Polymerization of cyclopropane

T his work concerns the study of the polymerization of cyclopropane, substituted cyclopropanes, and conjugated cyclopropanes in the presence of cationic and Ziegler-Natta polymerization. The unsaturation of cyclopropane has been described by several workers in the same way as unsaturated compounds. The unsaturation of cyclopropane compounds, which is the basis for the polymerization of these structures, can be explained by the electronic repartition on the three carbon atoms of the ring. Determination of the dipolar moment of chlorocyclopropane has shown that the carbonium ion resulting from the attack of the ring by a carbo cation is stabilized in a homoallylic structure. 

In anionic ring-opening polymerization of cyclopropane-1,1-dicarboxylates 22 using sodium and potassium thiophenolates as initiators, the nature of the cation was found to play an important role in the rate and selectivity of the polymerization. In particular, reaction rate increased in the order Na < C DCH18C6]... 

Table 13.3 Polymerization of cyclopropanes according to poorly defined or disputable mechanisms.

Tertiary bismuthines appear to have a number of uses in synthetic organic chemistry (32), eg, they promote the formation of 1,1,2-trisubstituted cyclopropanes by the iateraction of electron-deficient olefins and dialkyl dibromomalonates (100). They have also been employed for the preparation of thin films (qv) of superconducting bismuth strontium calcium copper oxide (101), as cocatalysts for the polymerization of alkynes (102), as inhibitors of the flammabihty of epoxy resins (103), and for a number of other industrial purposes. 

If stored as a liquid, even at —78°, cyclopropane undergoes a fairly rapid polymerization reaction. However, in the gas phase, at temperatures above 325° (in a stream of helium), it isomerizes smoothly to yield methylacetylene. This is clearly analogous to the isomerization of cyclopropane to propylene. 

In theoretical work, the initial steps in the polymerization of 1,1-dicyano-, 1,1-difluoro-, and 1,1-dimethyl-cyclopropanes by reaction with H, OH, and Me have been modelled by ab initio methods. " Other ab initio MO calculations for the reactions of H, Me, Ft, j-Pr, and r-Bu with a variety of silanes and germanes have been carried out. The results indicate that the attacking and leaving radicals adopt an almost co-linear arrangement. Bond distances and energy barriers were predicted for the reactions studied. 

Molecular mechanics and ab initio calculations on the cyclopentadienyl cation have been carried out an allylic stmcture is favoured. Calculations referring to the initiation of polymerization of 1,1-disubstituted cyclopropanes by cations (also neutrals and anions) are reported. Rate constants for the solvolyses of (69) show reasonable Yukawa-Tsuno correlations, interpreted in terms of the less reactive substituents... 

Both W(CO)5[C(C6Hs)2] and the analogous di-p-tolylmethylene complex have been used in model studies of the olefin metathesis reaction.2 3 In contrast to heteroatom-stabilized carbene complexes such as W(CO)s [C(OCH3)(C6Hs)], pentacarbonyl(diphenylmethylene)tungsten(0) reacts with alkenes to give cyclopropanes and 1,1-diphenylalkenes.2 The compound W(CO)5 [C(C6H5)2] is the best reported catalyst for the metathetical polymerization of 1-methylcyclo-butene.4... 

Recent advances in Cp-based catalyst technology made it possible to produce unique microstructure polymers from ethylene and BD. Longo and co-workers have reported in a series of publications the unprecedented cyclo-co-polymerization of ethylene and BD using a sterically encumbered isospecific metallocene F13-8 with MAO, which affords 1,2-cyclopropane rings together with 1,2-cyclopentane rings in the polymer chain, both with high trans-... 

Although cyclobutanes with varying substitution patterns are known, cyclopropanes present a much wider variety and much greater ease of synthesis. Ethyl 2-(p-methoxyphenyl)-l-cyanocyclopropanecarboxylate has been shown to thermally initiate the diradical polymerization of acrylonitrile [138]. In the presence of zinc chloride as activator, it also initiates the diradical polymerization of styrene [139]. On the other hand, this same initiator also initiates the thermal cationic polymerization of AT-vinylcarbazole [140]. This direction of tetra- and trimethylene chemistry is currently under active investigation. 

