Editing Polymerization (section)

==<span class="anchor" id="Step-growth v chain growth polymerization"></span>Step-growth vs. chain-growth polymerization==
Step-growth and chain-growth are the  main classes of polymerization reaction mechanisms.  The former is often easier to implement but requires precise control of stoichiometry.  The latter more reliably affords high molecular-weight polymers, but only applies to certain monomers.

[[File:Polymerization classification ENmod.png|thumb|A classification of the polymerization reactions|alt=|440x440px]]

===Step-growth===
{{Main|Step-growth polymerization}}

In step-growth (or step) polymerization, pairs of reactants, of any lengths, combine at each step to form a longer polymer molecule. The [[Molar mass distribution#Number average molar mass|average molar mass]] increases slowly. Long chains form only late in the reaction.<ref name=":0">{{Cite book |last1=Allcock |first1=H. R. |url= |title=Contemporary polymer chemistry. |last2=Lampe |first2=Frederick Walter |last3=Mark |first3=James E. |publisher=Pearson/Prentice Hall |others=Frederick Walter Lampe, James E. Mark |year=2003 |isbn=0-13-065056-0 |edition=3rd |location=Upper Saddle River, N.J. |pages=29–30 |oclc=51096012}}</ref><ref name=":1">{{Cite book |last=Fried |first=Joel R. |url= |title=Polymer science and technology |publisher=Prentice Hall Professional Technical Reference |year=2003 |isbn=0-13-018168-4 |edition=2nd |location=Upper Saddle River, NJ |pages=23 |oclc=51769096}}</ref>

Step-growth polymers are formed by independent reaction steps between functional groups of monomer units, usually containing [[heteroatoms]] such as nitrogen or oxygen. Most step-growth polymers are also classified as [[condensation polymer]]s, since a small molecule such as water is lost when the polymer chain is lengthened. For example, [[polyester]] chains grow by reaction of [[Alcohol (chemistry)|alcohol]] and [[carboxylic acid]] groups to form [[ester]] links with loss of water. However, there are exceptions; for example [[polyurethane]]s are step-growth polymers formed from [[isocyanate]] and alcohol bifunctional monomers) without loss of water or other volatile molecules, and are classified as [[addition polymer]]s rather than condensation polymers.

Step-growth polymers increase in molecular weight at a very slow rate at lower conversions and reach moderately high molecular weights only at very high conversion (i.e., >95%).  '''Solid state polymerization''' to afford polyamides (e.g., nylons) is an example of step-growth polymerization.<ref name=ullmannC2>{{Ullmann |doi=10.1002/14356007.a21_487.pub3|title=Polyethylene|year=2014|last1=Jeremic|first1=Dusan|pages=1–42|isbn=9783527306732}}</ref>

===Chain-growth===
{{Main|Chain-growth polymerization}}
In chain-growth (or chain) polymerization, the only chain-extension reaction step is the addition of a monomer to a growing chain with an active center such as a [[free radical]], [[cation]], or [[anion]]. Once the growth of a chain is initiated by formation of an active center, chain propagation is usually rapid by addition of a sequence of monomers. Long chains are formed from the beginning of the reaction.<ref name=":0" /><ref name=":1" />

Chain-growth polymerization (or addition polymerization) involves the linking together of unsaturated monomers, especially containing carbon-carbon [[double bonds]]. The pi-bond is lost by formation of a new sigma bond. Chain-growth polymerization is involved in the manufacture of polymers such as [[polyethylene]], [[polypropylene]], [[polyvinyl chloride]] (PVC), and [[acrylate]]. In these cases, the alkenes RCH=CH<sub>2</sub> are converted to high molecular weight alkanes (-RCHCH<sub>2</sub>-)<sub>n</sub>  (R = H, CH<sub>3</sub>, Cl, CO<sub>2</sub>CH<sub>3</sub>).

Other forms of chain growth polymerization include [[cationic addition polymerization]] and [[anionic addition polymerization]]. A special case of chain-growth polymerization leads to [[living polymerization]]. [[Ziegler–Natta polymerization]] allows considerable control of [[Branching (polymer chemistry)|polymer branching]].

