Editing Copolymer (section)

==Linear copolymers==

===Block copolymers===

Block copolymers comprise two or more [[Polymer#Monomers and repeat units|homopolymer]] subunits linked by covalent bonds. The union of the homopolymer subunits may require an intermediate non-repeating subunit, known as a '''junction block'''. '''Diblock copolymers''' have two distinct blocks; '''triblock copolymers''' have three. Technically, a block is a portion of a macromolecule, comprising many units, that has at least one feature which is not present in the adjacent portions.<ref name=goldbook1996/> A possible sequence of repeat units A and B in a triblock copolymer might be ~A-A-A-A-A-A-A-B-B-B-B-B-B-B-A-A-A-A-A~.<ref>Cowie, p.4</ref>

 {{Quote box|width = 35%
 |title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
 |quote = '''block copolymer''': A [https://doi.org/10.1351/goldbook.C01335 copolymer] that is a [https://doi.org/10.1351/goldbook.B00685 block polymer]. In the [https://doi.org/10.1351/goldbook.C01281 constituent] macromolecules of a [https://doi.org/10.1351/goldbook.B00682 block] [https://doi.org/10.1351/goldbook.C01335 copolymer], adjacent blocks are constitutionally different, i.e. adjacent blocks comprise [https://doi.org/10.1351/goldbook.C01288 constitutional unit] derived from different species of [https://doi.org/10.1351/goldbook.M04017 monomer] or from the same species of [https://doi.org/10.1351/goldbook.M04017 monomer] but with a different composition or [https://doi.org/10.1351/goldbook.ST06775 sequence] distribution of constitutional units. 
<ref name='Gold Book "block copolymer"'>{{cite web |title=block copolymer |url=https://goldbook.iupac.org/terms/view/B00683 |website=Gold Book |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book B00683 |doi=10.1351/goldbook.B00683}}</ref>
}}

Block copolymers are made up of blocks of different [[polymerized]] [[monomers]]. For example, polystyrene-b-poly(methyl methacrylate) or PS-b-PMMA (where b = block) is usually made by first polymerizing [[styrene]], and then subsequently polymerizing [[methyl methacrylate]] (MMA) from the reactive end of the polystyrene chains. This polymer is a "diblock copolymer" because it contains two different chemical blocks. Triblocks, tetrablocks, multiblocks, etc. can also be made. Diblock copolymers are made using [[living polymerization]] techniques, such as atom transfer free radical polymerization ([[ATRP (chemistry)|ATRP]]), reversible addition fragmentation chain transfer ([[RAFT (chemistry)|RAFT]]), [[ring-opening metathesis polymerization]] (ROMP), and living cationic or living anionic [[living polymerization|polymerization]]s.<ref>Hadjichristidis N., Pispas S., Floudas G. Block copolymers: synthetic strategies, physical properties, and applications – Wiley, 2003.</ref> An emerging technique is [[chain shuttling polymerization]].

The synthesis of block copolymers requires that both reactivity ratios are much larger than unity (r<sub>1</sub> >> 1, r<sub>2</sub> >> 1) under the reaction conditions, so that the terminal monomer unit of a growing chain tends to add a similar unit most of the time.<ref name=Fried>{{cite book |last=Fried |first=Joel R. |date=2003 |edition=2nd |title=Polymer Science and Technology |publisher=Prentice Hall |pages=41–43 |isbn=978-0-13-018168-8}}</ref>

The "'''blockiness'''" of a copolymer is a measure of the adjacency of comonomers vs their statistical distribution.  Many or even most synthetic polymers are in fact copolymers, containing about 1-20% of a minority monomer.  In such cases, blockiness is undesirable.<ref name=Chum>{{cite journal | last1 = Chum | first1 = P. S. | last2 = Swogger | first2 = K. W. | year = 2008 | title = Olefin Polymer Technologies-History and Recent Progress at the Dow Chemical Company | journal = Progress in Polymer Science | volume = 33 | issue = 8 | pages = 797–819 | doi = 10.1016/j.progpolymsci.2008.05.003 }}</ref> A ''block index'' has been proposed as a quantitative measure of blockiness or deviation from random monomer composition.<ref>{{cite journal |last1=Shan |first1=Colin Li Pi |last2=Hazlitt |first2=Lonnie G. |date=2007 |title=Block Index for Characterizing Olefin Block Copolymers |journal=Macromol. Symp. |volume=257 |pages=80–93 |citeseerx=10.1.1.424.4699 |doi=10.1002/masy.200751107 }}</ref>

