Editing Living polymerization (section)

====Living α-olefin polymerization====

[[Alpha-olefins|α-olefins]] can be polymerized through an anionic coordination polymerization in which the metal center of the catalyst is considered the counter cation for the [[anionic]] end of the alkyl chain (through a M-R coordination). Ziegler-Natta initiators were developed in the mid-1950s and are heterogeneous initiators used in the polymerization of alpha-olefins. Not only were these initiators the first to achieve relatively high molecular weight poly(1-alkenes) (currently the most widely produced thermoplastic in the world PE([[Polyethylene]]) and PP ([[Polypropylene]])<ref name=b1>{{cite book|last=Craver|first=C.|author2=Carraher, C.|title=Applied Polymer Science: 21st Century|year=2000|publisher=Elsevier|pages=1022–1023}}</ref> but the initiators were also capable of stereoselective polymerizations which is attributed to the [[Chirality (chemistry)|chiral]] [[Crystal structure]] of the heterogeneous initiator.<ref name=Principles /> Due to the importance of this discovery Ziegler and Natta were presented with the [https://www.nobelprize.org/nobel_prizes/chemistry/laureates/1963/ 1963 Nobel Prize in chemistry]. Although the active species formed from the Ziegler-Natta initiator generally have long lifetimes (on the scale of hours or longer) the lifetimes of the propagating chains are shortened due to several chain transfer pathways ([[Beta-Hydride elimination]] and transfer to the co-initiator) and as a result are not considered living.<ref name=Principles />

Metallocene initiators are considered as a type of Ziegler-Natta initiators due to the use of the two-component system consisting of a [[transition metal]] and a group I-III metal co-initiator (for example [[methylalumoxane]] (MAO) or other alkyl aluminum compounds). The [[metallocene]] initiators form homogeneous single site [[catalyst]]s that were initially developed to study the impact that the catalyst structure had on the resulting polymers structure/properties; which was difficult for multi-site heterogeneous Ziegler-Natta initiators.<ref name=b1/> Owing to the discrete single site on the metallocene catalyst researchers were able to tune and relate how the ancillary ligand (those not directly involved in the chemical transformations) structure and the symmetry about the chiral metal center affect the microstructure of the polymer.<ref>{{cite journal|last=Coates|first=Geoffrey W.|title=Precise Control of Polyolefin Stereochemistry Using Single-Site Metal Catalysts|journal=Chemical Reviews|date=April 2000|volume=100|issue=4|pages=1223–1252|doi=10.1021/cr990286u|pmid=11749265}}</ref> However, due to chain breaking reactions (mainly Beta-Hydride elimination) very few metallocene based polymerizations are known.<ref name=Principles />

By tuning the steric bulk and electronic properties of the ancillary ligands and their [[substituent]]s a class of initiators known as [[Chelation|chelate]] initiators (or post-metallocene initiators) have been successfully used for [[Stereospecificity|stereospecific]] living polymerizations of alpha-olefins. The chelate initiators have a high potential for living polymerizations because the ancillary ligands can be designed to discourage or inhibit chain termination pathways. Chelate initiators can be further broken down based on the ancillary ligands; ansa-cyclopentyadienyl-amido initiators, alpha-diimine chelates and phenoxy-imine chelates.<ref name=Principles />  
*Ansa-cyclopentadienyl-amido (CpA) initiators 
[[File:ACp initiators.png|thumb|300px|right|a.) Shows the general form of CpA initiators with one Cp ring and a coordinated Nitrogen b.) Shows the CpA initiator used in the living polymerization of 1-hexene (5)]]
CpA initiators have one [[cyclopentadienyl]] substituent and one or more nitrogen substituents coordinated to the metal center (generally a Zr or Ti) (Odian).  The dimethyl(pentamethylcyclopentyl)zirconium acetamidinate in figure___ has been used for a stereospecific living polymerization of 1-hexene at −10&nbsp;°C. The resulting poly(1-hexene) was [[Tacticity#Isotactic polymers|isotactic]] (stereochemistry is the same between adjacent repeat units) confirmed by <sup>13</sup>C-NMR. The multiple trials demonstrated a controllable and predictable (from catalyst to [[monomer]] ratio) M<sub>n</sub> with low Đ. The polymerization was further confirmed to be living by sequentially adding 2 portions of the monomer, the second portion was added after the first portion was already polymerized, and monitoring the Đ and M<sub>n</sub> of the chain. The resulting polymer chains complied with the predicted M<sub>n</sub> (with the total monomer concentration = portion 1 +2) and showed low Đ<ref name="Ansa chelates">{{cite journal|last1=Jayaratne|first1=K.|last2=Sita |first2=L|title= Stereospecific Living Ziegler−Natta Polymerization of 1-Hexene |journal=J. Am. Chem. Soc.|issue=5|pages=958–959|volume=122|doi=10.1021/ja993808w |year=2000}}</ref> suggesting the chains were still active, or living, as the second portion of monomer was added (5).
*α-diimine chelate initiators
α-diimine chelate initiators are characterized by having a [[diimine]] chelating ancillary ligand structure and which is generally coordinated to a late transition (i.e. Ni and Pd) metal center. 
[[File:Alphadiiminechelateinitiator.png|200px|center]]
Brookhart et al. did extensive work with this class of catalysts and reported living polymerization for α-olefins<ref name="Maurice Brookhart">{{Cite journal | doi = 10.1021/ja962516h| title = Living Polymerization of α-Olefins Using NiII−α-Diimine Catalysts. Synthesis of New Block Polymers Based on α-Olefins| journal = Journal of the American Chemical Society| volume = 118| issue = 46| pages = 11664–11665| year = 1996| last1 = Killian | first1 = C. M. | last2 = Tempel | first2 = D. J. | last3 = Johnson | first3 = L. K. | last4 = Brookhart | first4 = M. }}</ref> and demonstrated living α-olefin carbon monoxide alternating copolymers.<ref name="MAurice B 2">{{Cite journal | last1 = Brookhart | first1 = M. | last2 = Rix | first2 = F. C. | last3 = Desimone | first3 = J. M. | last4 = Barborak | first4 = J. C. | title = Palladium(II) catalysts for living alternating copolymerization of olefins and carbon monoxide | doi = 10.1021/ja00040a082 | journal = Journal of the American Chemical Society | volume = 114 | issue = 14 | pages = 5894–5895 | year = 1992 }}</ref>