Cossee-Arlman mechanism
Encyclopedia
The Cossee-Arlman mechanism in polymer chemistry
is the main pathway for the formation of C-C bonds in the polymerization of alkenes. The mechanism features an intermediate coordination complex that contains both the growing polymer chain and the monomer (alkene). These ligand
s combine within the coordination sphere
of the metal to form an polymer chain that is elongated by two carbons.
The details of this mechanism can be used to explain the stereoregularity of the polymerisation of alkene
s using Ziegler-Natta or metallocene
catalysts. Stereoregularity is relevant for unsymmetrical alkenes such as propylene
. The coordination sphere
of the metal
ligand
s sterically influences which end of the propylene attaches to the growing polymer chain and the relative stereochemistry of the methyl groups on the polymer. The stereoregularity is influenced by the ligands. For the metallocene
catalysts, the cyclopentadienyl ligands (or their surrogates) fulfill this role. For heterogeneous catalysts, the stereoregularity is determined by the surface structure around the active site on the catalyst particle, and can be influenced by additives such as succinates or phthalates, which tend to block specific sites, while leaving others (with different stereoreactivity) to catalyse the polymerization.
Polymer chemistry
Polymer chemistry or macromolecular chemistry is a multidisciplinary science that deals with the chemical synthesis and chemical properties of polymers or macromolecules. According to IUPAC recommendations, macromolecules refer to the individual molecular chains and are the domain of chemistry...
is the main pathway for the formation of C-C bonds in the polymerization of alkenes. The mechanism features an intermediate coordination complex that contains both the growing polymer chain and the monomer (alkene). These ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s combine within the coordination sphere
Coordination sphere
In coordination chemistry, the coordination sphere refers to a central atom or ion and an array of molecules or anions, the ligands, around.Molecules that are attached noncovalently to the ligands are called the second coordination sphere....
of the metal to form an polymer chain that is elongated by two carbons.
The details of this mechanism can be used to explain the stereoregularity of the polymerisation of alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s using Ziegler-Natta or metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
catalysts. Stereoregularity is relevant for unsymmetrical alkenes such as propylene
Propylene
Propene, also known as propylene or methylethylene, is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons, and it is also second in natural abundance.-Properties:At room temperature and...
. The coordination sphere
Coordination sphere
In coordination chemistry, the coordination sphere refers to a central atom or ion and an array of molecules or anions, the ligands, around.Molecules that are attached noncovalently to the ligands are called the second coordination sphere....
of the metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s sterically influences which end of the propylene attaches to the growing polymer chain and the relative stereochemistry of the methyl groups on the polymer. The stereoregularity is influenced by the ligands. For the metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
catalysts, the cyclopentadienyl ligands (or their surrogates) fulfill this role. For heterogeneous catalysts, the stereoregularity is determined by the surface structure around the active site on the catalyst particle, and can be influenced by additives such as succinates or phthalates, which tend to block specific sites, while leaving others (with different stereoreactivity) to catalyse the polymerization.