Polymerization group-transfer

Group tranter polymerization offers another route to LAP of (meth)acrylates without resorting to low temperatures [Hertler, 1994, 1996 Muller, 1990 Quirk et al., 1993 Reetz, 1988 Schubert and Bandermann, 1989, Sogah et al., 1987, 1990 Webster, 1987, 1992, 2000]. The initiator is a silyl ketene acetal (XXIV) that is synthesized from an ester enolate  

Group transfer polymerization (GTP) requires either a nucleophiUc or Lewis acid catalyst. Bifluoride (HF ) and fluoride ions, supplied by soluble reagents such as tris(dimethylamino)-sulfonium bifluoride, [(CH3)2N]3SHF2, and (n-C4H9)4NF, are the most effective nucleophilic catalysts, although other nucleophiles (CN , acetate, p-nitrophenolate) are also useful. Zinc 

The mechanisms for nucleophihc GTP and electrophihc GTP are not the same. Electrophihc GTP proceeds by an associative or concerted mechanism that does not involve anionic propagating centers. Initiation involves a concerted addition of methyl trimethylsilyl dimethyl ketene acetal to monomer to form species XXV. The overah effect is to transfer 

Nucleophilic GTP proceeds by a dissociative mechanism in which the propagating centers are ionic—similar to other anionic polymerizations [Hertler, 1994, 1996 Quirk and Bidinger, 1989 Quirk et al., 1993 Webster, 1992, 2000]. The anionic propagating species XXVII is generated in low concentrations by nucleophihc displacement of the trimethylsilyl group by the nucleophilic catalyst (W+Nu )  

Control of the side reactions is achieved through two factors (1) reversible complexation of the anionic propagating species XXVII hy the silyl ketene acetal polymer chain ends XXVI maintains the concentration of the anionic propagating species at a low concentration and (2) the bulky counterion W+ (e.g., tetra-n-butylammonium, tris(dimethylamino)sulfonium) decreases the reactivity of the anionic propagating centers toward the terminating side reactions. 

This technology offers considerable promise for conunercial preparations of living polymers of methyl methacrylate without resorting to low temperature anionic polymerizations. Although the mechanism or polymerization is not completely explained, the propagation is generally believed to be covalent in character. A silyl ketene acetal is the initiator. It forms from an ester enolate [391]  

The initiation, that is catalyzed by either a nucleophUic or by a Lewis acid catalyst, was explained as consisting of a concerted attack by the ketene acetal on the monomer [392]  

This results in a transfer of the silyl ketene acetal center to the monomer. The process is repeated in each step of the propagation. The ketone double bond acts as the propagating center [391]. 

Difunctional initiators cause chain growth to proceed from each end [395]. Because group-transfer polymerizations are living polymerization, once all the monomer has been consumed, a different monomer can be added and block copolymers can be formed. 

Water and compounds with active hydrogen must be excluded from the reaction medium. Oxygen, on the other hand, does not interfere with the reaction. Tetrahydrofuran, acetonitrile, and aromatic solvents are commonly used in polymerizations catalyzed by nucleophiles. Chlorinated solvents and dimethylformamide are utilized in many reactions catalyzed by electrophiles. Living polymerizations of methacrylate esters can be carried out at 0-50°C. The acrylate esters, however, require temperatures below 0°C for living, group-transfer polymerizations, because they are more reactive and can undergo side reactions. 

The most effective nucleophilic catalysts for this reaction are bifluoride (HF2) and fluoride ions. They can be generated from soluble reagents like tris(dimethylamino)sulfonium bifluoride. Other 

Propagation is believed to proceed via the following mechanism in which the catalyst (Cat) activates transfer of the trimethylsilyl group by association with the silicon atom 

Whilst polymers have been prepared from acrylates, acrylonitrile and other similar monomers, group transfer polymerization is most suited to the preparation of low molar mass ( 50kgmol ) functionalized methacrylate homopolymers and copolymers. 

More recently the workers at Du Pont have developed aldol group transfer polymerization, in which a silyl vinyl ether is polymerized using an aldehyde as the initiator to give a living silylated poly(vinyl alcohol), e.g. 

