Telomerisation

In all telomerisations, the distribution of molecular weights increases, i.e. n increases with (a) an increasing rate of free-radical addition to the alkene, (b) a decreasing rate of the atom-transfer step, i.e. the A—B bond is stronger, (c) a higher concentration of alkene relative to the telogen and (d) temperature. 

As explained earlier in this chapter (Section IIA), the propagation step for the homopolymerisation of tetrafluoroethene is approximately 71kJmol more exothermic than for ethene [2], in spite of adverse polar effects of fluorine substitution on the addition step. It is therefore easy to obtain telomers with tetrafluoroethene and this is also the case with, for example, trifluoroethene, chlorotrifluoroethene and 1,1-difluoroethene. 

The effect of the fluoroalkyl iodide on the telomer distribution is illustrated for CH2=CF2 in Table 7.12, as well as the effect of molar ratio of telogen to alkene and of the reaction temperature. 

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These three initial classes of product may then be converted to other chemicals by oxidation, halogenation, alkylation, hydration, carbonylation, telomerisation and many other reactions. There are nowadays few intermediates for plastics that cannot be produced more cheaply from petroleum than from other sources. 

This amino acid may also be produced via telomerisation reactions (see below). 

Since large tonnage production is desirable in order to minimise the cost of a polyamide and since the consumption of nylons as plastics materials remains rather small, it is important that any new materials introduced should also have a large outlet as a fibre. There are a number of polyamides in addition to those already mentioned that could well be very useful plastics materials but which would be uneconomical for all but a few applications if they were dependent on a limited outlet in the sphere of plastics. Both nylon 7 and nylon 9 are such examples but their availability as plastics is likely to occur only if they become established fibre-forming polymers. This in turn will depend on the economics of the telomerisation process and the ability to find outlets for the telomers produced other than those required for making the polyamides. 

A -Heterocyclic Carbene Complexes in Polymerisation, Oligomerisation and Telomerisation Reactions... 

Abstract Over the past decade significant advances have been made in the fields of polymerisation, oligomerisation and telomerisation with metal-NHC catalysts. Complexes from across the transition series, as well as lanthanide examples, have been employed as catalysts for these reactions. Recent developments in the use of metal-NHC complexes in a-olefin polymerisation and oligomerisation, CO/olefm copolymerisation, atom-transfer radical polymerisation (ATRP) and diene telomerisation are discnssed in subsequent sections. 

Telomerisation is an important, atom efficient reaction, which generates functionalised dienes from 1,3-diene feedstocks. The reaction, which typically employs a palladium based catalyst, comprises coupling two molecnles of a conjugated 1,3-diene... 

Scheme 4.4 Products from the telomerisation of 1,3-butadiene with a nucleophile...

Table 4.1 Comparative telomerisation reactions of butadiene with methanol catalysed by...

Preformed complexes of type 52, 53 and 59, and in situ catalyst systems based on IMesX (X = CO, MeS03H, HCl, HBr, HI) and IPrHCl, have been tested for telomerisation of butediene with primary and secondary amines. Under optimised conditions, and low catalyst loadings, excellent activities and selectivities were observed [82]. 

In situ derived systems, in general, performed similarly to preformed complexes, in telomerisation of butadiene with MeOH, Tables 4.1 and 4.2 [68,70,71,77,78], In situ systans may be generated from free NHC or from imidazolium salt in combination with an appropriate Pd(0) or Pd(ll) source. Typically, 2-4 equivalents of imidazolium salt relative to Pd have been nsed [68,70,77], In situ catalysts derived from mono- and bis-Fc-snbstituted (Fc = ferrocenyl) imidazohnm and benzimidazolium salts (64-68) (Table 4.2) showed interesting telomerisation activities ascribed to the steric bulk of the Fc substituents [70]. Unsymmetrical salts 65 and 66 bearing A -Fc and A -Me... 

It is clear that telomerisation is an exciting reaction with considerable promise for synthesis of functionalised organic molecules, as a stand-alone reaction or as part of a reaction scheme. NHCs have proved particularly effective as hgands in this reaction and many opportunities now exist for the exploitation of this chemistry. It can be expected that novel catalyst systems will be developed as new reaction schemes are designed aimed at specific target molecules. 

Mesnager J, Quettier C, Lambin A, Rataboul F, Pinel C (2009) ChemSusChem 2 1125-1129 Mesnager J, Kuntz E, Pinel C (2009) J Organomet Chem 694 2513-2518 Mesnager J (2008) Developpement d amidon hydrophobe par telomerisation du butadiene en milieu aqueux. Etudes et applications catalytiques de complexes de palladium, Universite Lyon n°215-2008... 

Amdduri B,Boutevin (1997) Telomerisation Reactions of Fluorinated Alkenes. 192 165-233 Anwander R (1996) Lanthanide Amides. 179 33-112... 

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