Cobalt Mediated Radical Polymerization
Encyclopedia
Cobalt
based catalysts, when used in radical polymerization
, have several main advantages especially in slowing down the reaction rate, allowing for the synthesis of polymers with peculiar properties. As starting the reaction does need a real radical initiator
, the cobalt species is not the only used catalyst, it is a mediator. For this reason this type of reaction is referred to as Cobalt mediated radical polymerization.
However, conventional (free) RP reactions suffer from a lack of control over the polymer molecular-weights and weight distributions. A relatively narrow molecular weight-distribution (Mw/Mw) is usually desirable, as broad distribution negatively influence the polymer properties of (generally atactic) polymers produced by RP. Common RP also does not allow the formation of block copolymers. This is where controlled (or living) radical polymerization
comes into play. Several CRP reactions have been developed over the past years, some of which capable of producing well-defined polymers with narrow molecular weight distributions.
Cobalt mediated radical polymerisation (CMRP) is one of these methods, which offers some specific advantages.
Most notably, CMRP allows RP of a broad substrate scope (among others acrylates, acrylic acid
, vinyl esters, vinyl acetate
, acrylonitrile
, vinylpyrrolidone) under various reaction conditions, and (for some catalysts) gives access to very fast CRP reactions with rates approaching those of conventional uncontrolled free radical polymerization reactions.
Most commonly applied cobalt compounds are cobaloximes, cobalt porphyrin
s and Co(acac
)2 derivatives, used in combination with various radical initiators (such as AIBN or V70).
Cobalt can control radical polymerization (RP) reactions by essentially three mechanisms:
CMRP through reversible termination (persistent radical effect) was introduced in 1994.
More recent developments focus on CMRP via degenerative transfer (DT) mechanisms, and combinations of RT and DT mechanisms.
In many cases, CMRP exploits the weak cobalt(III)-carbon bond to control the radical polymerization reaction. The Co-C bond containing radical initiator
easily breaks up (by heat or by light) in a carbon free radical and a cobalt(II) radical species. The carbon radical starts the growth of a polymer chain from the CH2=CHX monomer as in a free radical polymerization reaction. Cobalt is unusual in that it can reversibly
reform a covalent bond with the carbon radical terminus of the growing chain. This reduces the concentration of radicals to a minimum and thereby minimizes undesirable termination reactions by recombination of two carbon radicals. The cobalt trapping reagent is called a persistent radical and the cobalt-capped polymer chain is said to be dormant. This mechanism is called reversible termination and is said to operate via the "persistent radical effect". When the monomer lacks protons that can be easily abstracted by the cobalt radical, (catalytic) chain transfer
is also limited and the RP reaction becomes close to ‘living’.
The degenerative transfer mechanism is based on very fast exchange equilibria between small free radicals (being continuously injected into the solution) and dormant polymeryl radicals (protected as closed-shell cobalt species). Systems based on degenerative transfer do not proceed via the persistent radical effect (PRE). Instead an active propagating radical interchanges its role with a latent radical in a dormant complex. The activation of one polymer chain means the deactivation of another polymer chain. If the exchange process is much faster than the polymerisation rate (kp), effectively all polymer chains grow at the same rate. Because the large polymer chains diffuse much slower than the small organic radicals, and thereby terminate much slower via 2nd order radical-radical coupling or disproportionation, long chains effectively build-up at cobalt while the small radicals keep terminating. This leads to a desirable narrow molecular weight distribution of the polymer at high polymerization rates.
DT-CMRP is an associative process, which for CoIII(por)(alkyl) species implies formation of a 6-coordinate intermediate or transition state. Such Co(por)(alkyl)2 species formally have a Co(+IV) oxidation state
, but in reality their (electronic) structure is best described as a weak radical adduct of a CoIII(por)(alkyl) species.
A striking feature of DT-CMRP is the fact that even upon using a large excess of the radical initiator compared to the transfer agent, the radical polymerization reactions still remains controlled. A satisfactory explanation for this phenomenon seems to be lacking at the moment.
