View source for Branching (polymer chemistry)

{{short description|Attachment of side chains to the backbone chain of a polymer}}
{{More citations needed|date=August 2013}}
{{Quote box|width = 35%
 |title = [[International Union of Pure and Applied Chemistry|IUPAC]] definitions
 |quote = '''branched chain''': A chain with at least one branch point intermediate between the boundary units. 
<ref name='Gold Book "branched chain"'>{{cite web |title=branched chain |url=https://goldbook.iupac.org/terms/view/B00721 |website=Gold Book |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book B00721 |doi=10.1351/goldbook.B00721}}</ref><br/>
'''branch (side chain, pendant chain)''': An oligomer molecule or polymeric offshoot from a macromolecular chain. (See Gold Book entry for note.)
<ref name='Gold Book "branch (side chain, pendant chain)"'>{{cite web |title=branch (side chain, pendant chain) |url=https://goldbook.iupac.org/terms/view/B00720 |website=Gold Book |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book B00720 |doi=10.1351/goldbook.B00720}}</ref>
}}
[[Image:Polymer Branch.svg|thumb|right|200px|Branch point in a polymer]]
[[Image:Glycogen.svg|thumb|right|200px|[[Glycogen]], a branched [[polysaccharide]]]]

In [[polymer chemistry]], '''branching''' is the regular or irregular attachment of [[side chain]]s 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|covalently-bonded]] 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 conformation]]s, and exhibit intra-heterogeneous dynamical behavior with respect to the unbranched polymers.<ref name="article0">{{cite journal|author1=Alexandros Chremos|author2=Jack F. Douglas|title=When does a branched polymer become a particle?|date=2015|journal=J. Chem. Phys.|volume=143|issue=11|pages=111104|doi=10.1063/1.4931483|bibcode=2015JChPh.143k1104C|pmid=26395679|doi-access=free}}</ref><ref name="article-1">{{cite journal|author1=Alexandros Chremos|author2=E. Glynos|author3=P. F. Green |title=Structure and dynamical intra-molecular heterogeneity of star polymer melts above glass transition temperature|journal=Journal of Chemical Physics|date=2015|volume=142|issue=4|pages=044901|doi=10.1063/1.4906085|bibcode=2015JChPh.142d4901C|pmid=25638003}}</ref> In [[cross-link|crosslinking]] [[rubber]] by [[vulcanization]], short [[sulfur]] branches link [[isoprene|polyisoprene]] chains (or a [[chemical synthesis|synthetic]] variant) into a multiple-branched [[Thermosetting polymer|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 (tobacco)|smoking pipe]]. [[Polycarbonate]] chains can be crosslinked to form the hardest, most impact-resistant thermosetting [[plastic]], used in safety [[glasses]].<ref>{{cite web | url=http://www.pslc.ws/macrog/pc.htm | title=Polycarbonate|publisher=Pslc.ws | accessdate=October 22, 2012}}</ref>

Branching may result from the formation of [[carbon]]-carbon or various other types of [[chemical bond|covalent bonds]]. Branching by [[ester]] and [[amide]] bonds is typically by a [[condensation (chemistry)|condensation]] reaction, producing one [[molecule]] of [[water]] (or [[hydrogen chloride|HCl]]) for each bond formed.

Polymers which are branched but not crosslinked are generally [[thermoplastic]]. Branching sometimes occurs spontaneously during [[Polymerization|synthesis of polymers]]; e.g., by [[radical polymerization|free-radical polymerization]] of [[ethylene]] to form [[polyethylene]]. In fact, preventing branching to produce linear polyethylene requires special methods. Because of the way [[polyamide]]s are formed, [[nylon]] would seem to be limited to unbranched, straight chains. But "star" branched nylon can be produced by the condensation of [[dicarboxylic acid]]s with [[polyamine]]s having three or more [[amino group]]s. Branching also occurs naturally during [[enzyme|enzymatically-catalyzed]] [[polymerization]] of [[glucose]] to form [[polysaccharide]]s such as [[glycogen]] ([[animal]]s), and [[amylopectin]], a form of [[starch]] ([[plant]]s). The unbranched form of starch is called [[amylose]].