===Alternating copolymers===
 {{Quote box|width = 35%
 |title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
 |quote = '''alternating copolymer''': A [https://doi.org/10.1351/goldbook.C01335 copolymer] consisting of [https://doi.org/10.1351/goldbook.M03667 macromolecule] comprising two species of [https://doi.org/10.1351/goldbook.M04018 monomeric unit] in alternating [https://doi.org/10.1351/goldbook.ST06775 sequence]. (See Gold Book entry for note.)
<ref name='Gold Book "alternating copolymer"'>{{cite web |title=alternating copolymer |url=https://goldbook.iupac.org/terms/view/A00250 |website=Gold Book |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book A00250 |doi=10.1351/goldbook.A00250}}</ref>
}}
An alternating copolymer has regular alternating A and B units, and is often described by the formula: -A-B-A-B-A-B-A-B-A-B-, or -(-A-B-)<sub>n</sub>-. The molar ratio of each monomer in the polymer is normally close to one, which happens when the reactivity ratios r<sub>1</sub> and r<sub>2</sub> are close to zero, as can be seen from the Mayo–Lewis equation. For example, in the free-radical copolymerization of [[styrene maleic anhydride]] copolymer, r<sub>1</sub> = 0.097 and r<sub>2</sub> = 0.001,<ref name=Fried/> so that most chains ending in styrene add a maleic anhydride unit, and almost all chains ending in maleic anhydride add a styrene unit. This leads to a predominantly alternating structure.

A step-growth copolymer -(-A-A-B-B-)<sub>n</sub>- formed by the [[condensation reaction|condensation]] of two [[bifunctional]] monomers A–A and B–B is in principle a perfectly alternating copolymer of these two monomers, but is usually considered as a [[Polymer#Monomers and repeat units|homopolymer]] of the dimeric repeat unit A-A-B-B.<ref name=Cowie>{{cite book |last=Cowie |first=J.M.G. |date=1991 |edition=2nd |title=Polymers: Chemistry and Physics of Modern Materials |publisher=Blackie (USA: Chapman and Hall) |pages=[https://archive.org/details/polymerschemistr0000cowi/page/104 104–106] |isbn=978-0-216-92980-7 |url=https://archive.org/details/polymerschemistr0000cowi/page/104 }}</ref> An example is [[nylon 66]] with repeat unit -OC-( CH<sub>2</sub>)<sub>4</sub>-CO-NH-(CH<sub>2</sub>)<sub>6</sub>-NH-, formed from a [[dicarboxylic acid]] monomer and a [[diamine]] monomer.

===Periodic copolymers===

Periodic copolymers have units arranged in a repeating sequence. For two monomers A and B, for example, they might form the repeated pattern (A-B-A-B-B-A-A-A-A-B-B-B)<sub>n</sub>.

===Statistical copolymers===
 {{Quote box|width = 35%
 |title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
 |quote = '''statistical copolymer''': A [https://doi.org/10.1351/goldbook.C01335 copolymer] consisting of [https://doi.org/10.1351/goldbook.M03667 macromolecule] in which the sequential distribution of the [https://doi.org/10.1351/goldbook.M04018 monomeric unit] obeys known statistical laws. (See Gold Book entry for note.)
<ref name='Gold Book "statistical copolymer"'>{{cite web |title=statistical copolymer |url=https://goldbook.iupac.org/terms/view/S05955 |website=Gold Book |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book S05955 |doi=10.1351/goldbook.S05955}}</ref>
}}
In statistical copolymers the sequence of monomer residues follows a statistical rule. If the probability of finding a given type monomer residue at a particular point in the chain is equal to the mole fraction of that monomer residue in the chain, then the polymer may be referred to as a truly '''random copolymer'''<ref name="PC14">Painter P. C. and Coleman M. M., ''Fundamentals of Polymer Science'', CRC Press, 1997, p 14.</ref> (structure 3).

