Branching (polymer chemistry)

When the graft is a technique for the representation of polymers whose backbone forms a starting point for further chains of another type of monomer. Thus, a copolymer, comb connect to the main chain to chains of another type of monomer. This provides a further opportunity to develop new plastics with defined properties.

Schematic representation of:

(-A -A- A-A -A- A-A -A -A- A-)                  | |                  B B                  | |                  B B                  | |                  B B                  | |

Synthesis

Three methods are used mainly:

Grafting Onto

In the most commonly used method, an existing polymer is exposed to gamma radiation, the latter generates along the backbone free radical sites which can then form the basis for a further free-radical polymerization.

In this way, the very impact-resistant rubber HIPS ( " high-impact " polystyrene ) is produced by polymerization of styrene on a Polybutadienseitenkette example.

In another method, a pre-existing polymer to be added and the monomer in a non-polar solvent (eg, dichloromethane ) are dissolved and treated with a Lewis acid ( coinitiator ). The Lewis acid is then cut off the polymer at several points electrons. The - resulting - positively charged sites then provide the starting point for the cationic polymerization with the monomer dar.

Grafting from

This technique consists on the main chain already existing resources, which can be attacked by a growing chain. In an anionic polymerization effected can be the example electrophilic groups, as introduced by phosgene carbonyl or nitrile groups.

Via macromonomers

In this method, an existing polymer is functionalized subsequently along the side chain. An example is the reaction of a polymer with OH groups along the backbone, can be reacted with polymers having terminal carboxylic acid chloride groups to the respective esters. One advantage is that in contrast to the above two methods, both the points of attack for the newly growing chains, as well as the length of the side chains may be better defined in advance. The distance between two side chains is also known as a " spacer". Disadvantage is the higher technical and financial effort. In this way, for example, be prepared as comb polymers.

The graft polymerization is subject to the same laws as the copolymerization.

Swell

• JMG Cowie: Chemistry and Physics of synthetic polymers. London, 1997, ISBN 3-540-67052-1