Editing Copolymer (section)

===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>