Catalysts for GTP

It would be of interest to see if the more stable cesium carboxylates would be better catalysts for GTP than TBA carboxylates since Quirk showed that cesium 9-methylfluorenide works as well as TBA 9-methylfluorenide as a catalyst in his mechanism studies [6]. 

The preferred catalysts for GTP are nucleophilic anions. The most active catalysts are fluorides and bifluorides [1]. At above ambient temperatures, however, carboxylates and bicarboxlates are preferred [11]. A large counter ion is required for maximum efficiency. In the early work trisdimethylaminosul-fonium (TAS) was used, but later the more readily available tetrabutylam-monium (TBA) salts have gained favor. Since TBA slowly decomposes under the basic conditions used for GTP, other positive ions may work better. Quirk used cesium ion for his mechanistic studies and found it to be equivalent to TBA [6]. Bywater worked with the very stable Ph3PNPPh3+ bifluoride in his mechanistic probes [19] and Jenkins [21] showed that potassium com-plexed with 18-crown-6 was a possible alternative to TBA (Scheme 10). 

Ester Enolates Operate as Both Initiators and Catalysts for GTP... 

In a study that in itself almost lays to rest the associative mechanism for GTP, Quirk generated an ester enolate by addition of TBA 9-methylfluo-renide to MMA and used it as a catalyst for GTP of MMA initiated by MTS at 50 °C. The conversion was quantitative, the MWD was in the 1.2 range, and the molecular weight was controlled by the ratio of MTS to monomer [6]. In similar experiments without the MTS, conversions of only 14-52% were obtained, the MWD was broad and molecular weight control was lost. 

The fact that known anionic initiators for MMA can act as catalysts for GTP and the need for low amounts of catalysts in itself nearly puts to rest the associative mechanism. Seven of the other factors support the dissociative process. Except for the low temperature exchange studies, none supports the associative mechanism. Based on the lack of exchange of added silyl fluoride with silyl ketene acetal ends it looks like fluoride and bifluoride catalysts operate by irreversible generation of ester enolate chain ends [1] (Scheme 19b). On the other hand carboxylate catalysts appear to operate by reversible generation of ester enolate ends as evidenced by rapid exchange of silyl acetate with silyl ketene acetal ends [36] (Scheme 19c). 

Silyl ketene acetals are relatively stable species and require activation by a catalyst in order to initiate polymerization of a,3-unsaturated monomers. Numerous catalysts for GTP polymerizations have been examined " and these studies have revealed that bifluorides and bioxyanions such as tris(dimethylamino) sulfonium bifluoride (TASHF2) and tetra-n-butyl ammonium bibenzoate (TBABB), respectively, afford optimum polymerization characteristics. Note. Bifluoride based catalysts are not soluble in tetrahydrofuran (THF) and thus acetonitrile is used as the solvent in these cases in general TBABB is the optimum catalyst as it is readily soluble in THF and affords better control of molecular weight, conversion, and polydispersity.) GTP has been shown to be robust < 1.1) and is compatible with... 

Ester enolates operate as both catalysts and initiators for GTP... 

The original patent issued to Dupont describes several monomers for GTP [11]. However, methyl methacrylate (MMA) is the preferred monomer for most studies [12-16]. Typical examples of MMA polymerization using various anionic catalysts are shown in Table 1. 

GTP was employed for the synthesis of block copolymers with the first block PDMAEMA and the second PDEAEMA, poly[2-(diisopropylamino)e-thyl methacrylate], PDIPAEMA or poly[2-(N-morpholino)ethyl methacrylate], PM EM A (Scheme 33) [87]. The reactions took place under an inert atmosphere in THF at room temperature with l-methoxy-l-trimethylsiloxy-2-methyl-1-propane, MTS, as the initiator and tetra-n-butyl ammonium bibenzoate, TBABB, as the catalyst. Little or no homopolymer contamination was evidenced by SEC analysis. Copolymers in high yields with controlled molecular weights and narrow molecular weight distributions were obtained in all cases. The micellar properties of these materials were studied in aqueous solutions. 

In a process related to GTP, aldehydes initiate the polymerization of silyl vinyl ethers and silyl diene ethers. Here the silyl group is present in the monomer and transfers to the aldehyde ended chains regenerating aldehyde ends [17] (Scheme 8). A Lewis acid catalyst is required. terf-Butyldimethylsilyl works best as a transfer group for vinyl ether while trimethylsilyl is suitable for diene ethers [18]. Even though aldol GTP provides a route to polyvinyl alcohol segments in the subsequent block polymer synthesis, the projected cost of the monomers discouraged further research aimed at commercialization. 

Lewis acid catalysts were discussed in the acrylic monomer section. They are used at about 10% concentration vs monomer. Although no significant research has been done with them, other than looking for new ones, they most likely work by activating the monomers. Type Y zeolites catalyze GTP at about 25% concentration vs initiator. Conversions are quantitative and... 

The two main mechanistic routes for base catalyzed GTP under consideration are the dissociative pathway (Scheme 11) and the associative pathway (Scheme 12). In the dissociative route the nucleophilic catalyst complexes with the silyl ketene acetal end groups and in a reversible cleavage step gen-... 

In the absence of monomer, GTP catalysts are destroyed by the initiator. A one to one molar mixture of bifluoride and MTS, for example produces methyl isobuterate, trimethylsilylfluoride, trimethylsilylmethoxide, methyl 2,2,4-trimethyl-3-oxopentanoate, and other oligomers [19, 26]. Seebach has shown that ester enolates generate ketenes at room temperature [27] (Scheme 16). These reactions support the dissociative mechanism wherein... 

The stereochemistry of GTP of MMA polymerization was measured for Lewis acid as well as for bifluoride catalysis. Lewis acid catalysts gave a ratio of syndiotactic heterotactic triads of 2 1 while bifluoride catalysis gave ratios near 1 1 [9, 41]. The amount of isotactic triads was about 5%. The effect of temperature on triad and diad composition provided data to calculate the difference in activation enthalpy (AAH ) and activation entropy (AAS ) for... 

The fact that trimethylsilyl methacrylate is a sluggish monomer under GTP conditions [45, 46] also bodes well for a dissociative mechanism. The excess silyl carboxy groups are silylating enolate chain ends Thus lowering the rate of polymerization and changing the nature of the carboxylate catalyst (Scheme 23c). 

Inoue [56] has developed a method similar to GTP for polymerization of acrylic monomers. A methylaluminum porphyrin (MeAlTPP) is converted to a ketene acetal by in situ reaction of MMA and used to polymerize MMA (Scheme 24). A hindered Lewis acid catalyst is needed to activate the MMA. 

One main difference between anionic polymerization and GTP has to be found in the amount of enolates active in polymerization. In anionic polymerization, all the chains are end-capped by an enolate, which is the case for only a small part of the chains in GTP consistent with the very good control of GTP even at room temperature. In this respect, Brittain and Dicker showed that prop/ term is by far higher in GTP (250) than in classical anionic polymerization ( prop/ term = 8) . In line with slow termination compared to propagation in GTP, Bandermann and coworkers found that the amount of the nucleophilic catalyst is essential to the polymerization control. Indeed, as far as the tris(piperidino)sulfonium bifluoride-mediated GTP of MMA in THF is concerned, the polydispersity index increases with the amount of catalyst . 

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