Catalytic chain transfer

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Catalytic chain transfer (CCT) is a process that can be incorporated into radical polymerization to obtain greater control over the resulting products.

Contents

Introduction

Radical polymerization of vinyl monomers, like methyl (metha)acrylate 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.

Historical background

Boris Smirnov and Alexander Marchenko (USSR) discovered in 1975 that cobalt porphyrins are able to reduce the molecular weight of PMMA formed during radical polymerization of methacrylates. [1] [2] Later investigations showed that the cobalt dimethylglyoxime complexes were as effective as the porphyrin catalysts and also less oxygen sensitive. [3] [4] 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

Catalytic chain transfer 1.png

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). [5] 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. [6] Various transition metal species, including complexes of Cr(I), [7] [8] Mo(III), [9] Fe(I), [10] V(0), [11] Ti(III), [12] and Co(II) [13] [14] [15] have been demonstrated to control molecular weights in radical polymerization of olefins.

Cobalt mediated chain trans.png

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. [16] [17] The best recognized chain transfer catalysts are low spin cobalt(II) complexes [13] and organo-cobalt(III) species, which function as latent storage sites for organo-radicals required to obtain living radical polymerization by several pathways. [5]

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 macromonomers 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 monomers, 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 paints 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 pigments in the paints. The chemistry is very effective under emulsion polymerisation conditions and has been used in the printing industry since 2000. [18] The vinylic end group acts as an addition fragmentation agent and has been utilised to make multi block copolymers [19] and derivatives used as stress relief agents in dental restoration by 3M. [20]

See also

Related Research Articles

<span class="mw-page-title-main">Polymerization</span> Chemical reaction to form polymer chains

In polymer chemistry, polymerization, or polymerisation, is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many forms of polymerization and different systems exist to categorize them.

In polymer chemistry, living polymerization is a form of chain growth 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 also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar. Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups.

<span class="mw-page-title-main">Copolymer</span> Polymer derived from more than one species of monomer

In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are sometimes called bipolymers. Those obtained from three and four monomers are called terpolymers and quaterpolymers, respectively. Copolymers can be characterized by a variety of techniques such as NMR spectroscopy and size-exclusion chromatography to determine the molecular size, weight, properties, and composition of the material.

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<span class="mw-page-title-main">End-group</span> Functional group at the extremity of an oligomer or other macromolecule

End groups are an important aspect of polymer synthesis and characterization. In polymer chemistry, they are functional groups that are at the very ends of a macromolecule or oligomer (IUPAC). In polymer synthesis, like condensation polymerization and free-radical types of polymerization, end-groups are commonly used and can be analyzed by nuclear magnetic resonance (NMR) to determine the average length of the polymer. Other methods for characterization of polymers where end-groups are used are mass spectrometry and vibrational spectrometry, like infrared and raman spectroscopy. These groups are important for the analysis of polymers and for grafting to and from a polymer chain to create a new copolymer. One example of an end group is in the polymer poly(ethylene glycol) diacrylate where the end-groups are circled.

In polymer chemistry, free-radical polymerization (FRP) is a method of polymerization by which a polymer forms by the successive addition of free-radical building blocks. Free radicals can be formed by a number of different mechanisms, usually involving separate initiator molecules. Following its generation, the initiating free radical adds (nonradical) monomer units, thereby growing the polymer chain.

Atom transfer radical polymerization (ATRP) is an example of a reversible-deactivation radical polymerization. Like its counterpart, ATRA, or atom transfer radical addition, ATRP is a means of forming a carbon-carbon bond with a transition metal catalyst. Polymerization from this method is called atom transfer radical addition polymerization (ATRAP). As the name implies, the atom transfer step is crucial in the reaction responsible for uniform polymer chain growth. ATRP was independently discovered by Mitsuo Sawamoto and by Krzysztof Matyjaszewski and Jin-Shan Wang in 1995.

<span class="mw-page-title-main">Reversible addition−fragmentation chain-transfer polymerization</span>

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<span class="mw-page-title-main">Thiol-yne reaction</span>

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<span class="mw-page-title-main">Poly(methacrylic acid)</span> Chemical compound

Poly(methacrylic acid) (PMAA) is a polymer made from methacrylic acid, which is a carboxylic acid. It is often available as its sodium salt, poly(methacrylic acid) sodium salt. The monomer is a viscous liquid with a pungent odour. The first polymeric form of methacrylic acid was described in 1880 by Engelhorn and Fittig. The use of high purity monomers is required for proper polymerization conditions and therefore it is necessary to remove any inhibitors by extraction or via distillation. To prevent inhibition by dissolved oxygen, monomers should be carefully degassed prior to the start of the polymerization.

