Living polymerization

One of the key characteristics of a living polymerization is that the chain termination and transfer reactions are essentially eliminated from the four elementary reactions of [[chain-growth polymerization]] leaving only [[Initiation (chemistry)|initiation]] and (chain) propagation reactions.

One of the key characteristics of a living polymerization is that the chain termination and transfer reactions are essentially eliminated from the four elementary reactions of [[chain-growth polymerization]] leaving only [[Initiation (chemistry)|initiation]] and (chain) propagation reactions.

A key characteristic of living polymerization is that the rate of initiation (meaning the dormant chemical species generates the active chain propagating species) is much faster than the rate of chain propagation. Thus all of the chains grow at the same rate (the rate of propagation).

A key characteristic of living polymerization is that the rate of initiation (meaning the dormant [[chemical species]] generates the active chain propagating species) is much faster than the rate of chain propagation. Thus all of the chains grow at the same rate (the rate of propagation).

The high rate of initiation (together with absence of termination) results in low (or narrow) [[Dispersity|dispersity index (''Đ'')]], an indication of the broadness in the distribution of polymer chains.{{cite web |title=Living polymer |publisher=[[International Union of Pure and Applied Chemistry ]] |work=Gold Book |doi=10.1351/goldbook.LT07156 |url=http://goldbook.iupac.org/LT07156.html |access-date=January 4, 2023}} The extended lifetime of the propagating chain allowing for co-block polymer formation and end group functionalization to be performed on the living chain. These factors also allow predictable molecular weights, expressed as the number average molecular weight (M_n). For an ideal living system, assuming efficiency for generating active species is 100%, where each initiator generates only one active species the [[Kinetic chain length]] (average number of monomers the active species reacts with during its lifetime) at a given time can be estimated by knowing the concentration of monomer remaining. The number average molecular weight, [[Molar mass distribution#number average molecular mass|M_n]], increases linearly with percent conversion during a living polymerization

The high rate of initiation (together with absence of termination) results in low (or narrow) [[Dispersity|dispersity index (''Đ'')]], an indication of the broadness in the distribution of polymer chains.{{cite web |title=Living polymer |publisher=[[International Union of Pure and Applied Chemistry ]] |work=Gold Book |doi=10.1351/goldbook.LT07156 |url=http://goldbook.iupac.org/LT07156.html |access-date=January 4, 2023}} The extended lifetime of the propagating chain allowing for co-block polymer formation and end group functionalization to be performed on the living chain. These factors also allow predictable molecular weights, expressed as the number average molecular weight (M_n). For an ideal living system, assuming efficiency for generating active species is 100%, where each initiator generates only one active species the [[Kinetic chain length]] (average number of monomers the active species reacts with during its lifetime) at a given time can be estimated by knowing the concentration of monomer remaining. The number average molecular weight, [[Molar mass distribution#number average molecular mass|M_n]], increases linearly with percent conversion during a living polymerization

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

====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]]){{cite book|last=Craver|first=C.|author2=Carraher, C.|title=Applied Polymer Science: 21st Century|year=2000|publisher=Elsevier|pages=1022–1023}} but the initiators were also capable of stereoselective polymerizations which is attributed to the [[Chirality (chemistry)|chiral]] [[Crystal structure]] of the heterogeneous initiator. 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.

[[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]]){{cite book|last=Craver|first=C.|author2=Carraher, C.|title=Applied Polymer Science: 21st Century|year=2000|publisher=Elsevier|pages=1022–1023}} but the initiators were also capable of stereoselective polymerizations which is attributed to the [[Chirality (chemistry)|chiral]] [[Crystal structure]] of the heterogeneous initiator. 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.

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. 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.{{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}} However, due to chain breaking reactions (mainly Beta-Hydride elimination) very few metallocene based polymerizations are known.

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. 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.{{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}} However, due to chain breaking reactions (mainly Beta-Hydride elimination) very few metallocene based polymerizations are known.

{{Main|Living cationic polymerization}}

{{Main|Living cationic polymerization}}

Monomers for living cationic polymerization are electron-rich alkenes such as vinyl ethers, [[isobutylene]], [[styrene]], and N-vinylcarbazole. The initiators are binary systems consisting of an electrophile and a Lewis acid. The method was developed around 1980 with contributions from Higashimura, Sawamoto and Kennedy. Typically, generating a stable [[carbocation]] for a prolonged period of time is difficult, due to the possibility for the cation to be quenched by a β-protons attached to another monomer in the backbone, or in a free monomer. Therefore, a different approach is taken{{cite book|last=Cowie|first=J.M.G.|title=Polymers chemistry and physics of modern materials|year=2007|publisher=Taylor & Francis|location=Boca Raton|isbn=9780849398131|edition=3rd ed / J.M.G. Cowie and Valeria Arrighi}}

Monomers for living cationic polymerization are electron-rich alkenes such as vinyl ethers, [[isobutylene]], [[styrene]], and N-vinylcarbazole. The initiators are binary systems consisting of an [[electrophile]] and a Lewis acid. The method was developed around 1980 with contributions from Higashimura, Sawamoto and Kennedy. Typically, generating a stable [[carbocation]] for a prolonged period of time is difficult, due to the possibility for the cation to be quenched by a β-protons attached to another monomer in the backbone, or in a free monomer. Therefore, a different approach is taken{{cite book|last=Cowie|first=J.M.G.|title=Polymers chemistry and physics of modern materials|year=2007|publisher=Taylor & Francis|location=Boca Raton|isbn=9780849398131|edition=3rd ed / J.M.G. Cowie and Valeria Arrighi}}

[[File:Wiki Cation.png|right|400px|This is an example of a controlled/living cationic polymerization. Note that the "termination" step has been placed in equilibrium with an "initiation" step in either direction. Nu: is a weak nucleophile that can reversibly leave, while the MXn is a weak Lewis acid M bound to a halogen X to generate the carbocation.]]

[[File:Wiki Cation.png|right|400px|This is an example of a controlled/living cationic polymerization. Note that the "termination" step has been placed in equilibrium with an "initiation" step in either direction. Nu: is a weak nucleophile that can reversibly leave, while the MXn is a weak Lewis acid M bound to a halogen X to generate the carbocation.]]