Ti-X bonds

The broken bonds (boldface = dissociated fragment) BDEs (boldface = recommended data reference in parentheses) Methods (reference in parentheses) References 

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CpTiCl2X (X = NMe2, OEt) have been found to initiate the living polymerization of carbodiimides with reduced activity. The reactivity is largely determined by the steric demands of the monomer. Larger substituents inhibit or prevent the polymerization reaction. The mechanism for this polymerization has been shown to involve initiation by insertion of the monomer into the Ti-X bond to form an intermediate amidinate complex (Scheme 197).497... 

In the himetallic mechanism, both metal atoms participate in the bonding reaction [see (19-11)]. First, the Ti-electron system of the a-olefin interacts with the p or d orbitals of the transition metal [the titanium in (19-11)], thus producing a new electron-deficient bond. As a result of this, partial valences form at the a and p carbons (symbolized by A). Since the double bond character is only partly eliminated, however, and the 2p-3d overlapping (Q—Ti bond) is planar, there is no free rotation around this bond. This relatively rigid system Cp——Ti oscillates around the Ti-X bond, causing the partial valences at Cp and to develop into full single bonds. On subsequent hybridization at the Cp and the atoms, the Cy—A1... 

These tendencies are illustrated in the contour maps of the relevant MOs in Fig. 5.2, which clearly show the contraction of the wave function contours to the impurity core in the series C N O, and the lowering of the electronic density along the Ti-X bond. 

A number of industrial polymers are made from vinyl monomers of the general formula (CH =CHX (X = CO H, OCOCH, Cl, CN, etc.) that have polar groups. These polymers cannot be made by Zeigler-Natta type of catalysts mainly because titanium is highly oxophilic, and the resultant Ti-0 or Ti-X bonds are too strong. 

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