Alkene polymerisation

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

If one wants to use only a metal halide as the initiator for an alkene polymerisation, an analysis similar to ours can be made for the metal halide alone. In addition, there is a 1 2 equilibrium for the organic halide [35, 36] and a molecular aggregate <-> single molecule equilibrium associated with the metal halide. Thus, a solution of a carbocation salt is more exactly described by the series of linked equilibria summarised in Scheme 5. 

Finally, the above discussion has pointed to a number of gaps in the thermodynamic literature and the studies of BIE. Although the work of Arnett [21, 22, 40] has provided a sound basis for the comparative reactivity of carbocations, there are certain questions (such as the final state of the cations) which need to be clarified before these results can be applied more widely. Until these gaps in our experimental knowledge are filled, we are left with the theoretical approach described in this paper, if we wish to make a systematic choice of the optimum initiator for a chosen alkene polymerisation. 

Selection of those metals where the metal alkyl complexes are stable with respect to hydride and liberated alkene. For the metals on the left-hand side of the periodic table, the Early Transition Metals and the Lanthanides, the alkyls are relatively stable. Therefore it is not surprising that the best alkene polymerisation catalysts are found amongst these metals. 

Figure 7.3 gives an overview of the reactions involved in the hydroformylation of internal alkenes to linear products. It has been suggested that cobalt, once attached to an alkene, runs along the chain until an irreversible insertion of CO occurs. Thus, the alkene does not dissociate from the cobalt hydride during the isomerisation process. There is no experimental support for a clear-cut proof for this mechanism. In alkene polymerisation reactions this type of chain running has been actually observed. 

In this chapter we will discuss a few topics in the area of alkene polymerisations catalysed by homogeneous complexes of early and late transition metals (ETM, LTM). One of the main research themes for the ETM catalysts has been the polymerisation of propene, while industries have also paid a lot of attention to metallocenes giving LLDPE (linear low-density polyethylene, for thinner plastic bags). In less than a decade a completely new family of catalysts has been developed which enables one to synthesise regioselective and stereoselective polymers of a wide variety of monomers. These new catalysts are starting to find application on industrial scale, but as yet only reports on commercialisation of (branched) polyethylene and polystyrene have appeared. 

We will discuss some of the recent advances in the area of alkene polymerisations catalysed by homogeneous complexes of zirconium and... 

Having generated suitable (partially) cationic, Lewis acidic metal centers, several factors need to be considered to understand the progress of the alkene polymerisation reaction the coordination of the monomer, and the role (if any) of the counteranion on catalyst activity and, possibly, on the stereoselectivity of monomer enchainment. Since in d° metal systems there is no back-bonding, the formation of alkene complexes relies entirely on the rather weak donor properties of these ligands. In catalytic systems complexes of the type [L2M(R) (alkene)] cannot be detected and constitute structures more closely related to the transition state rather than intermediates or resting states. Information about metal-alkene interactions, bond distances and energetics comes from model studies and a combination of spectroscopic and kinetic techniques. 

The data discussed in Sections 8.5 and 8.6 make it clear that in the low-dielectric media typically employed for polymerisation reactions, the counteranions in metallocene ion pair catalysts are closely associated with the cationic complex as either inner-sphere or outer-sphere ligands. If anions are coordinated in the transition state, they must be expected to exert a significant influence on the stereochemistry of alkene polymerisation, even though the formation of syndiotactic and isotactic 1-alkenes have been readily explained by considering only the cationic metallo-cenium species and their ligand structure [21, 23, 122, 132, 133]. 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

With the sole exception of the random ethylene-propylene copolymers, for industrial applications heterogeneous catalysts have been used for alkene polymerisations. Ethylene-propylene statistical copolymerisation has been carried out using homogeneous vanadium-based catalysts [28]. 

The type of solvent or diluent should be specified in reporting a Ziegler-Natta catalyst system. Alkene polymerisations are usually carried out in inert solvents, such as aliphatic or aromatic hydrocarbons (e.g. some gasoline fractions or toluene). The use of protic or aprotic polar solvents or diluents instead of the hydrocarbon polymerisation medium can drastically alter the reaction mechanism. This usually results in catalyst deactivation for alkene coordination polymerisation. Modern alkene polymerisation processes are carried out in a gas phase, using fluidised-bed catalysts, and in a liquid monomer as in the case of propylene polymerisation [28,37]. 

It should be mentioned that many of the requirements necessary for the economic production of polyethylene and polypropylene have been achieved. However, catalysts of greater activity and of greater selectivity in the production of polymers and copolymers can be anticipated. This is of prime concern to alkene polymerisation processes in the presence of single-site metallocene catalysts. Such catalysts, undoubtedly of great scientific and commercial importance, have been developed on a large scale within recent years [29,30],... 

Organometallic compounds are used widely as homogeneous catalysts in the chemical industry. For example, if the alkene insertion reaction continues with further alkene inserting into the M C bond, it can form the basis for catalytic alkene polymerisation. Other catalytic cycles may include oxidative addition and reductive elimination steps. Figure above shows the steps involved in the Monsanto acetic acid process, which performs the conversion... 

Both of these reactions have very important industrial uses (Section 14.3.9). In order to obtain alkene streams of sufficient purity for further use, the products of steam-cracking or catalytic cracking of naphtha fractions must be treated to lower the concentration of alkynes and alkadienes to very low levels (<5ppm). For example, residual alkynes and dienes can reduce the effectiveness of alkene polymerisation catalysts, but the desired levels of impurities can be achieved by their selective hydrogenation (Scheme 9.4) with palladium catalysts, typically Pd/A Os with a low palladium content. A great deal of literature exists,13,37 particularly on the problem of hydrogenating ethyne in the presence of a large excess of... 

Tris(pentafluorophenyl)aluminium has been prepared by metathesis, [87] (Figure 10.38), and the toluene complex is used as a co-catalyst for alkene polymerisation. 

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