[[File:Ethylene polymerization.png|thumb|right|Polymerization of [[ethylene]]]]

Diverse methods are employed to manipulate the initiation, propagation, and termination rates during chain polymerization. A related issue is [[temperature control]], also called heat management, during these reactions, which are often highly exothermic. For example, for the polymerization of ethylene, 93.6 kJ of energy are released per mole of monomer.<ref name=ullmannC2/>

The manner in which polymerization is conducted is a highly evolved technology.  Methods include [[emulsion polymerization]], [[solution polymerization]], [[suspension polymerization]], and [[precipitation polymerization]]. Although the polymer [[dispersity]] and molecular weight may be improved, these methods may introduce additional processing requirements to isolate the product from a solvent.

=== Photopolymerization ===
{{Main|Photopolymer}}
Most '''photopolymerization''' reactions are chain-growth polymerizations which are initiated by the absorption of visible<ref>{{cite journal |last1=McKenzie |first1=Thomas G. |last2=Fu |first2=Qiang |last3=Wong |first3=Edgar H. H. |last4=Dunstan |first4=Dave E. |last5=Qiao |first5=Greg G. |date=2015-06-23 |title=Visible Light Mediated Controlled Radical Polymerization in the Absence of Exogenous Radical Sources or Catalysts |journal=Macromolecules |volume=48 |issue=12 |pages=3864–3872 |doi=10.1021/acs.macromol.5b00965 |issn=0024-9297 |bibcode=2015MaMol..48.3864M |url=https://figshare.com/articles/journal_contribution/2155795/files/3789646.pdf}}</ref> or ultraviolet light. Photopolymerization can also be a step-growth polymerization.<ref>{{Cite journal |last=Kaya |first=Kerem |date=January 2023 |title=A green and fast method for PEDOT: Photoinduced step-growth polymerization of EDOT |url=https://linkinghub.elsevier.com/retrieve/pii/S1381514822003091 |journal=Reactive and Functional Polymers |language=en |volume=182 |pages=105464 |doi=10.1016/j.reactfunctpolym.2022.105464|url-access=subscription }}</ref> The light may be absorbed either directly by the reactant monomer (''direct'' photopolymerization), or else by a ''photosensitizer'' which absorbs the light and then transfers energy to the monomer. In general, only the initiation step differs from that of the ordinary thermal polymerization of the same monomer; subsequent propagation, termination, and chain-transfer steps are unchanged.<ref name=":0" />
In step-growth photopolymerization, absorption of light triggers an addition (or condensation) reaction between two comonomers that do not react without light. A propagation cycle is not initiated because each growth step requires the assistance of light.<ref name="Soto 2014">{{cite journal |last=Soto |first=Marc |author2=Sebastián, Rosa María |author3=Marquet, Jordi |year=2014 |title=Photochemical Activation of Extremely Weak Nucleophiles: Highly Fluorinated Urethanes and Polyurethanes from Polyfluoro Alcohols |journal=J. Org. Chem. |volume=79 |issue=11 |pages=5019–5027 |doi=10.1021/jo5005789 |pmid=24820955}}</ref>

Photopolymerization can be used as a photographic or printing process because polymerization only occurs in regions which have been exposed to light. Unreacted monomer can be removed from unexposed regions, leaving a relief polymeric image.<ref name=":0" /> Several forms of [[3D printing#Photopolymerization|3D printing]]—including layer-by-layer [[stereolithography]] and [[two-photon absorption#3D photopolymerization|two-photon absorption 3D photopolymerization]]—use photopolymerization.<ref>{{cite journal |title=Additive manufacturing of ceramics from preceramic polymers |journal=Additive Manufacturing |volume=27 |pages=80–90 |doi=10.1016/j.addma.2019.02.012 |date=May 2019 |arxiv=1905.02060 |last1=Wang |first1=Xifan |last2=Schmidt |first2=Franziska |last3=Hanaor |first3=Dorian |last4=Kamm |first4=Paul H. |last5=Li |first5=Shuang |last6=Gurlo |first6=Aleksander |s2cid=104470679}}</ref>

[[Multiphoton polymerization]] using single pulses have also been demonstrated for fabrication of complex structures using a [[digital micromirror device]].<ref>{{cite journal |last1=Mills |first1=Benjamin |last2=Grant-Jacob |first2=James A |last3=Feinaeugle |first3=Matthias |last4=Eason |first4=Robert W |date=2013-06-17 |title=Single-pulse multiphoton polymerization of complex structures using a digital multimirror device |journal=Optics Express |language=EN |volume=21 |issue=12 |pages=14853–8 |doi=10.1364/oe.21.014853 |pmid=23787672 |issn=1094-4087 |bibcode=2013OExpr..2114853M |url=https://eprints.soton.ac.uk/356463/1/oe-21-12-14853.pdf |doi-access=free}}</ref>