Editing Polymer (section)

===Microstructure===
{{Main|Microstructure}}
The microstructure of a polymer (sometimes called configuration) relates to the physical arrangement of monomer residues along the backbone of the chain.<ref>Sperling, p. 30</ref> These are the elements of polymer structure that require the breaking of a covalent bond in order to change. Various polymer structures can be produced depending on the monomers and reaction conditions: A polymer may consist of linear macromolecules containing each only one unbranched chain. In the case of unbranched polyethylene, this chain is a long-chain ''n''-alkane. There are also branched macromolecules with a main chain and side chains, in the case of polyethylene the side chains would be [[alkyl groups]]. In particular unbranched macromolecules can be in the solid state semi-crystalline, crystalline chain sections highlighted red in the figure below.

While branched and unbranched polymers are usually thermoplastics, many [[elastomer]]s have a wide-meshed cross-linking between the "main chains". Close-meshed crosslinking, on the other hand, leads to [[thermosets]]. Cross-links and branches are shown as red dots in the figures. Highly branched polymers are amorphous and the molecules in the solid interact randomly.

:{| class="wikitable" style="text-align:center; font-size:90%;" width="60%"
|- class="hintergrundfarbe2"
|[[File:Polymerstruktur-linear.svg|130px]]<br /> Linear, unbranched macromolecule
|[[File:Polymerstruktur-verzweigt.svg|130px]]<br /> Branched macromolecule
|[[File:Polymerstruktur-teilkristallin.svg|150px]]<br />Semi-crystalline structure of an unbranched polymer
|[[File:Polymerstruktur-weitmaschig vernetzt.svg|130px]]<br /> Slightly [[cross-link]]ed polymer ([[elastomer]])
|[[File:Polymerstruktur-engmaschig vernetzt.svg|130px]]<br /> Highly cross-linked polymer ([[Thermosetting polymer|thermoset]])
|}

====Polymer architecture{{Anchor|Intermolecular forces}}====
{{Main|Polymer architecture}}
[[File:Polymer Branch.svg|thumb|right|upright=0.9|Branch point in a polymer]]

An important microstructural feature of a polymer is its architecture and shape, which relates to the way branch points lead to a deviation from a simple linear chain.<ref name="PP6">{{cite book |last1= Rubinstein |first1= Michael |last2= Colby |first2= Ralph H. |title= Polymer physics |url= https://archive.org/details/polymerphysics00rubi_825 |url-access= limited |year= 2003 |publisher= Oxford University Press |location= Oxford; New York |isbn= 978-0-19-852059-7 |page= [https://archive.org/details/polymerphysics00rubi_825/page/n14 6]}}</ref> A [[branching (polymer chemistry)|branched polymer]] molecule is composed of a main chain with one or more substituent side chains or branches. Types of branched polymers include [[star polymer]]s, [[comb polymers]], [[polymer brush]]es, [[dendronized polymer]]s, [[ladder polymer]]s, and [[dendrimer]]s.<ref name="PP6"/> There exist also [[two-dimensional polymer]]s (2DP) which are composed of topologically planar repeat units. A polymer's architecture affects many of its physical properties including solution viscosity, melt viscosity, solubility in various solvents, [[glass transition|glass-transition]] temperature and the size of individual polymer coils in solution. A variety of techniques may be employed for the synthesis of a polymeric material with a range of architectures, for example [[living polymerization]].