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
based catalysts, when used in radical polymerization
Radical polymerization
Free radical polymerization is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. Free radicals can be formed via a number of different mechanisms usually involving separate initiator molecules...
, have several main advantages especially in slowing down the reaction rate, allowing for the synthesis of polymers with peculiar properties. As starting the reaction does need a real radical initiator
Radical initiator
In chemistry, radical initiators are substances that can produce radical species under mild conditions and promote radical reactions . These substances generally possess weak bonds—bonds that have small bond dissociation energies. Radical initiators are utilized in industrial processes such...
, the cobalt species is not the only used catalyst, it is a mediator. For this reason this type of reaction is referred to as Cobalt mediated radical polymerization.
Introduction
About half of all commercial polymers are produced by radical polymerization. Radical polymerization (RP) reactions have several advantageous properties:- A wide variety of monomers can be polymerized.
- RP reactions are tolerant to various functional groups.
- RP reactions allow a large temperature range of operation (–100 to >200 °C).
- RP reactions are generally compatibility with several reactions conditions (bulk, solution, (mini)emulsion and suspension).
- RP reactions allow a relatively simple reactor set-up, and are hence costs effective.
However, conventional (free) RP reactions suffer from a lack of control over the polymer molecular-weights and weight distributions. A relatively narrow molecular weight-distribution (Mw/Mw) is usually desirable, as broad distribution negatively influence the polymer properties of (generally atactic) polymers produced by RP. Common RP also does not allow the formation of block copolymers. This is where controlled (or living) radical polymerization
Living polymerization
In polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is...
comes into play. Several CRP reactions have been developed over the past years, some of which capable of producing well-defined polymers with narrow molecular weight distributions.
Cobalt mediated radical polymerisation (CMRP) is one of these methods, which offers some specific advantages.
Most notably, CMRP allows RP of a broad substrate scope (among others acrylates, acrylic acid
Acrylic acid
Acrylic acid is an organic compound with the formula CH2=CHCO2H. It is the simplest unsaturated carboxylic acid, consisting of a vinyl group connected directly to a carboxylic acid terminus. This colorless liquid has a characteristic acrid or tart smell. It is miscible with water, alcohols,...
, vinyl esters, vinyl acetate
Vinyl acetate
Vinyl acetate is an organic compound with the formula CH3COOCH=CH2. A colorless liquid with a pungent odor, it is the precursor to polyvinyl acetate, an important polymer in industry.-Production:...
, acrylonitrile
Acrylonitrile
Acrylonitrile is the chemical compound with the formula C3H3N. This pungent-smelling colorless liquid often appears yellow due to impurities. It is an important monomer for the manufacture of useful plastics. In terms of its molecular structure, it consists of a vinyl group linked to a nitrile...
, vinylpyrrolidone) under various reaction conditions, and (for some catalysts) gives access to very fast CRP reactions with rates approaching those of conventional uncontrolled free radical polymerization reactions.
Porphyrin
Porphyrins are a group of organic compounds, many naturally occurring. One of the best-known porphyrins is heme, the pigment in red blood cells; heme is a cofactor of the protein hemoglobin. Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at...
s and Co(acac
Acac
ACAC may refer to:* American Council for Accredited Certification, a private, non-profit certifying body* Atlanta Contemporary Art Center, a contemporary art museum in Atlanta* ACAC consortium, a subsidiary of China Aviation Industry Corporation...
)2 derivatives, used in combination with various radical initiators (such as AIBN or V70).
Cobalt can control radical polymerization (RP) reactions by essentially three mechanisms:
- Catalytic chain transferCatalytic Chain TransferCatalytic chain transfer is a process that can be incorporated into radical polymerization to obtain greater control over the resulting products.-Introduction:...
(CCT) - Reversibile termination (RT), leading to the persistent radical effect (PRE)
- Degenerative transfer (DT).
Historical background
Cobalt mediated control of RP reactions through CCT was discovered by Smirnov and Marchenko in 1975.CMRP through reversible termination (persistent radical effect) was introduced in 1994.
