Product Code Database
In polymer chemistry, branching is the regular or irregular attachment of to a polymer's backbone chain. It occurs by the replacement of a substituent (e.g. a hydrogen atom) on a monomer subunit by another covalent bond chain of that polymer; or, in the case of a graft copolymer, by a chain of another type. Branched polymers have more compact and symmetrical molecular conformations, and exhibit intra-heterogeneous dynamical behavior with respect to the unbranched polymers. In cross-link rubber by vulcanization, short sulfur branches link isoprene chains (or a synthetic variant) into a multiple-branched thermosetting elastomer. Rubber can also be so completely vulcanized that it becomes a rigid solid, so hard it can be used as the bit in a smoking pipe. Polycarbonate chains can be crosslinked to form the hardest, most impact-resistant thermosetting plastic, used in safety glasses.
Branching may result from the formation of carbon-carbon or various other types of chemical bond. Branching by ester and amide bonds is typically by a condensation reaction, producing one molecule of water (or HCl) for each bond formed.
Polymers which are branched but not crosslinked are generally thermoplastic. Branching sometimes occurs spontaneously during Polymerization; e.g., by free-radical polymerization of ethylene to form polyethylene. In fact, preventing branching to produce linear polyethylene requires special methods. Because of the way are formed, nylon would seem to be limited to unbranched, straight chains. But "star" branched nylon can be produced by the condensation of dicarboxylic acids with having three or more . Branching also occurs naturally during enzyme polymerization of glucose to form such as glycogen (), and amylopectin, a form of starch (). The unbranched form of starch is called amylose.
The ultimate in branching is a completely crosslinked network such as found in Bakelite, a phenol-formaldehyde thermoset resin.
The problem of branching occurs during propagation, when a chain curls back on itself and breaks - leaving irregular chains sprouting from the main carbon backbone. Branching makes the polymers less dense and results in low tensile strength and melting points. Developed by Karl Ziegler and Giulio Natta in the 1950s, Ziegler–Natta catalysts (triethylaluminium in the presence of a metal(IV) chloride) largely solved this problem. Instead of a free radical reaction, the initial ethene monomer inserts between the aluminium atom and one of the in the catalyst. The polymer is then able to grow out from the aluminium atom and results in almost totally unbranched chains. With the new catalysts, the tacticity of the polypropene chain, the alignment of alkyl groups, was also able to be controlled. Different metal chlorides allowed the selective production of each form i.e., syndiotactic, isotactic and atactic polymer chains could be selectively created.
However, there were further complications to be solved. If the Ziegler–Natta catalyst was poisoned or damaged then the chain stopped growing. Also, Ziegler–Natta monomers have to be small, and it was still impossible to control the molecular mass of the polymer chains. Again new catalysts, the , were developed to tackle these problems. Due to their structure they have less premature chain termination and branching.
|
|
