Aluminium isopropoxide, initiation

When using the aluminium isopropoxide catalyst to promote the polymerisation of lactide, all three isopropoxide groups have been found to initiate the polymerisation [137,139]. Ring cleavage in the polymerisation with aluminium isopropoxide and diethylaluminium isopropoxide occurred at the C(0)-0 bond, leading to aluminium alcoholate propagating species [139], Lactide polymerisation with other catalysts containing multinuclear species proceeded similarly [138-140]. Analysis of stereosequences of lactide polymers obtained with catalysts such as aluminium isopropoxide, diethylaluminium alcoholate and methylaluminoxane indicates that none of these catalysts favours the formation of isotactic blocks [139]. 

Aluminium isopropoxide was used to initiate L- and DJ>-lactide homopolymerization at 70 °C. All alkoxide groups were found to initiate polymerization. The presence of the different aggregates does not influence the polymerization to a large extent as in the case of e-caprolactone, probably because reactions are carried out at higher temperatures and the reactivity of lactide in polymerization is lower than that of e-caprolactone. 

Degee, P., Dubois, P. and Jerome, R. (1997) Bulk polymerization of lactides initiated by aluminium isopropoxide, 3 Thermal stabUity and viscoelastic properties. Mactomolecular Chemistry and Physics, 198,1985-1995. 

Recently, Varela et al. [73] obtained similar results from 2,3,4,5-tetra-O-methyl-D-galactono-1,6-lactone. The homopolymerization of this lactone, promoted by aluminium isopropoxide [Al(0 Pr)3] or scandium triflate [Sc(OTf)3], was attempted, but failed in both cases. The copolymerization with -caprolactone, using scandium triflate as initiator, gave a low-molecular weight copolymer ( 3000) with an incorporation of the sugar to the polymeric chain of about 10 per cent, as estimated by NMR. 

Aluminium alkoxides have also been utilized as efficient catalysts for the lactide polymerization. The most popular one is aluminium isopropoxide (Al(0 Pr)3). Duda verified that Al(0 Pr)3 exists in two types of aggregates a trimer (A3) and a tetramer (A4) as shown in Scheme 1.9. The A3, consisting of penta-coordinate aluminium ion, can initiate the lactide polymerization whilst A4, having hexa-coordinate aluminium ion, is ineffective for the ROP. Since A4 is equilibrated with A3, the ROP of lactide is slowly initiated in the presence of A4. All of the alkojq l ligands in the aluminium alkoxides are involved in the initiation because interligand exchange of the propagating metal alkoxides is fast. 

Table 3 presents the results obtained when the Ti02/(Ti02+Al203) molar ratio (R) is varied from 0.0 to 1.0. The experiments in this table were performed by the method B of preparation using a 0.52 M. ammonium carbonate concentration, 150 ml of water to produce the hydrolysis of the titanium and aluminium isopropoxides and at a pH of 9. It is possible to observe in table 3 that when the ratio R is increased, the surface area is increased initially from 319 m /g (R= 0.0) to 394 m /g (R= 0.5) and then decreased continuously to 42 rtfi/g at R= 1.0. On the other hand, the cumulative pore volume did not change much between R= 0.0 to R= 0.5 and then decreased sharply at R= 0.75 and R= 1.0. The mean pore diameter dropped from 142 A (R= 0.0) to around 96 A for all other values of R. 

Lou, X.D., Detrembleur, C., Lecomte, P., Jerome, R., 2002. Two-step backbiting reaction in the ring-opening pol3fmerization of gamma-acryloyloxy-epsilon-caprolactone initiated with aluminium isopropoxide. Macromol. Rapid Commun. 23, 126—129. 

Another popular metal-based initiator-catalyst is aluminium tri-isopropoxide. While aluminium-based compounds are less active compared to other catalysts, they offer good control over the polymerisation. This occurs through a living polymerisation, a chain polymerisation where no termination or chain transfer occurs, resulting in well-defined polymers (Quirk and Lee, 1992). 

Contrary to the facile straightforward reactions of ethoxides and isopropoxides of different metals according to Eq. (2.268), an entirely different comse of reaction, resulting in the final formation of metal acetate, can be further illustrated by the reaction of aluminium tertiary butoxide with acetyl chloride. In this case, the first stage of the reaction is fast, but the aluminium monochloride di-fert-butoxide formed initially reacts with tertiary butylacetate also formed during the reaction, to produce the corresponding mixed alkoxide-acetate ... 

The proposed structure has been supported by the observation that only five isopropoxide groups [(1) to (5)] are replaced by treatment with excess Et3COH, whereas on treatment with the less sterically demanding species Mc3COH, eight isopropoxide groups, (1) to (5) initially, then (6), (7), and (8), are finally replaced, when each of the three aluminium atoms is left bonded with three tertiary butoxy and one isopropoxy group. 

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