Catalytic chain transfer
{{Short description|Chemical process}}
Catalytic chain transfer (CCT) is a process that can be incorporated into radical polymerization to obtain greater control over the resulting products.
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.{{cite journal|author1=Enikolopyan, N. S. |author2=Smirnov, B. R. |author3=Ponomarev, G. V. |author4=Belgovskii, I. M. |title = Catalyzed chain transfer to monomer in free radical polymerization|journal = Journal of Polymer Science: Polymer Chemistry Edition|year = 1981|volume = 19| pages = 879–889|doi=10.1002/pol.1981.170190403|issue = 4|bibcode = 1981JPoSA..19..879E }}{{cite journal |title =Historic perspective| author = Gridnev, A. J.| journal = J. Polym. Sci. A Polym. Chem.|year = 2000|volume = 38|issue=10|page = 1753|doi=10.1002/(SICI)1099-0518(20000515)38:10<1753::AID-POLA600>3.0.CO;2-O |bibcode = 2000JPoSA..38.1753G |doi-access = free}} Later investigations showed that the cobalt dimethylglyoxime complexes were as effective as the porphyrin catalysts and also less oxygen sensitive.{{ref patent|inventor= Melby, Lester Russell; Janowicz, Andrew Henry; Ittel, Steven Dale|number=199436|country=EP|pridate=1986 }}Janowicz, Andrew H. "Molecular weight control in free radical polymerizations" {{US patent|4886861}} Issue date: Dec 12, 1989 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
File: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).{{cite journal|doi = 10.1021/ma061643n|author1=Wayland, B. B. |author2=Peng, C.-H. |author3=Fu, X. |author4=Lu, Z. |author5=Fryd, M. |title = Degenerative Transfer and Reversible Termination Mechanisms for Living Radical Polymerizations Mediated by Cobalt Porphyrins|journal = Macromolecules |year = 2006|volume = 39|pages = 8219–8222|issue = 24|bibcode = 2006MaMol..39.8219W }}
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.{{cite journal|author = Poli, R.|title = Minireview|journal = Angew. Chem. Int. Ed.|year = 2006|volume = 45|issue = 31|pmid = 16821230|pages = 5058–5070|doi = 10.1002/anie.200503785| url=https://hal.archives-ouvertes.fr/hal-03195911/file/224-10.1002-anie.200503785-Accepted.pdf }} Various transition metal species, including complexes of Cr(I),{{cite journal|doi = 10.1021/ma9914523|author1=Abramo, G. P. |author2=Norton, J. R. |title = Catalysis by C5Ph5Cr(CO)3• of Chain Transfer during the Free Radical Polymerization of Methyl Methacrylate|journal = Macromolecules |year = 2000|volume = 33|pages = 2790–2792|issue = 8|bibcode = 2000MaMol..33.2790A }}{{cite journal|doi = 10.1021/ma035612t|author1=Tang, L. |author2=Norton, J. R. |title = Effect of Steric Congestion on the Activity of Chromium and Molybdenum Metalloradicals as Chain Transfer Catalysts during MMA Polymerization |journal = Macromolecules|year = 2004|volume = 37|pages = 241–243|issue = 2|bibcode = 2004MaMol..37..241T }} Mo(III),{{cite journal|doi = 10.1021/ja010998d|author1=Le Grognec, E. |author2=Claverie, J. |author3=Poli, R. |title = Radical polymerization of styrene controlled by half-sandwich Mo(III)/Mo(IV) couples: all basic mechanisms are possible |journal =J. Am. Chem. Soc.