Trimer alkoxide initiator

Figure 3. HFPO polymerization using cesium HFPO trimer alkoxide initiator...

In the presence of chiral lithium alkoxides as initiators, chloral forms completely isotactic, crystalline polymers that are insoluble in all solvents (Scheme 71) (166). The polychloral film displays rotation values as high as 3000-5000°. The stereoregular helicity comes about at the stage of the trimer (167). A pure enantiomer of a tert-butoxy-initiated, acetate end-capped pentamer of chloral, which has 4r-helical conformation in both chloroform solution at 35 °C and in the crystalline state, can be obtained by HPLC resolution on a chiral stationary phase (168). 

Phenylsodium is formed initially in the reaction between benzonitrile and sodium (Scheme 79) (59JOC208). Trimerizations of aryl or heterocyclic nitriles catalyzed by amines or alkoxides resemble the reactions of the perfluoronitriles described above (Schemes 77 and 78 respectively). Kurabayashi et al. (71BCJ3413) studied the trimerization of benzonitrile under pressure in methanol, and have shown that the rate determining step is the reaction of the benzimino ether with benzonitrile. Steric factors exert an important influence in the trimerizations of aryl cyanides. The cyclizations of ort/zo-substituted aryl cyanides need more severe conditions than either the corresponding meta or para derivatives (Table 14). 

Reactions with Isocyanates, TYZOR TPT catalyzes the trimerization of isocyanates and polyisocyanates to isocyanurates and polyisocyanurates (38). Titanium alkoxides of the type Cl3TiOR initiate the living polymerization of isocyanates. Polyisocyanates possessing controlled molecular weights and narrow polydispersities can be synthesized using these catalysts (39) ... 

Attempts to prepare alkyl cyanates 17 from alkoxides and cyanogen halides led to 2,4,6-tri-alkoxy-l,3,5-triazines 18.238 240 The alkyl cyanates are assumed to be the initial intermediates, which add another alcohol molecule to form dialkyl imidocarbonates. These then trimerize to the 2,4,6-trialkoxy-l,3,5-triazines241 (cf. Houben-Weyl, Vol. E4, p925ff). 

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. 

However, the trimeric form, although amorphous with the zinc atoms coordinated tetrahedrally, was a relatively low-activity initiator apparently having no readily available coordination sites for monomer. The trimeric form and higher associated forms of alkoxide tend to form with time as the alkoxide, formed in situ, associates or flocculates on standing, and it was this observation that was believed to explain the reduction in activity observed with some initiators on standing for some time before use. 

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