Anionic transfer polymerization, olefins

The outcome of charge-transfer polymerizations has been systematized by Iwatsuki and Yamashita in their penetrating early review [130]. They arrived at a correlation of polymerization behavior with the value of the EDA complex equilibrium constant, Keq, With weak donor and acceptor olefins, no spontaneous polymerization takes place, while the addition of a radical initiator results in a random or an alternating copolymer depending on the value of Keq. As the donor and acceptor strength of the olefins increases, spontaneous initiation rates for radical copolymerization increase and with even stronger donor and acceptor olefins, ionic homopolymerization takes place (cationic and/or anionic). 

Propylene monomer, like ethylene, is obtained from petroleum sources. Free-radical polymerizations of propylene and other a-olefins are completely controlled by chain transferring. It is therefore polymerized by anionic coordination polymerization. At present, mainly isotactic polypropylene is being used in large commercial quantities. There is some utilization of atactic polypropylene as well. Syndiotactic polypropylene, on the other hand, still remains mainly a laboratory curiosity. 

The next step in both MAO-activated and MAO-free systems is believed to involve the transfer of an alkylide group to a Lewis acid (eqs. 8 and 9) or a proton (eq. 10), which creates an electron-deficient (14 e ) metal center with a site for the potential coordination of an olefin. The nature of the counterion strongly affects the abihty of the site to polymerize olefins, and indeed may change the active-site geometry enough to drastically change the selectivity of polymerization as well as the chain-transfer rate. Cation-anion interactions have been reviewed by Bochmann (125). The reactivity of anion-free cationic metallocenes has been probed by ion-cyclotron resonance (ICR) spectroscopy (126). 

Block copolymers can be obtained by copolymerization of cycloolefins of entirely different reactivities or by applying adequate sequential addition of the monomer. They also arise from cycloolefins and vinylic monomers, including linear olefins, in the presence of Ziegler-Natta catalysts [5] [Eq. (3)] or of metathesis catalysts. In the latter case it is usual to change the reaction mechanism to Ziegler Natta [6] and group transfer polymerization [7] or from anionic-coordina-tive to metathesis polymerization [8] [Eq. (4)]. 

M. Szwarc et al., tbid. 78, 2656 (1956), and N. D. Scott, U.S. Patent 2,181,771 (1939), have shown that these aromatic anions can initiate polymerization of olefins by a mechanism of electron transfer to form a radical-anion, followed by dimerization of two radical-anions to form di-anions which can propagate indefinitely ... 

Mulhaupt and coworkers have reported the details of several studies related to the preparation of block copolymers from thiol, maleic acid and hydroxy-functional polypropylene prepared by a metallocene catalyst [157, 158]. The same group also reported the transformation of metallocene-mediated olefin polymerization to anionic polymerization by a novel consecutive chain-transfer reaction for the preparation of polypropylene-based block copolymers [159]. The latter were also... 

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