Internal Lewis base

Upon oxidation, alkoxy bridges seem to be invariably formed and this may be viewed as an internal Lewis base association reaction in response to the increased Lewis acidity of the metal ions which accompanies the changes (MiM)6 (M=M)8 ... 

The role of Lewis bases in MgCl2-supported Ziegler-Natta catalysts for x-olefin polymerisation is still a subject of continuous interest. Recently [67], the use of 1,3-diethers [of the general formula R1OCH2C(R2)(R3)CH20R4], e.g. 2,2-diisobutyl-l,3-dimethoxypropane, as the internal Lewis base during... 

One has to realise the extreme chemical complexity of catalysts such as MgCl2/internal Lewis base/TiCU AlEt3/external Lewis base. Moreover, the... 

The formation of heterocycles 25-35, which are internal Lewis base-borane complexes, is dependent on the borane substituents. An increase in the Lewis acidity of the borane can result in ring formation. For example, bis(methoxy)borane 101 has no B-P bond, but treatment with boron trichloride results in the formation of 1,2-phosphoniaboratole 156, presumably via 155 (Scheme 12) <1997ZFA1093>. 

The enhancement in reactivity and selectivity induced by an internal Lewis base is such that -PrMgCl and n-BuMgCl, normally significantly less reactive than EtMgCl, can be used in the metal-catalyzed process with the more reactive bicyclic homoallylic alcohols and ethers shown in Table 4.5. Whereas reactions of exo-5-norbornen-2-ol with -PrMgCl and -BuMgCl yield <15% of the two e.vo alkyl isomers non-selectiveiy, enrfo-5-norbornen-2-ol (4 equiv of /i-alkylMgCl, 10 mol % Cp.ZrCL, 25°C)... 

Lewis base is called the internal Lewis base. This is followed by addition of Group I-in metal-alkyl component with an additional Lewis base. The base that is added the second time may be the same or a different one from that used in the original milling. In either case it is called the external Lewis base. It is not unconunon to use an ester as the internal base and an oiganosilane compound, like phenyltriethoxysilane, as the external one. 

On the other hand, the traditional catalytic system designed by Natta, i.e. TlCl,-AlRjCl, is now to be replaced by the more active catalytic system supported on NgClj, including a solid precatalyst with the support, TiCl and a so-called internal Lewis base (ILB), and a cocatalytic solution with AIR,and an external Lewis base (ELB). Two main families of these supported catalytic systems are to be considered, according to the nature of the Lewis bases. In the most studied family, the two Lewis bases are aromatic esters in a more recent, but more efficient family, the ILB is an aromatic diester. 

Internal Lewis bases (ILB) and external Lewis bases (ELB) are both necessary with the supported Ziegler-Natta catalysts to achieve the requirements of high activity and stereospecificity. The ILB is not directly involved in the active center. During the catalyst preparation the ILB masks the magnesium chloride surface, blocking the acidic defects. The catalyst is then formed by a selective replacement of the weakest bonded ILB by the titanium tetrachloride, giving thus access to low acidic active centers. The scheme can be considered as an active center selection. High activity is observed when a dynamic selection process results in an accumulation of trapped defects. 

Catalysts detailed preparation of a catalyst containing dlbutylphtalate (DBP) as an internal Lewis base have been given earl ier The preparation comprises 4 steps ... 

All the catalysts comprising an internal Lewis base have been prepared according to the scheme. The molar ratios used are MgClj/OBP = 16 in step (2) and TiCl /DBP - 1 in step (3). 

Table 9. Gas phase ethylene butene polymerization (85 C 10 bar total pressure) effect of butene content and ELB on the melt-flow ratio. The solid catalyst used cotnains dlbutylphtalate as an internal Lewis base.

Addition of suitable internal Lewis bases to the catalytic systems was found to cause a strong increase of the content in isotactic sequences in the polymerization of propene. Indeed, it is admitted that the crystal defects where the active sites lie exhibit various degrees of Lewis acidities, depending upon their position on the crystal edge. The most acidic sites, which are also the nonstereospecific ones, would be inhibited upon addition of a weak Lewis base (ester, ether, amine, etc.), leaving unaffected the less acidic isospecific sites. 

The mode of action of external Lewis base—so called because it is added to the catalytic system together with the organoaluminum cocatalyst—is more difficult to account for. This base generally consists of a dialkoxysilane molecule, and its role would be to inhibit certain nonspecific sites in addition to the internal Lewis base or transform some of them into isospecific ones by the steric effect. It is admitted that it coordinates simultaneously to a titanium atom and an aluminum atom via its two oxygen atoms, thus corroborating the Rodriguez and Van Looy mechanism. 

---