Catalytic Chain Transfer
Encyclopedia
Catalytic chain transfer (CCT) is a process that can be incorporated into radical polymerization
to obtain greater control over the resulting products.
of vinyl acetate
is a common (industrial) method to prepare polymeric materials. One of the problems associated with this method is, however, that the radical polymerisation reaction rate
is so high that even at short reaction times the polymeric chains are exceedingly long. This has several practical disadvantages, especially for polymer processing (e.g. melt-processing). A solution to this problem is catalytic chain transfer, which is a way to make shorter polymer chains in radical polymerisation processes. The method involves adding a catalytic chain transfer agent to the reaction mixture of the monomer
and the radical initiator
.
porphyrin
s are able to reduce the molecular weight of PMMA formed during radical polymerization
of methacrylate
s. Later investigations showed that the cobalt dimethylglyoxime
complexes were as effective as the porphyrin catalysts and also less oxygen sensitive. Due to their lower oxygen sensitivity these catalysts have been investigated much more thoroughly than the porphyrin catalysts and are the catalysts actually used commercially.
These organo-radical reactions with metal complexes provides several mechanisms to control radical polymerization of monomers CH2=CH(X). A wide range of metal-centered radicals and organo-metal complexes manifest at least a portion of these reactions. Various transition metal species, including complexes of Cr(I), Mo(III), Fe(I), V(0), Ti(III), and Co(II) have been demonstrated to control molecular weights in radical polymerization of olefins.
The olefin generating reaction 2 can become catalytic, and such catalytic chain transfer reactions are generally used to reduce the polymer molecular weight during the radical polymerization
process. Mechanistically, catalytic chain transfer involves hydrogen atom transfer from the organic growing polymeryl radical to cobalt(II), thus leaving a polymer vinyl-end group and a cobalt-hydride species. The Co(por)(H) species has no cis-vacant site for direct insertion of a new olefinic monomer into the Co-H bond to finalize the chain-transfer process, and hence the required olefin insertion also proceeds via a radical pathway.
The best recognized chain transfer catalysts are low spin cobalt(II) complexes and organo-cobalt(III) species, which function as latent storage sites for organo-radicals required to obtain living radical polymerization by several pathways.
The major products of catalytic chain transfer polymerization are vinyl
terminated polymer chains. One of the major drawbacks of the process is that catalytic chain transfer polymerization does not produce macromonomer
s of use in free radical polymerizations, but instead produces addition-fragmentation agents. When a growing polymer chain reacts with the addition fragmentation agent the radical end-group
attacks the vinyl bond and forms a bond. However, the resulting product is so hindered that the species undergoes fragmentation, leading eventually to telechelic species
.
These addition fragmentation chain transfer agents do form graft copolymers with styrenic and acrylate
species however they do so by first forming block copolymers and then incorporating these block copolymers into the main polymer backbone. While high yields of macromonomers are possible with methacrylate monomer
s, low yields are obtained when using catalytic chain transfer agents during the polymerization of acrylate and styrenic monomers. This has been seen to be due to the interaction of the radical centre with the catalyst during these polymerization reactions.
s for the automotive industry
. Federally-mandated VOC
restrictions are leading to the elimination of solvents from the automotive finishes and the lower molecular weight chain transfer products are often fluids. Incorporation of monomers such as glycidyl methacrylate
or hydroxyethylmethacrylate (HEMA) into the macromonomers aid curing processes. Macromonomers incorporating HEMA
can be effective in the dispersion
of pigment
s in the paints.
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...
to obtain greater control over the resulting products.
Introduction
Radical polymerization of vinyl monomers, like methyl (metha)acrylateMethyl methacrylate
Methyl methacrylate is an organic compound with the formula CH2=CCOOCH3. This colourless liquid, the methyl ester of methacrylic acid is a monomer produced on a large scale for the production of poly .-Production:...
of 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:...
is a common (industrial) method to prepare polymeric materials. One of the problems associated with this method is, however, that the radical polymerisation reaction rate
Reaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
is so high that even at short reaction times the polymeric chains are exceedingly long. This has several practical disadvantages, especially for polymer processing (e.g. melt-processing). A solution to this problem is catalytic chain transfer, which is a way to make shorter polymer chains in radical polymerisation processes. The method involves adding a catalytic chain transfer agent to the reaction mixture of the monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
and the 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...
.
Historical background
Boris Smirnov and Alexander Marchenko (USSR) discovered in 1975 that cobaltCobalt
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....
porphyrin
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 are able to reduce the molecular weight of PMMA formed during radical polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
of methacrylate
Methyl methacrylate
Methyl methacrylate is an organic compound with the formula CH2=CCOOCH3. This colourless liquid, the methyl ester of methacrylic acid is a monomer produced on a large scale for the production of poly .-Production:...
s. Later investigations showed that the cobalt dimethylglyoxime
Dimethylglyoxime
Dimethylglyoxime is a chemical compound described by the formula CH3CCCH3. This colourless solid is the dioxime derivative of the diketone diacetyl . DmgH2 is used in the analysis of palladium or nickel. Its coordination complexes are of theoretical interest as models for enzymes and as catalysts...
complexes were as effective as the porphyrin catalysts and also less oxygen sensitive. Due to their lower oxygen sensitivity these catalysts have been investigated much more thoroughly than the porphyrin catalysts and are the catalysts actually used commercially.