More recent developments focus on CMRP via degenerative transfer (DT) mechanisms, and combinations of RT and DT mechanisms.
Control via reversible termination (persistent radical effect)
Radical initiator
In chemistry, radical initiators are substances that can produce radical species under mild conditions and promote radical reactions . These substances generally possess weak bonds—bonds that have small bond dissociation energies. Radical initiators are utilized in industrial processes such...
easily breaks up (by heat or by light) in a carbon free radical and a cobalt(II) radical species. The carbon radical starts the growth of a polymer chain from the CH2=CHX monomer as in a free radical polymerization reaction. Cobalt is unusual in that it can reversibly
Reversible reaction
A reversible reaction is a chemical reaction that results in an equilibrium mixture of reactants and products. For a reaction involving two reactants and two products this can be expressed symbolically as...
reform a covalent bond with the carbon radical terminus of the growing chain. This reduces the concentration of radicals to a minimum and thereby minimizes undesirable termination reactions by recombination of two carbon radicals. The cobalt trapping reagent is called a persistent radical and the cobalt-capped polymer chain is said to be dormant. This mechanism is called reversible termination and is said to operate via the "persistent radical effect". When the monomer lacks protons that can be easily abstracted by the cobalt radical, (catalytic) chain transfer
Chain transfer
Chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule.Chain transfer reactions reduce the average molecular weight of the final polymer...
is also limited and the RP reaction becomes close to ‘living’.
Control via catalytic chain transfer
Catalytic chain transfer is a way to make shorter polymer chains in a radical polymerization process. The method involves adding a catalytic chain transfer agent to the reaction mixture of the monomer and the radical initiator. Catalytic chain transfer proceeds via hydrogen atom transfer from the organic growing polymeryl radical to cobalt(II), producing a polymer vinyl-end group and a cobalt-hydride species (equilibrium 2). The Co-H species then reacts with the monomer to start a new Co(III)-alkyl species, which re-initiates a new growing polymeric radical (reversible termination, equilibrium 1). The main products of catalytic chain transfer polymerization are therefore vinyl terminated polymer chains which are shorter than in conventional (free) radical polymerization.Control via degenerative transfer
One of the disadvantages of controlled radical polymerization reactions is that they tend to become rather slow. Controlled polymerization conditions are usually achieved by extending the life-time of the growing polymer chain radical, by keeping it in a dormant state for most of the time (known as the Persistent Radical Effect). Thereby the control agent substantially slows-down the over-all radical polymerisation reaction. However, some CMRP reactions proceed via a different mechanism, called degenerative transfer (DT), which allows controlled radical polymerization reactions to proceed at roughly the same rate as any uncontrolled free radical polymerisation.The degenerative transfer mechanism is based on very fast exchange equilibria between small free radicals (being continuously injected into the solution) and dormant polymeryl radicals (protected as closed-shell cobalt species). Systems based on degenerative transfer do not proceed via the persistent radical effect (PRE). Instead an active propagating radical interchanges its role with a latent radical in a dormant complex. The activation of one polymer chain means the deactivation of another polymer chain. If the exchange process is much faster than the polymerisation rate (kp), effectively all polymer chains grow at the same rate. Because the large polymer chains diffuse much slower than the small organic radicals, and thereby terminate much slower via 2nd order radical-radical coupling or disproportionation, long chains effectively build-up at cobalt while the small radicals keep terminating. This leads to a desirable narrow molecular weight distribution of the polymer at high polymerization rates.
DT-CMRP is an associative process, which for CoIII(por)(alkyl) species implies formation of a 6-coordinate intermediate or transition state. Such Co(por)(alkyl)2 species formally have a Co(+IV) oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
, but in reality their (electronic) structure is best described as a weak radical adduct of a CoIII(por)(alkyl) species.
A striking feature of DT-CMRP is the fact that even upon using a large excess of the radical initiator compared to the transfer agent, the radical polymerization reactions still remains controlled. A satisfactory explanation for this phenomenon seems to be lacking at the moment.