Transfer with hydrogen

Relatively few other compounds transfer growing polymer chains with coordination catalysts, but molecular hydrogen and its isotopes are effective. The transfer mechanism is 

In agreement with this mechanism terminal groupings (methyl from polyethylene and isopropyl from polypropene) increase in concentration. When deuterium is used the end groups are CH2D [34] and with tritium two atoms per molecule have been found one from initiation by the metal hydride bond and the other on chain transfer [82]. 

Spontaneous deactivation to give a stable molecule will also produce a hydride derivative of the catalyst, XVII, viz. 

After reaction with the monomer to form a new propagating chain the position is formally the same as transfer with monomer. However, the two mechanisms can be distinguished kinetically if realkylation of the catalyst is slow compared with propagation. There is no direct evidence for this reaction although it is well established that the relatively stable alkyls of ms nesium and aluminium form metal hydride bonds on decomposition at elevated temperatures [83]. The existence of spontaneous termination has been deduced from a consideration of the kinetics, and by analogy with the effects of hydrogen on the polymerization. 

A true termination reaction with destruction of the catalyst site occurs usually with the metal reduced to a lower valency state. From simple organo-metallic compounds the decomposition products result from combination and disproportionation of the organic groupings [37]. The reaction would appear to be scission of the transition metal—carbon bond. 

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For titanium trichloride k, estimated in ref. by taking into account kf obtained by use of CO, is practically independent of the cocatalyst nature. is strongly influenced by the monomer nature. For propylene polymerization k is much lower (almost 20 times) than for ethylene polymerization (Table 11). kf, however, differs more significantly (by two orders of magnitude). Thus, under similar reaction conditions, the polymer molecular mass is apparently lower in propylene than in ethylene polymerization. The rate constant of the chain transfer with hydrogen, k, in the case of ethylene and propylene polymerization differs only by the factor four (Table 11) this is much lower than the differences in kf. Hence, for a similar decrease of the molecular mass of polypropylene the hydrogen concentration should be much lower than in ethylene polymerization. 

An essential difference is observed for the chain transfer with hydrogen in the polymerization on bulk TiClj (the chain transfer is 0.5th order with respect to [Hj]) and on catalysts supported on MgClj (first — order chain transfer with respect to [Hj]). This difference leads to higher values of the melt index of polyethylene prepared on the TiClyMgClj catalyst in the presence of H in comparison with non-supported titanium chloride catalysts... 

The donors present in the catalyst system play an active role in the formation or modification of isospecific sites, and the polymer molecular weight distribution depends on the relative contribution and hydrogen response (i.e., sensitivity to chain transfer with hydrogen) of each type of active site. The characteristics of different catalyst systems with regard to PP molecular weight distribution are as follows ... 

The chain transfer with hydrogen reactivates inactive centers Mt-CH(CH3)2 with the formation of a Ti-H bond, followed by a primary propene insertion. 

Recent work by Terano and co-workers (88) has shown that, under stopped-flow conditions, hydrogen is only effective as chain-transfer agent when catalyst and cocatalyst have been precontacted. These and subsequent (89,90) results indicated that effective chain transfer with hydrogen requires the presence of species able to promote the dissociation of H2 to atomic hydrogen. 

Chain transfer with hydrogen to ethylene coordinated to the active site produces a saturated polymer end group and a Ti-Et bond (or a Ti-H bond if chain transfer proceeds without an ethylene molecule coordinated to the active site) that is able to initiate the growth of another polymer molecule. 

Consider a Ziegler-Natta polymerization in which chains are terminated only by chain transfer with hydrogen. The rate of transfer is given by rtr = kffC [H2 -... 

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