Everything about Branching Chemistry totally explained
In
polymer chemistry,
branching occurs by the replacement of a
substituent, e.g, a
hydrogen atom, on a
monomer subunit, by another
covalently bonded chain of that
polymer; or, in the case of a
graft copolymer, by a
chain of another type. In
crosslinking rubber by
vulcanization, short
sulfur branches link
polyisoprene chains (or a
synthetic variant) into a multiply-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.
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Branching may result from the formation of
carbon-carbon or various other types of
covalent bonds. 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 synthesis of polymers; for example, by
free-radical polymerization of
ethylene to form
polyethylene. In fact, preventing branching to produce
linear polyethylene requires special methods. Because of the way
polyamides 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
polyamines having three or more
amino groups. Branching also occurs naturally during
enzymatically-catalyzed polymerization of
glucose to form
polysaccharides such as
glycogen (
animals), and
amylopectin, a form of
starch (
plants). 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.
Special 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
network in which all polymer chains are interconnected to form a single macroscopic entity by many
crosslinks . See for example
thermosets.
Branching in radical polymerization
In
Chemistry In
free-radical polymerization, branching occurs when a chain curls back and bonds to an earlier part of the chain. When this curl breaks, it leaves small chains sprouting from the main carbon backbone. Branched carbon chains can't line up as close to each other as unbranched chains can. This causes less contact between atoms of different chains, and fewer opportunities for
induced or permanent dipoles to occur. A low density results from the chains being further apart. Lower melting points and
tensile strengths 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 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
ethyl groups 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 for example,
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
metallocenes, were developed to tackle these problems. Due to their structure they've less premature chain termination and branching.
Branching index
The branching index measures the effect of long-chain branches on the size of a macromolecule in solution. It is definedas g =
b2>/l2>, where sb is the mean square radius of gyration of 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.
Further Information
Get more info on 'Branching Chemistry'.
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