Lewis acids, living polymerization

Recently, it was found that Lewis acid assisted polymerizations of methyl methacrylate with aluminum porphyrin initiators yield living polymers. The polymerizations of methacrylate esters with al laluminum porphyrin initiators occur through formations of enolate aluminum porphyrin intermediate as the growing species. For the sake of illustration, the methylaluminum porphyrin molecules (see Fig. 3.3) can be designated as... 

Our research has focused on polymerization reactions that employ nonolefin monomers, more specifically the polymerization of yhdes and diazoalkanes. The carbon backbone is built one carbon at a time (Cl polymerization). These studies draw upon earlier investigations of the Lewis-acid-catalyzed polymerization of diazoalkanes (Imoto and Nakaya, 1972 Maria and Bernhard, 1974 Bawn et al, 1959) and build upon our discovery of the trialky Iborane-initiated living polymerization of dimethylsulfoxonium methylide 1 (Shea et al., 1997). 

The success of the HI/I, system suggested the effectiveness of a combination of a nudeophilic counteranion and a relatively mild Lewis add for living polymerization. Thus, cationic polymerization of alkyl VEs was examined using various protonic adds (or an addud of a VE with a protonic acid) with weak Lewis adds. Living polymerization was achieved using hydrogen halides " and acetic adds in conjunction with zinc... 

Lewis acid effects 435 1UPAC recommendations copolymer depiction 335 living polymerization 452 polymer structure 2... 

However, in the presence of a suitable Lewis base the polymerization becomes living, due to the nucleophihc stabilization of the growing cation generated by the added base. (3) Initiator, strong Lewis acid and onium salt as additive The previous method cannot be easily applied in polar media. In this case the living cationic polymerization is promoted by the addition of salts with nucleophihc anions, such as ammonium and phosphonium derivatives. 

The rate of polymerization may be dramatically accelerated upon addition of a bulky Lewis acid. For example, addition of (184) to a sample of living PMMA generated by irradiation of (181)/MMA causes an increase in polymerization rate by a factor of >45,000.444 The dualcomponent systems (181)/(184), and (181)/(185), have been used to prepare monodisperse, ultra-high-molecular-weight samples of PMMA (Mn > 106, Mw/Mn= 1.2).445... 

The first report on living carbocationic surface-initiated polymerization (LCSIP) using a defined surface modification is by Vidal and Kennedy [268-270]. They prepared poly(isobutene) (PIB) brushes from silica surfaces using a silane functionalized benzylchloride activated by a Lewis acid. 

Fig, 4, Schematic illustration of high-speed living polymerization of methacrylate esters accelerated by steric separation of the aluminum porphyrin nucleophile and bulky Lewis acid. ... 

Group-transfer polymerizations make use of a silicon-mediated Michael addition reaction. They allow the synthesis of isolatable, well-characterized living polymers whose reactive end groups can be converted into other functional groups. It allows the polymerization of alpha, beta-unsaturated esters, ketones, amides, or nitriles through the use of silyl ketenes in the presence of suitable nucleophilic catalysts such as soluble Lewis acids, fluorides, cyanides, azides, and bifluorides, HF. ... 

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

Keywords Anionic polymerization. Living Polymerization, Immortal polymerization, Metalloporphyrin, Lewis acid... 

One-Shot Lewis Acid Promoted Living Polymerization... 

Of key importance in the Lewis acid promoted living anionic polymerization of methacrylic esters with aluminum porphyrin is how to suppress the undesired reaction between the nucleophile (2j ) and the Lewis acid, leading to termination of polymerization (Fig. 11). As mentioned in previous sections, one of our approaches was to make use of sterically crowded Lewis acids such as methyla-luminum bis(ort/zo-substituted phenolates). This section focuses attention on the steric bulk of the nucleophile component (2 ), by using strategically designed aluminum porphyrins and some other methacrylates, for the purpose of understanding the scope and limitation of this method (Fig. 12). 

Fig. 11. The basic concept of the Lewis acid assisted high-speed living anionic polymerization of methacrylic esters...

For Lewis acid promoted living polymerization of MMA with (TPP)AlMe (1,X= Me) as initiator, a photoinitiation prior to the addition of the Lewis acid is required. This is because (1) 1 (X=Me) without irradiation does not have the ability to initiate the polymerization even in the presence of Lewis acid, and (2) all-at-once polymerization by direct irradiation of a mixture of MMA, 1 (X=Me), and the Lewis acid results in the formation of a relatively broad MWD PMMA with Mn much higher than expected. In this sense, the procedure using 1 (X= Me) as initiator is not convenient for practical application. In this section, we report on aluminum porphyrins with various axial ligands which were tested as initiators in order to realize a more convenient, one-shot high-speed living polymerization of methyl methacrylate with no need for irradiation with visible light. 

For the anionic polymerization of methacrylonitrile (MAN), many initiators have been developed, which include alkali-metal alkyls such as butyllithium [42], triphenylmethylsodium [43], phenylisopropylpotassium [43], the disodium salt of living a-methylstyrene tetramer [44], alkali-metal amides [45], alkoxides [46], and hydroxide [47], alkali metal in liquid NH3 [48], quaternary ammonium hydroxide [49], and a silyl ketene acetal coupled with nucleophilic or Lewis acidic catalysts [50]. However, only a single example of the synthesis of PMAN with narrow molecular-weight distribution can be cited, and the reported number-average molecular weights were much higher than those calculated from the stoichiometry of the butyllithium initiator [42]. 

As mentioned above, the new method Lewis acid promoted living polymerization of methacrylic esters, by using enolatealuminum porphyrin (2) as nucleophilic initiator in conjunction with organoaluminum compounds, such as methylaluminum bis(2,6-di-tert-butyl-4-methylphenolate) (3e), as Lewis acids has enabled us to synthesize poly(methacrylic ester) of narrow molecular-weight distribution [51]. On the other hand, some reactions of aluminum por-... 

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