Coordinate anionic polymerization

Polymers with much higher average molecular weights, from 90,000 to 4 x 10 , are formed by a process of coordinate anionic polymerization (43—45). The patent Hterature describes numerous organometaUic compounds, aLkaline-earth compounds, and mixtures as polymerization catalysts. Iron oxides that accumulate in ethylene oxide storage vessels also catalyze polymerization. This leads to the formation of nonvolatile residue (NVR) no inhibitor has been found (46). 

For the (coordination) anionic polymerization, metal alkoxides are often employed as initiators. In this system, the ring opening of epoxide takes place by a nucleophilic attack of an alkoxide on the (activated) epoxide carbon to generate another metal alkoxide which behaves as the propagating species (Scheme 3), The nature of metal-alkoxide... 

Coordinated Anionic Polymerization with Alkyl Metal Catalysts. . . 545... 

In the coordinated anionic polymerizations with Group I—III metal alkyls alone, monomer coordination involves overlap of the olefinic jr-electrons with vacant sp3 hybrid orbitals. Since this interaction is very weak it is most effective with easily polarized monomers. In the coordination polymerizations with Ziegler type catalysts, stronger monomer coordination is obtained by overlap of jr-electrons with vacant -orbitals of the transition metal component. The complexes have structures of the type proposed by Dewar (199b) and by Chatt and Duncanson (200) and applied to Ziegler type catalysts by Cossee (201) (Fig. 6). The olefin yr-electrons overlap with the orbital of... 

A coordinated anionic polymerization can be defined as one in which each monomer unit is complexed and polarized by a positive center prior to its addition to the growing anion. Thus, the main features which distinguish coordinated anionic catalysts from simple anionic catalysts are ... 

In line with the general concept of coordinated anionic polymerizations, the above results could be explained by the different abilities of the... 

Note Added in Proof Coordinated anionic polymerization of dienes by alkali metal alkyl catalysts and the effect of cation solvation on mechnism have been discussed in a similar manner by Medvedev and Gantmakher (231a). 

Boron alkyls are expected to be inactive for coordinated anionic polymerization of olefins because the boron-carbon bond is not sufficiently ionic. The diazomethane polymerization with boron alkyl catalyst reported by Bawn, Ledwith and Matthies (275) is a special case of the growth reaction. A coordination mechanism seems most probable, but it has not been ascertained whether the polymer chain migrates as a car-banion or as a radical. If the complex between diazomethane and boron decomposes into a boron-carbene complex, then the polymer chain could migrate as a carbanion with the driving force provided by the electrophilic carbonium ion ... 

There is no concrete evidence that tin or lead alkyls can initiate coordinated anionic polymerization, and this is in accord with their covalent bond character and low electrophilicity. Polymerizations which have been initiated with these catalysts are most probably free radical in nature. 

Stereospecific coordinated anionic polymerizations of unhindered monomers follow Hammett s rule (305). The negative slope indicated that the addition reaction is favored by electron-releasing substituents in the same manner as in cationic polymerizations (306). The opposite results were obtained with conventional anionic catalysts (Na-K alloy or LiC6Hs in ether) and with a free radical catalyst (benzoyl peroxide). 

The relatively low angle slope of the Hammett plot indicates that the polarization induced by complexing the monomer with the electrophilic site involves only a partial charge. Weak polarization of monomer is supported by the fact that vinylcyclopropane does not isomerize during coordinated anionic polymerization but does so during conventional cationic polymerization (307). 

We have carried the theme of a primary electrophilic attack on monomer through the discussions on coordinated anionic polymerizations with simple alkyl metals, and on coordinated anionic and radical polymerizations with Ziegler type catalysts. When a Ziegler type catalyst is comprised of one or more strongly electrophilic components, the Lewis acidity can become great enough to initiate polymerization by a cationic mechanism. Naturally, this will occur most readily for those monomers which are prone to cationic initiation because of their ability to stabilize the carbonium ion 278). 

Since butadiene can also undergo coordinated anionic polymerizations, some of the differences in polymer microstructure are attributable to changes in mechanism. Based on the catalysts reported to date, the isotactic and syndiotactic 1,2-polybutadienes appear to arise from coordinated anionic mechanisms. Qs and trans 1,4-polybutadienes can probably be made by all mechanisms, with cis arising from soluble catalysts which are capable of multi-coordination at one metal site. Trans structure is favored by cationic mechanism and by anionic mechanism involving coordination at two metal centers. 

Precatalyst 4(Sm) was utilized as a standard system [60]. The mechanism follows a coordination anionic polymerization via an eight-membered transition state (Scheme 3, see p. 985). Formation of a metal enolate turned out to be essential for the initiation of the MMA polymerization and was confirmed by the initiation activity of the enolate complex [(C5H4SiMe3)2Y(OCH=CH2)]2- The rate of polymerization is directed by steric factors depending on the metal (Sm > Y > Yb > Lu) and the auxiliary ligand (Cp > Cp ). Ethyl, isopropyl and f-butyl methacrylates are also stereospecifically polymerized, but the rate of poly-... 

Although numerous mechanism proposals have been offered for the coordinated anionic polymerization of olefins, no direct proof exists to confirm the proposed structures. Although the structures have value in stimulating future research to verify certain features of the mechanism, noncritical acceptance of the proposal as fact could hinder progress. 

The catalysts for these polymerizations can be separated into two groups. To the first belong the so-called Ziegler-Natta catalysts, and to the second, transition metal oxides on special supports, like carbon black or silica-alumina, etc. Besides the two, there are related catalysts, like transition metal alkyls or metal halides that also catalyze some coordinated anionic polymerization. This group also includes transition metal-TT-allylic compounds and transition metal hydrides. 

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