The Bond-Forming Initiation Theory gives a good interpretation of the observed spontaneous polymerizations of captodative monomers. The tetramethylene diradicals already implicated as initiators in the thermal (spontaneous) polymerizations of vinyl monomers can be particularly stabilized by captodative substituents. For comparison, and to initiate the polymerization of third monomers, captodative cyclobutanes and cyclopropanes are particularly appropriate precursors for generating tetra- and trimethylene diradicals. In particular the extensive work of Viehe [3,45,46] showed that thermolysis of captodative substituted cyclopropanes leads to trimethylene captodative diradicals at reasonable temperatures. Their initiating abilities for polymerization have not yet been determined. 

Prior knowledge has shown the value of introducing cyclopropane systems into macromolecules. A number of isolated studies have been carried out on the polymerization of such structures (10, 14, 28), principally on cyclopropane (28) and isopropylcyclopropane. Attention was directed toward three types of structures—i.e., the 1,1-dichlorocyclopro-panes, the bicyclo[n.l.0]alkanes, and the spiro [2.n] alkanes. In each case, the effects involved appeared highly complex the polymers formed have not yet all been characterized, and it is thought that a comparison with the model structures expected from rupture of one or the other of the cyclopropane bonds may be of value. 

Polymerization of 1,1 -Dichlorocyclopropanes. Experiments were first carried out on vinyldichlorocyclopropanes derived from isoprene and butadiene. The polymers obtained were very different from the cyclopropane macromolecular products already described (17), and it was found that cyclopropane participates in polymerization reactions. The study was therefore repeated in the simplest case—i.e., that of saturated cyclopropane monomers. 

Polymerization of the Bicyclo [w.1.0] alkanes. Some cyclopropane type hydrocarbons have already been polymerized—i.e., cyclopropane (28), 1,1-dimethyl- (10) and isopropylcyclopropane (14), particularly in presence of cationic catalysts. This work has established that a it complex is formed between the active site in the chain and a new molecule of monomer the complex then develops via opening of the cyclopropane ring in certain cases this opening is accompanied by transfer of a hydride ion (10). 

Polymerization of the bicyclo [n. 1.0] alkanes studied therefore occurs by opening of the cyclopropane ring and results in the appearance of one methyl group per structural unit. 

Cationic polymerization of XII may therefore be visualized in terms of Figure 9 according to which the ir complex initially formed between the active site and the monomer is converted into a carbocation with rupture of a C—C bond in the cyclopropane. This cation may be Xlla, b, or c, but only the latter can give rise to Structure M, alone compatible with the experimental data. This change necessitates the transfer of a hydride ion to transform the primary cation XIIc into the more stable tertiary cation Xlld. On this assumption, the termination reaction probably occurs as the result of the displacement of a proton in the alpha position with respect to the C+, which is relatively easy, whereas the steric hindrance around the active site does not favor continued poly-... 

Polymerization of the methylenecyclanes XIII and methylcyclenes XIV, the isomers of XII, was studied independently (19). The oligomers obtained from XIII or XIV have the same structure M as those described in this paper. These results therefore confirm the hypotheses advanced for XII—i.e.y opening of the cyclopropane ring and transfer of hydride ion with formation of Xlld, the hindered intermediate which polymerizes poorly and undergoes deprotonation easily, which interrupts the polymerization process. 

In this multi-authored monograph, several experts and leaders in the field bring the reader up to date in these various areas of research (synthesis and reactivity of zirconaaziridine derivatives, zirconocene-silene complexes, ste-reodefined dienyl zirconocenes complexes, octahedral allylic and heteroallylic zirconium complexes as catalysts for the polymerization of olefins and finally the use of zirconocene complexes for the preparation of cyclopropane derivatives). It is their expertise that will familiarize the reader with the essence of the topic. 

Equation 7.39 describes a transformation with first the C-H bond of cyclopropane adding to the Rh complex, followed by RE of H2, and then rearrangement to give the rhodacyclobutane.82 Metallacyclobutanes are thought to be intermediates in some alkene polymerization and metathesis reactions these compounds will appear again in Chapter 11. 

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