The ultimate in branching is a completely crosslinked [[Graph (discrete mathematics)|network]] such as found in [[Bakelite]], a [[phenol]]-[[formaldehyde]] thermoset resin.
{{-}}

==Special types of branched polymer==
[[File:DendrimerNewkome1985.png|thumb|Dendrimer synthesis first generation Newkome 1985]]
:* A '''graft polymer''' molecule is a branched polymer molecule in which one or more of the side chains are different, structurally or configurationally, from the main chain.
:* A '''[[star-shaped 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 '''[[Polymer brush|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 [[wikt:network|network]] in which all polymer chains are interconnected to form a single macroscopic entity by many [[crosslink]]s.<ref>{{cite web|url=https://goldbook.iupac.org/terms/view/N04112 |title=Network (in polymer chemistry)|publisher=Iupac.org|access-date=2013-08-17}}</ref> See for example [[thermoset]]s or [[interpenetrating polymer network]]s.
:* A '''[[dendrimer]]''' is a repetitively branched compound.

==In radical polymerization==
[[Image:1,3-Butadiene Polymerization.svg|thumb|Polymerization of [[1,3-butadiene]]]]
In [[radical polymerization|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 cannot line up as close to each other as unbranched chains can. This causes less contact between atoms of different chains, and fewer opportunities for [[Molecular dipole moment|induced or permanent dipoles]] to occur. A low density results from the chains being further apart. Lower melting points and [[tensile strength]]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 Ziegler]] and [[Giulio Natta]] in the 1950s, [[Ziegler–Natta catalyst]]s ([[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 group]]s 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 [[metallocene]]s, were developed to tackle these problems. Due to their structure they have less premature chain termination and branching.

==Branching index==
{{Quote box|width = 35%
 |title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
 |quote = '''Branching index''': A parameter, {{mvar|g}}, characterizing the effect of long-chain branches on the size of a branched macromolecule in solution and defined as the ratio of the mean-square radius of gyration of a branched molecule, {{math|&lang;''s''{{su|b=''b''|p=2}}&rang;}}, to that of an otherwise identical linear molecule, {{math|&lang;''s''{{su|b=''l''|p=2}}&rang;}}, with the same relative molecular mass in the same solvent and at the same temperature:<ref name='Gold Book "branching index"'>{{cite web |title=branching index |url=https://goldbook.iupac.org/terms/view/B00726 |website=Gold Book |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book B00726 |doi=10.1351/goldbook.B00726}}</ref><br>
:<math>g = \tfrac{\langle s_b^2 \rangle}{\langle s_l^2 \rangle }</math> 
}}

The branching index measures the effect of long-chain branches on the size of a macromolecule in solution. It is defined<ref>{{Cite web |last=IUPAC |first= |date=2008-10-07 |others=Compiled by A. D. McNaught and A. Wilkinson; Online version (2019-) created by S. J. Chalk. |title=Compendium of Chemical Terminology (the "Gold Book") |url=https://goldbook.iupac.org/terms/view/B00726 |access-date=2023-02-20 |website= |publisher=Blackwell Scientific Publications |place=Oxford |publication-date=1997 |doi=10.1351/goldbook.b00726 |isbn=0-9678550-9-8 |edition=2nd}}</ref> as <math display=block>g = \frac{\langle s_b^2 \rangle}{\langle s_l^2 \rangle }</math>
where {{mvar|s<sub>b</sub>}} is the mean square [[radius of gyration]] of the branched macromolecule in a given solvent, and {{mvar|s<sub>l</sub>}} 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.

==References==
{{Reflist}}

==External links==
*{{Commons category-inline|Branching (polymer chemistry)}}

{{DEFAULTSORT:Branching (Chemistry)}}
[[Category:Polymer chemistry]]
[[Category:Organic chemistry]]