- Branching (chemistry)
Branching may result from the formation of
carbon-carbon or various other types of covalent bonds. Branching by esterand amidebonds is typically by a condensation reaction, producing one moleculeof water(or HCl) for each bond formed.
The ultimate in branching is a completely crosslinked network such as found in
Bakelite, a phenol- formaldehydethermoset resin.
pecial types of branched polymer
:* A graft polymer molecule is a branched polymer molecule in which one or more the side chains are different, structurally or configurationally, from the main chain.:* A star polymer molecule is a branched polymer molecule in which a single branch point gives rise to multiple linear chains or arms. If the arms are identical the star polymer molecule is said to be regular. If adjacent arms are composed of different repeating subunits, the star polymer molecule is said to be variegated.:* A comb polymer molecule consists of a main chain with two or more three-way branch points and linear side chains. If the arms are identical the comb polymer molecule is said to be regular.:* A brush polymer molecule consists of a main chain with linear, unbranched side chains and where one or more of the branch points has four-way functionality or larger.:* A polymer network is a
networkin which all polymer chains are interconnected to form a single macroscopic entity by many crosslinks [ [http://www.iupac.org/goldbook/N04112.pdf network (in polymer chemistry) ] ] . See for example thermosets.
Branching in radical polymerization
s are evident, because the intermolecular bonds are weaker and require less energy to break.
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 Zieglerand Giulio Nattain the 1950s, Ziegler-Natta catalysts ( triethylaluminiumin the presence of a metal(IV) chloride) largely solved this problem. Instead of a free radical reaction, the initial ethene monomer inserts between the aluminiumatom and one of the ethylgroups 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 tacticityof the polypropene chain, the alignment of alkylgroups, was also able to be controlled. Different metal chlorides allowed the selective production of each form i.e., syndiotactic, isotacticand atacticpolymer 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
metallocenes, were developed to tackle these problems. Due to their structure they have less premature chain termination and branching.
The branching index measures the effect of long-chain branches on the size of a macromolecule in solution. It is defined [http://www.iupac.org/goldbook/B00726.pdf] as g =
b2>/ l2>, where sb is the mean square radius of gyrationof the branched macromolecule in a given solvent, and sl is the mean square radius of gyration of an otherwise identical linear macromolecule in the same solvent at the same temperature. A value greater than 1 indicates an increased radius of gyration due to branching.
* [http://www.pslc.ws/macrog/pc.htm Polycarbonate]
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