Living oligomers

A Ta vinylalkylidene complex 6, confirmed by a single crystal X-ray analysis, was revealed to polymerize 2-butyne in a manner of living polymerization.The initiation efficiency is quantitative, and the living end can be end-capped with aromatic aldehydes. As polymers from symmetric acetylenes are generally insoluble, soluble poly(2-butyne) is accessible if the degree of polymerization is suppressed below 200. The NMR analysis of living oligomers of 2-butyne clearly indicates that both cis- and 7ra r-structures exist in the main chain. 

In order to demonstrate such an assumption, we hydrolyzed a concentrated solution of living oligomers and we obtained an amount of toluene corresponding to one benzyllc carbanlon per living chain. The dlsymetrlcal structure of the species Indicates that the reactivity of the second benzyllc carbanlon of the Initiator Is affected by the substitution of the first one. This behavior was already observed by other authors. 

Tsvetanov et al. (55) found evidence for this allylic anion from IR spectroscopic measurements on living oligomers in THF. However,... 

Fig. 2. Equilibrium constant for polymerization of a-methylstyrene to living oligomers and to a high molecular weight polymer. (O) Vrancken, SmiD, and Szwarc ( ) Worsfold and Bywater (A) McCormick. Reproduced, with permission, from Vrancken, Smid, and Szwarc Trans. Faraday Soc. 58, 2036 (1962).

The equilibrium constants for the addition of monomer to low-molecular weight living oligomers have been determined by a method devel-... 

Alternatively, we may look at the state of ultimate equilibrium of a system containing a monomer, the polymerization of which involves no termination and yields living polymers. In such a discussion, the modes of initiation of the polymerization and the concentration of the initiator or of the lowest living oligomer must be specified. 

The system resulting from addition of monomer to living oligomer, is represented by the following set of equations ... 

Propagation by dicarboanionic polystyryl barium has also been examined in some detail.Meticulous ultraviolet/visible absorption and conductance studies of THF and tetrahydropyran solutions have proved the dicarboanionic nature of the system. Furthermore, the absorption maximum of the anions varies with the degree of polymerization (D.P.) of the living oligomers present, and together with other evidence points to the existence of cyclic species (22). Investigations of fluorenyl barium in THF show a sandwich-type structure with Ba " ions located between cyclopentadienyl rings, and a... 

Asymmetric Polymerization via Optical Resolution of Living Oligomers. . . 123... 

One of the important features of this polymerization process is its perfect living nature, which permits block copolymerization [53]. The isolated living oligomers 33 (Pn=10, 20, and 30), derived from (—)-8, were used as initiators for the polymerization of achiral aryl isocyanides 7 and 35 (Scheme 31, Table 4). The intensity of the specific rotation largely depended... 

The hindrance of desorption does not affect the mobility of radical-anions on the metal surface. Hence, their dimerization with formation of still adsorbed dimeric dianions is very likely, and these may grow and form living oligomers. Degree of polymerization of the attached oligomers depends on their lifetime on the surface, and the lifetime is shortened by a cationsolvating solvent that facilitates removal of the cation from the metal lattice and therefore the desorption. This is demonstrated by Overberger (13), who studied the co-polymerization of styrene and methyl methacrylate initiated by a fine suspension of particles of metallic lithium. 

Fig. 4. Concentration of a-methyl styrene, Me, in equilibrium with its living oligomer or polymer as a function of added amount of the monomer, Mo, at constant temperature in THF solution. increases with Mo since the initially present dimeric dianions are converted into oligomers and eventually polymers. Me should reach a plateau as high molecular weight polymer is formed. However, increasing concentration of the polymer increases the activity coefficient of the monomer, causing a decrease of M,. The point denoted by a A results from the addition of a dead polymer to a solution corresponding to a point . Hence, M decreased...

Fig. 9. Plot of the number average degree of polymerization, j, of a living oligomer or polymer vs. temperature at constant total amount of supplied monomer, Mo, and a constant I...

Fig. 10. The fraction of monomer converted into a living oligomer or polymer, (Mo-MeJ/Mo, as a function of temperature at constant I...

Big. 34. Molecular weight distribution of polymers formed by continuous addition of monomer to living oligomers solution containing some terminating impurities... 

Recently Heitz et at. 20) have used cationic oligomerization of THF in the presence of acetic anhydride to calculate K, and K. They have assumed that the distribution of the oligomers carrying acetoxy end groups, determined by gpc, corresponds to the distribution of the living oligomers. The GPC chromatogram is shown in Fig. 2.1. 

The determination of the redox potential of short-lived oligomers is accessible only by kinetics methods using pulse radiolysis. In the couple Mn /Mn, reducing properties of M as electron donor as well as oxidizing properties of as electron acceptor are deduced from the occurrence of an electron transfer reaction with a reference reactant of known potential. The unknown potential E°(M "/M ) is derived in comparing the action of several reference systems of different potential. Number of rate constants for reactions of transients from metal ions and metal complexes were determined by pulse radiolysis. ... 

Though this review focus on homogeneous catalyzed reactions between unsatured hydrocarbons and carbon dioxide, also some related reactions without transition metal catalysts will be considered. It appears suitable to compare the different possibilities of catalytic and non-catalytic methods in the field of C-C linkage. For instance ionic reactions are well known routes to attach CO2 on a hydrocarbon chain. Living oligomers of ethene obtained with n-BuLi complexed by tertiary amines react with carbon dioxide and yield long-chain car xylic acids [27] (Equation 3). 

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