers may be naturally occuring or synthetic.
The nature of the chemical bonds is the same.

type of bonding examples  examples
  natural synthetic
carbon-carbon polyolefins
(e.g. rubber)
polyolefins
(e.g. polyethylene)
ester nucleic acids
(e.g. DNA, RNA)
polyesters
(e.g. Diolen)
amide polypeptides
(e.g. wool, silk; enzymes)
polyamides
(e.g. Nylon)
ether polysaccharides
(e.g. starch, cellulose)
special plastics
(e.g. Delrin)

Naturally occurring polymers

Rubber is made from latex, the "milk" of hevea brasiliensis, by curing ("vulcanizing") it using sulphur or . "Curing" means cross linking the polymer chains to a certain extent.

Stretching a rubber band means stretching its molecular chains.

There are also synthetic substances with rubberlike properties (e.g. "Buna"). Usually, these are polymers of butadiene , while natural rubber is a polymer of isoprene, .

There are two different polymers of butadiene; the way the chain proceeds after the double bond is of extreme importance.

Starch and cellulose are polymers of the sugar, glucose.

Glucose forms several natural polymers. It is extremely important if the alpha- or beta-form is polymerized.

Peptides and other natural poly(amino acids) are polymers with a homogeneous backbone and different side chains. In nature, these are of extreme importance. For example, one flaw may lead to an incurable deadly disease.

Look for examples of amino acids.


"Semi-synthetic" polymers
(chemically treated polymers of natural origin) are quite common and of great practical importance.

Cellulose, for example, is used in two different ways:



Synthetic polymers

Methods of synthesis
Basically there are three chemical principles to form polymers:


Formation of polymers
 [Full screen 47 KB]
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Neue Werkstoffe")



Polymers are chain molecules
chain molecules
 [Full screen 52 KB]
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Neue Werkstoffe")


Molecular architecture

Array of the
monomers
Chain structure

 [Full screen 42 KB]
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Neue Werkstoffe")


Superstructures and dimensions of polymers
 [Full screen 60 KB]
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Neue Werkstoffe")


Important examples
of monomers used in the production of plastics

acrylic acid
acrylonitrile
butadiene
ethene, or ethylene
methacrylic acid
propene, or propylene
styrene
tetrafluoroethene
vinyl acetate, or acetic acid vinyl ester
vinyl alcohol (not stable)
vinyl chloride
etc., etc., etc.  

The number of monomers forming a polymer chain (the "degree of polymerisation") may vary from rather small (e.g., 10) to very high (e.g., 100,000). Substances with small monomer chains are often referred to as oligomers or prepolymers.

Even within a certian batch of a polymer, the chains are not all the same size. Two different quantities are used to express the average molecular weight:

  • the number-average, Mn
  • the weigth average, Mw
  • When the viscosity of a solution of a polymer is employed to measure the average molecular weight, something in between results: the viscosity average, Mv.



    Technical division of polymers

    From a more practical point of view, there are

    Thermoplastic polymers
    At a certain temperature, the melting temperature , the polymer becomes more or less a liquid, can be given a new physical shape, and will remain so after cooling. This procedure may be repeated as often as you like. Thermoplastic substances usually are soluble in some suitable solvent. As a rule, they are flexible or hornlike; below another transition temperature, the glass temperature , they are hard and brittle. Thermoplastic polymers consist of linear or branched chains of monomers, but these are not cross-linked.

        thermoplastic polymers    

    Elastomers
    Elastomers are made up of slightly cross-linked chains. So they are no more soluble, but swell in appropriate solvents. At their temperature of use, they resemble rubber; they are very flexible and may be extended several times the original size. Elastomers also have a glass temperature.

     elastomer

    Thermosetting polymers, or duromers
    Thermosetting polymers consist of highly cross-linked chains and hence usually are rigid and brittle. They must be given their shape before their final chemical structure is produced, which can only be altered mechanically. They cannot be molten, but will decompose at high temperatures; they are entirely insoluble and will not swell when treated with solvents.

     thermosetting polymer


    Crystallized polymers

    Polymers may be crystallized to a certain extent. The small crystals present are called crystallites.

    possible structures of thermoplastic substances

    Different types of polyethylene.

    Low-density polyethylene is made at elevated temperatures by radical polymerisation, while the more regular high-density type is prepared using Ziegler-Natta catalysts.

    The more and the larger the crystallites are, the higher the glass and the melting temperatures of the polymer will be. If desired, the glass temperature of the polymer can be lowered by employing a placticizer.

    Effect of the plasticizer, DOP, on polyvinyl chloride

    Plasticizers are high-boiling solvents, preferably phthalic acid esters. They alter (diminsh) the size and amount of crystallites formed; roughly speaking, they can be compared to grease.


    Practical examples

    The Cromalin™ process

    Preparing offset litho printing plates

    Letterpress formes for newspaper printing




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