Thus the active species is an aldehyde group and propagation is believed to proceed via a mechanism of the type 

As well as viscosity, other factors to be aware of include the purity of the ionic liquids. The presence of residual halide ions in neutral ionic liquids can poison transition metal catalysts and different levels of proton impurities in chloroalumi-nate(iii) ionic liquids can alter the product distribution of the reaction. The reduced temperatures required for many polymerization reactions in ionic liquids, together with the reduced solubility of oxygen in ionic liquids compared to conventional solvents, means that two of the most common quenching methods are reduced in effectiveness. When detailed studies are being carried out, in particular kinetic studies, it is necessary to completely stop further reaction so that accurate data are obtained. 

One of the common observations across several of the techniques used is that the precipitation of the polymer product from the ionic liquid terminates the reaction. The advantage of this is that it leads to narrower polydispersities than are often found. However, it can lead to lower molecular weights than other methodologies. With the synthetic flexibility that ionic liquids have it may prove possible to manipulate the solubility of polymers in the ionic liquids in a sufficiently controlled 

Weissermel, H.-. Arpe, Industrial Organic Chemistry, VCH, Weinheim, 

Biedron. M. Bednarek, P. Kubisa, Macromol. Rapid Commun. 2004, 25, 

Solomon, The Chemistry of Free Radical Polymerization, Pergamon, Oxford, 1995. 

In the 1980s, the DuPont Company developed and patented [10] a new type of polymerization that mechanistically is similar to anionic polymerization. Group-transfer polymerization (GTP) has been defined as polymerization of a,/3-unsaturated esters, ketones, nitriles, or amides, initiated by silyl ketene acetals [11]. It has most commonly been used to polymerize acrylate and methacrylate monomers with the aid of anionic catalysts (they are true catalysts here), such as the bifluoride ion, [FHF] , or bioxyanions. GTP is illustrated below for the polymerization of methyl methacrylate (MMA) with silyl ketene acetal (SKA)  

The initiating functionality is transferred to the growing end of the chain as each new monomer unit is added. They are living chains as in anionic addition and can likewise be used to produce monodisperse polymers, block copolymers, and, with the addition of appropriate reagents, chains with desired terminal groups. They can also control stereoregularity in the chain. 

Unlike with anionic addition, chain transfer can occur  

As in anionic addition polymeri23tion, chain length can be reduced by using more initiator, but because these initiators are rather expensive, it is often preferable to use a chain-transfer agent instead. Also, at low monomer concentrations, termination can occur through cyclization of the chain end. 

This has recently been described as a new method of polymerizing a. /8-unsaturated esters, amides and nitriles with organosilicon derivatives of ketene as initiators and Lewis acids or bases as catalysts. The products formed have the characteristics of living polymers [9]. 

for example. Terminology of Plastics and Rubber. Czechoslovak State Standard C SN 640 001, 

2 1UPAC, Macromolecular Division, Nomenclature Commission, Basic Definitions of Terms Relating to Polymers. Pure Appl. Chem.. 40 (1974) 477. 

3 1UPAC, Macromolecular Division, Commission on Macromolecular Nomenclature. Source-Based Nomenclature for Copolymers, Pure Appl. Cliem.. 57 (1985) 1427. 

Szwarc, Carbanions, Living Polymers and Electron Transfer Processes, Interscience, Wiley, New York, 1968. 

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A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

The range of uses of mercuric iodide has increased because of its abiUty to detect nuclear particles. Various metals such as Pd, Cu, Al, Tri, Sn, Ag, and Ta affect the photoluminescence of Hgl2, which is of importance in the preparation of high quaUty photodetectors (qv). Hgl2 has also been mentioned as a catalyst in group transfer polymerization of methacrylates or acrylates (8). 

Polymerization of methacrylates is also possible via what is known as group-transfer polymerization. Although only limited commercial use has been made of this technique, it does provide a route to block copolymers that is not available from ordinary free-radical polymerizations. In a prototypical group-transfer polymerization the fluoride-ion-catalyzed reaction of a methacrylate (or acrylate) in the presence of a silyl ketene acetal gives a high molecular weight polymer (45—50). 

