Telomer, defined

Thermal initiation has the advantage to lead to the formation of well-defined telomers with either selective production of monoadduct or higher DPn than those obtained from photochemical initiation. It appears as an easy and attractive way since, except the autoclave, no special equipment or solvent or expensive reactants (e. g., initiators) is required. In addition, this route provides clean reactions without the formation of any by-products. 

This concept was also exploited in order to prepare living and well defined tetrafluoroethylene telomers in which the telomer produced acts as a further telogen as follows ... 

The novel catalyst system based on palladium(O) /V-heterocyclic carbene complexes was developed by the group of Beller, in part prompted by the strong patent position of Dow on phosphine-based palladium catalysts [8]. The catalyst [37], either generated in situ from the corresponding imidazolium salt or the molecularly defined divinyldisiloxane complex [Pd(Imes)(dvds)] (Fig. 3), was used in the telomerization of 1,3-butadiene with methanol [38—40]. The /V-heterocyclic carbenes are in general better a-donor ligands and come with considerably different steric requirements than the phosphines. The [Pd(Imes)(dvds)] complex resembles the final telomer-palladium product complex and thus offers a facile and clean entry into the catalytic cycle. The metal carbene complex was shown to be... 

Peroxides also initiate telomerization, which is a process also occurring by way of radicals.15 In this reaction compounds such as halomethanes participate to form short-chain polymers with both ends of the chain defined. In the overall reaction the taxogen (e.g., ethylene) combines with the telogen (e.g., carbon tetrachloride) to afford a telomer ... 

Studies of this kind fall broadly into two groups as indicated in Table 15.1. Those labelled P are mainly concerned with the effect of small quantities of acyclic olefin M2 on the MW, yield, and cis content of the polymer produced from the cyclic olefin Mi in this case the ratio [M2]/[Mi] is usually 0-0.05. Those labelled T involve the use of much higher proportions of M2 (with [M2]/[Mi] 0.5-2), with the object of producing telomers with well-defined end-groups, in some cases for synthetic purposes. For example, highly substituted cyclopentane and tetrahydro-fiiran derivatives can be readily made by the cross-metathesis of substituted norbomenes or 7-oxanorbomenes with hex-3-ene or l,4-dimethoxybut-2-ene catalyzed by ruthenium carbene complexes (Schneider, M.F. 1996). If the acyclic olefin is unsymmetrical and represented by Q Q, where Q and are the alkylidene moieties, three series of telomers may be produced, Q (Mi) Q, Q (Mi) Q, Q (Mi) Q, where ti is the number of Mi units. The lowest members of the series (n= 1) are dienes, and it is sometimes possible to deteet, separate, and identify the cc, ct, and tt isomers. 

The terms telogen and telomerization were defined many years ago to describe an innovative method for the free-radical polymerization of ethylene (49,50). The essence of telomerization, described in a 1942 patent (50), involves reaction of a telogen XY (also called a chain-transfer agent) with an ethylenic molecule to give a telomer of structure X—(A) —Y. In the product telomer, fragments of the chain-transfer agent are attached to the termini of the molecule (49,50). Figure 15 illustrates the method. 

PPP derivatives with a controlled number of side-chains have been obtained by copolymerization of functionalized telomers. This approach allows the construction of PPP derivatives with well defined blocks of unsubstituted units of variable length linked to each other by only one alkyl-substituted aromatic unit [74] (Scheme 6.22). 

In homogeneous catalysis telomerization is defined as the oligomerization of dienes with incorporation of a nucleophile. For example, two molecules of butadiene react with one nucleophile HY to form telomers. This reaction is catalyzed by various organometallic compounds of the transition metals, especially by palladium and nickel complexes. 

Then, the Pd-catalyzed coupling reaction of diboronic acids 29 or 32 with dibromo compounds 28, 30, or 31 gave telomer copolymer 33 with defined blocks of one to six nonsubstituted phenylene units (Scheme 13). A vapor pressure osmometry measurement of 33 (y = 2) gave = 21,000. Thus,... 

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