Editing Living polymerization (section)

==="Living" free radical polymerization===
{{Main|Reversible-deactivation radical polymerization}}  
Starting in the 1970s several new methods were discovered which allowed the development of living polymerization using [[radical (chemistry)|free radical]] chemistry. These techniques involved [[catalytic chain transfer]] polymerization, iniferter mediated polymerization, stable free radical mediated polymerization (SFRP), [[atom transfer radical polymerization]] (ATRP), [[reversible addition-fragmentation chain transfer]] ([[RAFT (chemistry)|RAFT]]) polymerization, and iodine-transfer polymerization.

In "living" radical polymerization (or controlled radical polymerization (CRP)) the chain breaking pathways are severely depressed when compared to conventional radical polymerization (RP) and CRP can display characteristics of a living polymerization. However, since chain termination is not absent, but only minimized, CRP technically does not meet the requirements imposed by IUPAC for a living polymerization (see introduction for IUPAC definition). This issue has been up for debate the view points of different researchers can be found in a special issue of the Journal of Polymer Science titled   [http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1099-0518(20000515)38:10%3C%3E1.0.CO;2-C/issuetoc Living or Controlled ?]. The issue has not yet been resolved in the literature so it is often denoted as a "living" polymerization, quasi-living polymerization, pseudo-living and other terms to denote this issue.

There are two general strategies employed in CRP to suppress chain breaking reactions and promote fast initiation relative to propagation. Both strategies are based on developing a dynamic equilibrium amongst an active propagating radical and a dormant species.<ref name="Braunecker CRP" />

The first strategy involves a reversible trapping mechanism in which the propagating radical undergoes an activation/deactivation (i.e. [[Atom-transfer radical-polymerization]]) process with a species X. The species X is a persistent radical, or a species that can generate a stable radical, that cannot terminate with itself or propagate but can only reversibly "terminate" with the propagating radical (from the propagating polymer chain) P*. P* is a radical species that can propagate (k<sub>p</sub>) and irreversibly terminate (k<sub>t</sub>) with another P*.  X is normally a nitroxide (i.e. [[TEMPO]] used in [[Nitroxide Mediated Radical Polymerization]]) or an organometallic species. The dormant species (P<sub>n</sub>-X) can be activated to regenerate the active propagating species (P*) spontaneously, thermally, using a catalyst and optically.<ref name="Braunecker CRP">{{cite journal|last=Braunecker|first=Wade A.|author2=Matyjaszewski, Krzysztof |title=Controlled/living radical polymerization: Features, developments, and perspectives|journal=Progress in Polymer Science|date=2007|volume=32|issue=1|pages=93–146|doi=10.1016/j.progpolymsci.2006.11.002}}</ref><ref name=Matyjaszewski>{{cite web|last=Matyjaszewski|title=Features of Controlled "Living" Polymerization|url=http://www.cmu.edu/maty/crp/feature-development-crp/features.html#feature%202|url-status=dead|archiveurl=https://web.archive.org/web/20140314175012/http://www.cmu.edu/maty/crp/feature-development-crp/features.html#feature%202|archivedate=14 March 2014}}</ref>  
[[File:reversible trapping.png|center|400px]]

The second strategy is based on a degenerative transfer (DT) of the propagating radical between transfer agent that acts as a dormant species (i.e. [[Reversible addition−fragmentation chain-transfer polymerization]]). The DT based CRP's follow the conventional kinetics of radical polymerization, that is slow initiation and fast termination, but the transfer agent (Pm-X or Pn-X) is present in a much higher concentration compared to the radical initiator. The propagating radical species undergoes a thermally neutral exchange with the dormant transfer agent through atom transfer, group transfer or addition fragment chemistry.<ref name="Braunecker CRP" />             
[[File:radical degenerative transfer.png|center|500px]]