The anionic polymerization of methacrylates using a silyl ketene acetal initiator has been termed group-transfer polymerization (GTP). First reported by Du Pont researchers in 1983 (100), group-transfer polymerization allows the control of methacrylate molecular stmcture typical of living polymers, but can be conveniendy mn at room temperature and above. The use of GTP to prepare block polymers, comb-graft polymers, loop polymers, star polymers, and functional polymers has been reported (100,101). 

Group-Transfer Polymerization. Du Pont has patented (29) a technique known as group-transfer polymerization and appHed it primarily to the polymerization of acrylates and methacrylates. It is mechanistically similar to anionic polymerization, giving living chains, except that chain transfer can occur (30). 

Group-Transfer Polymerization. Living polymerization of acrylic monomers has been carried out using ketene silyl acetals as initiators. This chemistry can be used to make random, block, or graft copolymers of polar monomers. The following scheme demonstrates the synthesis of a methyl methacrylate—lauryl methacrylate (MMA—LMA) AB block copolymer (38). LMA is CH2=C(CH2)COO(CH2) CH2. 

GTP is a safe operation. A runaway polymerization can be quickly quenched with a protonic solvent. Since the group transfer polymerization goes to completion, no unwanted toxic monomer remains the silicone group on the living end after hydroxylation is removed as inactive siloxane. The living polymer in GTP is costlier than traditional polymerization techniques because of the stringent reaction conditions and requirements for pure and dry monomers and solvents. It can be used in fabrication of silicon chips, coating of optical fibers, etc. 

The previous two systems resemble in some way the interesting group-transfer polymerization discovered by the DuPont team 13). The initiator, asilyl ketene acetal, l,... 

These TMS-carbamate-mediated NCA polymerizations resemble to some extent the group-transfer polymerization (GTP) of acrylic monomers initiated by organo-silicon compounds [40]. Unlike GTPs that typically require Lewis acid activators or nucelophilic catalysts to facilitate the polymerization [41], TMS-carbamate-mediated NCA polymerizations do not appear to require any additional catalysts or activators. However, it is still unclear whether the TMS transfer proceeds through an anionic process as in GTP [41] or through a concerted process as illustrated in Scheme 14. 

Webster OW, Hertler WR, Sogah DY, Eamham WB, Rajanbabu TV (1983) Group-transfer polymerization. 1. A New concept for addition polymerization with organo-silicon initiators. J Am Chem Soc 105 5706-5708... 

Webster OW (2004) Group transfer polymerization mechanism and comparison with other methods for controlled polymerization of acrylic monomers. In New synthetic methods. Advances in polymer science, vol 167. Springer, Berlin, pp 1-34... 

Synthesis of Block Copolymers by Group Transfer Polymerization, GTP... 

The controlled polymerization of (meth)acrylates was achieved by anionic polymerization. However, special bulky initiators and very low temperatures (- 78 °C) must be employed in order to avoid side reactions. An alternative procedure for achieving the same results by conducting the polymerization at room temperature was proposed by Webster and Sogah [84], The technique, called group transfer polymerization, involves a catalyzed silicon-mediated sequential Michael addition of a, /f-unsaluralcd esters using silyl ketene acetals as initiators. Nucleophilic (anionic) or Lewis acid catalysts are necessary for the polymerization. Nucleophilic catalysts activate the initiator and are usually employed for the polymerization of methacrylates, whereas Lewis acids activate the monomer and are more suitable for the polymerization of acrylates [85,86]. 

Webster, O. IV Group Transfer Polymerization Mechanism and Comparison with Other Methods of Controlled Polymerization of Acrylic Monomers. Vol. 167, pp. 1-34. 

Webster OW, Anderson BC (1992) Group transfer polymerization, in new methods for polymer synthesis. Mijs WJ Ed, Plenum, p.l... 

The anionic polymerization of methylmethacrylate at room temperature (originally called group transfer polymerization) [75-77] has provided a means for preparing star poly(methylmethacrylate) via the block polymerization with ethyleneglycoldimethacrylate ... 

Group transfer polymerization was employed by Herder and coworkers [348] as well as Huber et al. [349]. 

There are many examples where commercial success of a new product and process are not achieved even when all the elements of an industrial business team are pulling together. The latest appears to be Group Transfer Polymerization. This is unfortunate for it is a beautiful piece of technology, but it is understandable from the laws of industrial polymer science presented previously. 

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