Statistical copolymers are dictated by the reaction kinetics of the two chemically distinct monomer reactants, and are commonly referred to interchangeably as "random" in the polymer literature.<ref name="Chanda">Chanda, M. ''Introduction to Polymer Science and Chemistry''. Second Edition. CRC Press, 2013.</ref> As with other types of copolymers, random copolymers can have interesting and commercially desirable properties that blend those of the individual homopolymers. Examples of commercially relevant random copolymers include [[rubber]]s made from styrene-butadiene copolymers and resins from styrene-acrylic or [[methacrylic acid]] derivatives.<ref>Overberger, C. ″Copolymerization: 1. General Remarks; 2: Selective Examples of Copolymerizations″. ''Journal of Polymer Science: Polymer Symposium'' 72, 67-69 (1985).</ref>  Copolymerization is particularly useful in tuning the [[glass transition]] temperature, which is important in the operating conditions of polymers; it is assumed that each monomer occupies the same amount of free volume whether it is in a copolymer or homopolymer, so the [[glass transition]] temperature (T<sub>g</sub>) falls between the values for each homopolymer and is dictated by the mole or mass fraction of each component.<ref name="Chanda" />

A number of parameters are relevant in the composition of the polymer product; namely, one must consider the reactivity ratio of each component. Reactivity ratios describe whether the monomer reacts preferentially with a segment of the same type or of the other type. For example, a reactivity ratio that is less than one for component 1 indicates that this component reacts with the other type of monomer more readily. Given this information, which is available for a multitude of monomer combinations in the "Wiley Database of Polymer Properties",<ref>Greenley, Robert. ″Free Radical Copolymerization Reactivity Ratios″. ''The Wiley Database of Polymer Properties''. 2003. {{doi|10.1002/0471532053.bra007}}</ref> the [[Mayo-Lewis equation]] can be used to predict the composition of the polymer product for all initial mole fractions of monomer. This equation is derived using the [[Markov model]], which only considers the last segment added as affecting the kinetics of the next addition; the Penultimate Model considers the second-to-last segment as well, but is more complicated than is required for most systems.<ref>{{Cite journal | doi = 10.1021/ma971043b| pmid = 9680398| title = Ethene−Norbornene Copolymerization with Homogeneous Metallocene and Half-Sandwich Catalysts: Kinetics and Relationships between Catalyst Structure and Polymer Structure. 3. Copolymerization Parameters and Copolymerization Diagrams| journal = Macromolecules| volume = 31| issue = 15| pages = 4681–3| year = 1998| last1 = Ruchatz| first1 = Dieter| last2 = Fink| first2 = Gerhard| bibcode = 1998MaMol..31.4681R}}</ref> When both reactivity ratios are less than one, there is an azeotropic point in the Mayo-Lewis plot. At this point, the mole fraction of monomer equals the composition of the component in the polymer.<ref name="Chanda" />

There are several ways to synthesize random copolymers. The most common synthesis method is [[free radical polymerization]]; this is especially useful when the desired properties rely on the composition of the copolymer rather than the molecular weight, since free radical polymerization produces relatively disperse polymer chains. Free radical polymerization is less expensive than other methods, and produces high-molecular weight polymer quickly.<ref>Cao, Ti and Stephen E. Webber. ″Free-Radical Copolymerization of Fullerenes with Styrene″. ''Macromolecules'', 1996, 28, pp 3741-3743.</ref>  Several methods offer better control over [[dispersity]]. [[Anionic polymerization]] can be used to create random copolymers, but with several caveats: if [[carbanion]]s of the two components do not have the same stability, only one of the species will add to the other. Additionally, anionic polymerization is expensive and requires very clean reaction conditions, and is therefore difficult to implement on a large scale.<ref name="Chanda" /> Less disperse random copolymers are also synthesized by ″living″ [[controlled radical polymerization]] methods, such as [[atom-transfer radical-polymerization]] (ATRP), [[nitroxide mediated radical polymerization]] (NMP), or [[reversible addition−fragmentation chain-transfer polymerization]] (RAFT). These methods are favored over anionic polymerization because they can be performed in conditions similar to free radical polymerization. The reactions require longer experimentation periods than free radical polymerization, but still achieve reasonable reaction rates.<ref>{{Cite journal | doi = 10.1016/S1359-0286(96)80101-X| title = Controlled radical polymerization| journal = Current Opinion in Solid State and Materials Science| volume = 1| issue = 6| pages = 769–776| year = 1996| last1 = Matyjaszewski| first1 = Krzysztof| bibcode = 1996COSSM...1..769M}}</ref>

===Stereoblock copolymers===

[[File:Stereobl.png|thumb|right|A stereoblock vinyl copolymer]]

In stereoblock copolymers the blocks or units differ only in the [[tacticity]] of the monomers.

===Gradient copolymers===

In gradient copolymers the monomer composition changes gradually along the chain.