|year = 2001|volume = 123|issue = 39|pages = 9513–9524|pmid = 11572671|bibcode=2001JAChS.123.9513L |url=https://hal.archives-ouvertes.fr/hal-03294509/file/160-10.1021-ja010998d-Accepted.pdf }} Fe(I),{{cite journal|doi = 10.1021/ma034046z|author1=Gibson, V. C. |author2=O'Reilly, R. K. |author3=Wass, D. F. |author4=White, A. J. P. |author5=Williams, D. J. |title = Polymerization of Methyl Methacrylate Using Four-Coordinate (α-Diimine)iron Catalysts: Atom Transfer Radical Polymerization vs Catalytic Chain Transfer|journal =Macromolecules|year = 2003|volume = 36|pages = 2591–2593|issue = 8|bibcode = 2003MaMol..36.2591G }} V(0),{{cite journal|doi = 10.1016/j.ica.2008.01.014|author1=Choi, J. |author2=Norton, J. R. |title = Chain-transfer catalysis by vanadium complexes during methyl methacrylate polymerization|journal = Inorg. Chim. Acta|year = 2008|volume = 361|pages = 3089–3093|issue = 11}} Ti(III),{{cite journal|doi = 10.1021/ma0618833|author1=Asandei, A. D. |author2=Saha, G. |title = Cp2TiCl-Catalyzed Epoxide Radical Ring Opening: A New Initiating Methodology for Graft Copolymer Synthesis|journal = Macromolecules|year = 2006|volume = 39|pages = 8999–9009|issue = 26|bibcode = 2006MaMol..39.8999A |s2cid=97128699 }} and Co(II){{cite journal | vauthors = Gridnev AA, Ittel SD | title = Catalytic chain transfer in free-radical polymerizations | journal = Chemical Reviews | volume = 101 | issue = 12 | pages = 3611–60 | date = December 2001 | pmid = 11740917 | doi = 10.1021/cr9901236}}{{cite journal|vauthors = Gridnev AA, Ittel SD, Fryd M, Wayland BB |title = 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| journal =Organometallics|year = 1993|volume = 12|pages = 4871–4880|issue = 12|doi = 10.1021/om00036a029}}{{cite journal|doi = 10.1021/ma061574c|author1=Tang, L. |author2=Norton, J. R. |title = Factors Affecting the Apparent Chain Transfer Rate Constants of Chromium Metalloradicals: Mechanistic Implications|journal = Macromolecules|year = 2006|volume = 39|pages = 8229–8235|issue = 24|bibcode = 2006MaMol..39.8229T }} have been demonstrated to control molecular weights in radical polymerization of olefins.
File: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.{{cite journal|doi = 10.1002/chem.200802022|author1=de Bruin, B. |author2=Dzik, W. I. |author3=Li, S. |author4=Wayland, B. B |title = Hydrogen-Atom Transfer in Reactions of Organic Radicals with [CoII(por)]. (por=Porphyrinato) and in Subsequent Addition of [Co(H)(por)] to Olefins|pmid = 19266521| journal = Chemistry: A European Journal|year = 2009|volume = 15|issue=17|pages = 4312–4320}}{{cite journal|vauthors = Gridnev AA, Ittel SD, Fryd M, Wayland BB |title = Isotopic Investigation of Hydrogen Transfer Related to Cobalt-Catalyzed Free-Radical Chain Transfer|journal = Organometallics|year = 1996|volume = 15|issue=24|page =5116|doi = 10.1021/om960457a}}
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 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.{{Cite web|title=Espacenet – search results|url=https://worldwide.espacenet.com/patent/search?q=pn=WO9504759A1|access-date=2021-10-05|website=worldwide.espacenet.com}} The vinylic end group acts as an addition fragmentation agent and has been utilised to make multi block copolymers{{Cite journal |last1=Engelis |first1=Nikolaos G. |last2=Anastasaki |first2=Athina |author-link2=Athina Anastasaki |last3=Nurumbetov |first3=Gabit |last4=Truong |first4=Nghia P. |last5=Nikolaou |first5=Vasiliki |last6=Shegiwal |first6=Ataulla |last7=Whittaker |first7=Michael R. |last8=Davis |first8=Thomas P. |last9=Haddleton |first9=David M. |date=February 2017 |title=Sequence-controlled methacrylic multiblock copolymers via sulfur-free RAFT emulsion polymerization |url=https://www.nature.com/articles/nchem.2634 |journal=Nature Chemistry |language=en |volume=9 |issue=2 |pages=171–178 |doi=10.1038/nchem.2634 |pmid=28282058 |s2cid=3418399 |issn=1755-4349}} and derivatives used as stress relief agents in dental restoration by 3M.{{Cite web|title=Espacenet – search results|url=https://worldwide.espacenet.com/patent/search/family/046924527/publication/EP2748206A1?q=pn=EP2748206A1&queryLang=en:de:fr|access-date=2021-10-05|website=worldwide.espacenet.com|language=en}}