Ligand effects on chain length

As a measure for the growth rate we have simply taken the overall observed rate of production. This may not be the intrinsic rate of each catalyst, as part of the catalyst may be in an inactive state. The effect of the ligand bridge on the rate is moderate, but distinct. The effect on the number averaged molecular weight is much larger The approximate value of the rate of termination (chain transfer in this instance) is shown in the last column. 

Note that these rates are not true rate constants as these overall rates will contain concentration and pre-equilibria parameters. Nevertheless, longer bridges, and thus wider bite angles lead to a relative increase in the rate of chain transfer. Ester formation for the wide bite angle ligands were assigned to the formation of trans complexes as mentioned above. 

For entries 3-5 the increase in molecular weight observed can be assigned to the increase in the rate of insertion and the rate of termination remains practically the same. An increase of the rate of polymerisation with the steric bulk of the ligand is usually ascribed to the destabilisation of the alkene adduct while the energy of the transition state remains the same. As a chain transfer reaction presumably P-hydride elimination takes place or traces of water might be chain transfer agents. Chain transfer does occur, because a Schulz-Flory molecular weight distribution is found (PDI 2, see Table 12.2). Shorter chains are obtained with a polar ortho substituent (OMe, entry 2) and in methanol as the solvent, albeit that most palladium is inactive in the latter case. 

It is important to note that in methanol as the solvent the reaction is much slower and also the molecular weight is much lower. Apparently a major part of the palladium complex occurs in an inactive state and the termination reaction is relatively accelerated by methanol. This suggests that ester formation is the dominant chain transfer mechanism in methanol, although P-hydride elimination will still occur. 

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Table 12.2. Effect of chain length of ligand bridge on rate and M,i [9]a...

Figure 11. Effect of packing density on ligand chain length of surfactant molecules for 3 nm diameter FePt spheres, cylinders and cubes. Current state-of-the-art spherical 3nm diameter particles use / = lnm oleic acid surfactant molecules and have a packing fraction of only p — 19%. Cubic or cylindrical structures with shorter chains are needed to increase p to acceptable levels.

Peralkylated (I and II) derivatives have been synthesized that act as bridging ligands in analogous dicationic dinuclear complexes (Fig. 4) The effective chain lengths (population of rotamers) and flexibility (rotational barriers, nature of thiourea bridging unit) in these species depend on the degree of alkylation and the nature of the substituents on nitrogen. Reactions... 

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