====Chain length====
A common means of expressing the length of a chain is the [[degree of polymerization]], which quantifies the number of monomers incorporated into the chain.<ref>McCrum, p. 30</ref><ref name="PP33">Rubinstein, p. 3</ref> As with other molecules, a polymer's size may also be expressed in terms of [[molecular weight]]. Since synthetic polymerization techniques typically yield a statistical distribution of chain lengths, the molecular weight is expressed in terms of weighted averages. The [[number-average molecular weight]] (''M''<sub>n</sub>) and [[weight-average molecular weight]] (''M''<sub>w</sub>) are most commonly reported.<ref>McCrum, p. 33</ref><ref name="PP233">Rubinstein, pp. 23–24</ref> The ratio of these two values (''M''<sub>w</sub> / ''M''<sub>n</sub>) is the [[dispersity]] (''Đ''), which is commonly used to express the width of the molecular weight distribution.<ref>Painter, p. 22</ref>

The physical properties<ref>{{cite book |last1= De Gennes |first1= Pierre Gilles |title= Scaling concepts in polymer physics |year= 1979 |publisher= Cornell University Press |location= Ithaca, N.Y. |isbn= 978-0-8014-1203-5}}</ref> of polymer strongly depend on the length (or equivalently, the molecular weight) of the polymer chain.<ref name="PP5">Rubinstein, p. 5</ref> One important example of the physical consequences of the molecular weight is the scaling of the [[viscosity]] (resistance to flow) in the melt.<ref>McCrum, p. 37</ref> The influence of the weight-average molecular weight (<math>M_w</math>) on the melt viscosity (<math>\eta</math>) depends on whether the polymer is above or below the onset of [[reptation|entanglements]]. Below the entanglement molecular weight{{clarify|date=December 2018}}, <math>\eta \sim {M_w}^{1}</math>, whereas above the entanglement molecular weight, <math>\eta \sim {M_w}^{3.4}</math>. In the latter case, increasing the polymer chain length 10-fold would increase the viscosity over 1000 times.<ref>Introduction to Polymer Science and Chemistry: A Problem-Solving Approach By Manas Chanda</ref>{{page needed|date=December 2018}} Increasing chain length furthermore tends to decrease chain mobility, increase strength and toughness, and increase the glass-transition temperature (T<sub>g</sub>).<ref>{{cite journal |last1=O'Driscoll |first1=K. |last2=Amin Sanayei |first2=R. |date=July 1991 |title=Chain-length dependence of the glass transition temperature |journal=Macromolecules |volume=24 |issue=15 |pages=4479–4480 |doi= 10.1021/ma00015a038|bibcode=1991MaMol..24.4479O}}</ref> This is a result of the increase in chain interactions such as [[Van der Waals force|van der Waals attractions]] and [[reptation|entanglements]] that come with increased chain length.<ref>{{cite book|last1=Pokrovskii|first1=V. N.|year=2010|title=The Mesoscopic Theory of Polymer Dynamics|series=Springer Series in Chemical Physics|volume=95|doi=10.1007/978-90-481-2231-8|isbn=978-90-481-2230-1|bibcode=2010mtpd.book.....P|url=https://cds.cern.ch/record/1315698}}</ref><ref>{{cite journal|last1=Edwards|first1=S. F.|year=1967|title=The statistical mechanics of polymerized material|journal=Proceedings of the Physical Society|volume=92|issue=1|pages=9–16|bibcode=1967PPS....92....9E|doi=10.1088/0370-1328/92/1/303}}</ref> These interactions tend to fix the individual chains more strongly in position and resist deformations and matrix breakup, both at higher stresses and higher temperatures.

====Monomer arrangement in copolymers====
{{Main|Copolymer}}

Copolymers are classified either as statistical copolymers, alternating copolymers, block copolymers, graft copolymers or gradient copolymers. In the schematic figure below, <span style="color:#F46C2C">Ⓐ</span> and <span style="color:#00AAC5">Ⓑ</span> symbolize the two [[repeat unit]]s.