Process
In general, reactions of organic free radicals (•C(CH3)(X)R) with metal-centered radicals (M•) either produce an organometallic complex (reaction 1) or a metal hydride (M-H) and an olefin (CH2=C(X)R) by the metallo radical M• abstracting a β-hydrogen from the organic radical •C(CH3)(X)R (reaction 2).These organo-radical reactions with metal complexes provides several mechanisms to control radical polymerization of monomers CH2=CH(X). A wide range of metal-centered radicals and organo-metal complexes manifest at least a portion of these reactions. Various transition metal species, including complexes of Cr(I), Mo(III), Fe(I), V(0), Ti(III), and Co(II) have been demonstrated to control molecular weights in radical polymerization of olefins.
The olefin generating reaction 2 can become catalytic, and such catalytic chain transfer reactions are generally used to reduce the polymer molecular weight during the radical polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
process. Mechanistically, catalytic chain transfer involves hydrogen atom transfer from the organic growing polymeryl radical to cobalt(II), thus leaving a polymer vinyl-end group and a cobalt-hydride species. The Co(por)(H) species has no cis-vacant site for direct insertion of a new olefinic monomer into the Co-H bond to finalize the chain-transfer process, and hence the required olefin insertion also proceeds via a radical pathway.
The best recognized chain transfer catalysts are low spin cobalt(II) complexes and organo-cobalt(III) species, which function as latent storage sites for organo-radicals required to obtain living radical polymerization by several pathways.
The major products of catalytic chain transfer polymerization are vinyl
Vinyl
A vinyl compound is any organic compound that contains a vinyl group ,which are derivatives of ethene, CH2=CH2, with one hydrogen atom replaced with some other group...
terminated polymer chains. One of the major drawbacks of the process is that catalytic chain transfer polymerization does not produce macromonomer
Macromonomer
A macromonomer is a macromolecule with one end-group that enables it to act as a monomer. Macromonomers will contribute a single monomeric unit to a chain of the completed macromolecule....
s of use in free radical polymerizations, but instead produces addition-fragmentation agents. When a growing polymer chain reacts with the addition fragmentation agent the radical end-group
End-group
An end-group in polymer chemistry is a constitutional unit that is an extremity of a macromolecule or oligomer molecule. For example the end-group of a PET polyester may be an alcohol group or a carboxylic acid group...
attacks the vinyl bond and forms a bond. However, the resulting product is so hindered that the species undergoes fragmentation, leading eventually to telechelic species
Telechelic polymer
A telechelic polymer or oligomer is a prepolymer capable of entering into further polymerization or other reactions through its reactive end-groups. It can be used for example to synthesize block copolymers....
.
These addition fragmentation chain transfer agents do form graft copolymers with styrenic and acrylate
Acrylate
The acrylate ion is the ion of acrylic acid.Acrylates are the salts and esters of acrylic acid. They are also known as propenoates ....
species however they do so by first forming block copolymers and then incorporating these block copolymers into the main polymer backbone. While high yields of macromonomers are possible with methacrylate monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
s, low yields are obtained when using catalytic chain transfer agents during the polymerization of acrylate and styrenic monomers. This has been seen to be due to the interaction of the radical centre with the catalyst during these polymerization reactions.
Utility
The catalytic chain transfer process was commercialized very soon after its discovery. The initial commercial outlet was the production of chemically-reactive macromonomers to be incorporated into paintPaint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
s for the automotive industry
Automotive industry
The automotive industry designs, develops, manufactures, markets, and sells motor vehicles, and is one of the world's most important economic sectors by revenue....
. Federally-mandated VOC
Volatile organic compound
Volatile organic compounds are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and...
restrictions are leading to the elimination of solvents from the automotive finishes and the lower molecular weight chain transfer products are often fluids. Incorporation of monomers such as glycidyl methacrylate
Glycidyl methacrylate
Glycidyl methacrylate is an ester of methacrylic acid and a common monomer used in the creation of epoxy resins. While typical home epoxies contain diglycidyl ether of bisphenol A , glycidyl methacrylate is instead used to provide epoxy functionalization to polyolefins and other acrylate resins...
or hydroxyethylmethacrylate (HEMA) into the macromonomers aid curing processes. Macromonomers incorporating HEMA
Hema
Hema may refer to:* Hema , an ethnic group in the eastern Democratic Republic of the Congo* Hema , a figure from Polynesian mythology* HEMA , a Dutch chain of stores* Historical European martial arts...
can be effective in the dispersion
Dispersion (chemistry)
A dispersion is a system in which particles are dispersed in a continuous phase of a different composition . See also emulsion. A dispersion is classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not...
of pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
s in the paints.
See also
- Radical polymerizationRadical polymerizationFree 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...
- Living polymerizationLiving polymerizationIn 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...
- Cobalt Mediated Radical PolymerizationCobalt Mediated Radical PolymerizationCobalt 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...