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<span class="mw-page-title-main">Sequence-controlled polymer</span>

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<span class="mw-page-title-main">Graft polymer</span> Polymer with a backbone of one composite and random branches of another composite

In polymer chemistry, graft polymers are segmented copolymers with a linear backbone of one composite and randomly distributed branches of another composite. The picture labeled "graft polymer" shows how grafted chains of species B are covalently bonded to polymer species A. Although the side chains are structurally distinct from the main chain, the individual grafted chains may be homopolymers or copolymers. Graft polymers have been synthesized for many decades and are especially used as impact resistant materials, thermoplastic elastomers, compatibilizers, or emulsifiers for the preparation of stable blends or alloys. One of the better-known examples of a graft polymer is a component used in high impact polystyrene, consisting of a polystyrene backbone with polybutadiene grafted chains.

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Functionalized polyolefins are olefin polymers with polar and nonpolar functionalities attached onto the polymer backbone. There has been an increased interest in functionalizing polyolefins due to their increased usage in everyday life. Polyolefins are virtually ubiquitous in everyday life, from consumer food packaging to biomedical applications; therefore, efforts must be made to study catalytic pathways towards the attachment of various functional groups onto polyolefins in order to affect the material's physical properties.

β-Butyrolactone Chemical compound

β-Butyrolactone is the intramolecular carboxylic acid ester (lactone) of the optically active 3-hydroxybutanoic acid. It is produced during chemical synthesis as a racemate. β-Butyrolactone is suitable as a monomer for the production of the biodegradable polyhydroxyalkanoate poly(3-hydroxybutyrate) (PHB). Polymerisation of racemic (RS)-β-butyrolactone provides (RS)-polyhydroxybutyric acid, which, however, is inferior in essential properties to the (R)-poly-3-hydroxybutyrate originating from natural sources.

References

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  4. ↑ Janowicz, Andrew H. "Molecular weight control in free radical polymerizations" U.S. Patent 4,886,861 Issue date: Dec 12, 1989
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  8. ↑ Tang, L.; Norton, J. R. (2004). "Effect of Steric Congestion on the Activity of Chromium and Molybdenum Metalloradicals as Chain Transfer Catalysts during MMA Polymerization". Macromolecules. 37 (2): 241–243. Bibcode:2004MaMol..37..241T. doi:10.1021/ma035612t.
  9. ↑ Le Grognec, E.; Claverie, J.; Poli, R. (2001). "Radical polymerization of styrene controlled by half-sandwich Mo(III)/Mo(IV) couples: all basic mechanisms are possible" (PDF). J. Am. Chem. Soc. 123 (39): 9513–9524. doi:10.1021/ja010998d. PMID   11572671.
  10. ↑ Gibson, V. C.; O'Reilly, R. K.; Wass, D. F.; White, A. J. P.; Williams, D. J. (2003). "Polymerization of Methyl Methacrylate Using Four-Coordinate (α-Diimine)iron Catalysts: Atom Transfer Radical Polymerization vs Catalytic Chain Transfer". Macromolecules. 36 (8): 2591–2593. Bibcode:2003MaMol..36.2591G. doi:10.1021/ma034046z.
  11. ↑ Choi, J.; Norton, J. R. (2008). "Chain-transfer catalysis by vanadium complexes during methyl methacrylate polymerization". Inorg. Chim. Acta. 361 (11): 3089–3093. doi:10.1016/j.ica.2008.01.014.
  12. ↑ Asandei, A. D.; Saha, G. (2006). "Cp2TiCl-Catalyzed Epoxide Radical Ring Opening: A New Initiating Methodology for Graft Copolymer Synthesis". Macromolecules. 39 (26): 8999–9009. Bibcode:2006MaMol..39.8999A. doi:10.1021/ma0618833. S2CID   97128699.
  13. 1 2 Gridnev AA, Ittel SD (December 2001). "Catalytic chain transfer in free-radical polymerizations". Chemical Reviews. 101 (12): 3611–60. doi:10.1021/cr9901236. PMID   11740917.
  14. ↑ Gridnev AA, Ittel SD, Fryd M, Wayland BB (1993). "Formation of organocobalt porphyrin complexes from reactions of cobalt(II) porphyrins and dialkylcyanomethyl radicals with organic substrates: chemical trapping of a transient cobalt porphyrin hydride". Organometallics. 12 (12): 4871–4880. doi:10.1021/om00036a029.
  15. ↑ Tang, L.; Norton, J. R. (2006). "Factors Affecting the Apparent Chain Transfer Rate Constants of Chromium Metalloradicals: Mechanistic Implications". Macromolecules. 39 (24): 8229–8235. Bibcode:2006MaMol..39.8229T. doi:10.1021/ma061574c.
  16. ↑ de Bruin, B.; Dzik, W. I.; Li, S.; Wayland, B. B (2009). "Hydrogen-Atom Transfer in Reactions of Organic Radicals with [CoII(por)]. (por=Porphyrinato) and in Subsequent Addition of [Co(H)(por)] to Olefins". Chemistry: A European Journal. 15 (17): 4312–4320. doi:10.1002/chem.200802022. PMID   19266521.
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