:{| class="wikitable" style="text-align:center; font-size:90%;"
|- class="hintergrundfarbe2"
| [[File:Statistical copolymer 3D.svg|270px|Statistisches Copolymer]]<br />Random copolymer
| [[File:Gradient copolymer 3D.svg|270px|Gradientcopolymer]]<br />Gradient copolymer
| rowspan="2" | [[File:Graft copolymer 3D.svg|270px|Pfropfcopolymer]]<br /> [[Graft copolymer]]
|- class="hintergrundfarbe2"
| [[File:Alternating copolymer 3D.svg|270px|Alternierendes Copolymer]]<br /> Alternating copolymer
| [[File:Block copolymer 3D.svg|250px|Blockcopolymer]]<br /> [[Block copolymer]]
|}

*'''Alternating copolymers''' possess two regularly alternating monomer residues:<ref name="PC14">Painter, p. 14</ref> {{chem|(AB)|n}}. An example is the equimolar copolymer of [[styrene]] and [[maleic anhydride]] formed by free-radical chain-growth polymerization.<ref name=Rudin18>Rudin, p. 18–20</ref> A step-growth copolymer such as [[Nylon 66]] can also be considered a strictly alternating copolymer of diamine and diacid residues, but is often described as a homopolymer with the dimeric residue of one amine and one acid as a repeat unit.<ref name=Cowie104>Cowie, p. 104</ref>
*'''Periodic copolymers''' have more than two species of monomer units in a regular sequence.<ref>{{cite journal |title=Periodic copolymer |url=https://goldbook.iupac.org/terms/view/P04494 |website=IUPAC Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). |year=2014 |publisher=International Union of Pure and Applied Chemistry |doi=10.1351/goldbook.P04494 |access-date=9 April 2020|doi-access=free }}</ref>
*'''Statistical copolymers''' have monomer residues arranged according to a statistical rule. A statistical copolymer in which the probability of finding a particular type of monomer residue at a particular point in the chain is independent of the types of surrounding monomer residue may be referred to as a truly '''random copolymer'''.<ref name="PC15">Painter, p. 15</ref><ref>Sperling, p. 47</ref> For example, the chain-growth copolymer of [[vinyl chloride]] and [[vinyl acetate]] is random.<ref name=Rudin18/>
*'''Block copolymers''' have long sequences of different monomer units.<ref name=Rudin18/><ref name=Cowie104/> Polymers with two or three blocks of two distinct chemical species (e.g., A and B) are called diblock copolymers and triblock copolymers, respectively. Polymers with three blocks, each of a different chemical species (e.g., A, B, and C) are termed triblock terpolymers.
*'''Graft or grafted copolymers''' contain side chains or branches whose repeat units have a different composition or configuration than the main chain.<ref name=Cowie104/> The branches are added on to a preformed main chain macromolecule.<ref name=Rudin18/>

Monomers within a copolymer may be organized along the backbone in a variety of ways. A copolymer containing a controlled arrangement of monomers is called a [[sequence-controlled polymer]].<ref>{{cite journal|last1=Lutz|first1=Jean-François|last2=Ouchi|first2=Makoto|last3=Liu|first3=David R.|last4=Sawamoto|first4=Mitsuo|date=9 August 2013|title=Sequence-Controlled Polymers|journal=Science|language=en|volume=341|issue=6146|pages=1238149|doi=10.1126/science.1238149|issn=0036-8075|pmid=23929982|s2cid=206549042}}</ref> Alternating, periodic and block copolymers are simple examples of [[sequence-controlled polymer]]s.

====Tacticity====
{{Main|Tacticity}}
Tacticity describes the relative [[stereochemistry]] of [[chirality (chemistry)|chiral]] centers in neighboring structural units within a macromolecule. There are three types of tacticity: [[isotactic]] (all substituents on the same side), [[atactic]] (random placement of substituents), and [[syndiotactic]] (alternating placement of substituents).

:{| class="wikitable" style="text-align:center; font-size:90%;" width="60%"
|- class="hintergrundfarbe2"
|[[File:Isotactic-A-2D-skeletal.png|240px]]<br />Isotactic
|[[File:Syndiotactic-2D-skeletal.png|280px]]<br /> Syndiotactic
|[[File:Atactic-2D-skeletal.png|240px]]<br /> Atactic (i. e. random)
|}