Cyclopolymerization symmetrical

A vast range of symmetrical and unsymmetrieal 1,6-diene monomers has now been prepared and polymerized and the generality of the process is well established.98,1 A summary of symmetrical 1,6-dienc structures, known to give cyclopolymerization, is presented in Table 4.4 In many cases, the structure of the repeat units has not been rigorously established. Often the only direct evidence for cyclopolymerization is the solubility of the polymer or the absence of residual unsaturalion. In these cases the proposed repeat unit structures are speculative. 

Table 4.4 Ring Sizes Formed in Cyclopolymerization of Symmetrical 1,6-Diene...

To explain the formation of non-crosslinked polymers from the diallyl quaternary ammonium system, Butler and Angelo proposed a chain growth mechanism which involved a series of intra- and inter-molecular propagation steps (15). This type of polymerization was subsequently shown to occur in a wide variety of symmetrical diene systems which cyclize to form five or six-membered ring structures. This mode of propagation of a non-conjugated diene with subsequent ring formation was later called cyclopolymerization. 

Ethylene/butadiene cyclopolymerization was used to probe the dual-site nature of Cj-symmetric ansa-metallo-... 

The polymers with trans-fused five-membered rings linked with a diisotactic head-to-tail sequence have chirality, although the polymers composed of the cis-fused ring are achiral. Scheme 10 summarizes the structures of the stereoisomeric polymers. The optically active zirconocene complex with a C2 symmetric structure catalyzes the enantioselective cyclopolymerization of 1,5-hexadiene (Eq. 20) [98, 99]. Although the polymer contains not only trans-fused ring but also cis-fused ring units (ca. 68 32), it shows optical rotation due to the main chain chirality. 

On the effect of the modification of the benzene rings in St-C3-St on the cyclopolymerization, we have still not had much information. It will be interesting to polymerize monomers substituted at benzene rings symmetrically or unsymmetrically by some groups. At the present moment, we have only one example, the polymerization of VN-Cj-VN, which has more bulky groups but wider TT-systems than St-Cj-St (22). Although more detailed works are still needed to compare the data of the cyclopolymerization of VN-C3-VN with those of St-C3 St, the monomer seems to be cyclo-polymerized equally readily to St-C3-St The rc values, the cyclization constant defined by Aso and his associates (41), for the cyclopolymerizations of St-C3-St and VN-C3-VN were 0.15 mol/... 

The cyclopolymerization of symmetrical disub-situted dipropargyl derivative bis(3-trimethylsilyl-2-propynyl) ether (20) was attempted as shown in Scheme 4. The polymer yields were generally very... 

As the example of 2-vinyl butadiene has already shown, the free radical polymerization of dienes is particularly interesting. Because of the two bifunctional double bonds, 1,4-dienes, CH2=CH—R—CH=CH2, are normally tetrafunctional and therefore lead to branching and cross-linking even at relatively low yield. With certain initiators, however, 1,3-dienes such as CH2=CH—CH=CH2, butadiene, or CH2==C(CH3)—CH=CH2, isoprene, react bifunctionally to form unbranched 1,4-poly (dienes). Symmetric 1,6-dienes, and frequently also symmetric 1,5-dienes, likewise give more or less unbranched molecules. In contrast to the l,4-poly(dienes), however, the 1,6- and l,5-poly(dienes) only possess slight unsaturation. That is, intramolecular ring formation by a cyclopolymerization occurs, for example, in the 1,6-diene acrylic acid anhydride. 

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