Cyclobutenes ring-opening metathesis polymerization

Figure 6. Degree of polymerization ( ) and polydispersity (O) resulting from ring-opening metathesis polymerizations of (a) 5- [8 -[4"-(4" -nitrostilbeneoxy)octyl]carbonyllbicyclo[2.2.1]hept-2-ene and (b) poly[3-((8 -(4"-(4" -nitrostilbeneoxy)octyl)carbonyl)methyleneoxy)methyl]cyclobutene) initiated by RuCl2(CHPh)(PPh3)2 in CH2CI2 at 25 and 45 °C, respectively [187].

Similar Ru-O coordination has been observed in both the crystal structure of an r -methyl vinyl ketone complex [34] and in the DFT calculations of a ruthenium ester carbene by Parker, Sampson, and co-workers [35]. The stability of the ester carbene intermediate led to the inactivity of a 1-substituted cyclobutene ester in the formation of homopolymers via ROMP (ring-opening metathesis polymerization) (Scheme 7.2) [36]. 

Metallocene/methylaluminoxane (MAO) and other single site catalysts for olefin polymerization have opened a new field of synlhesis in polymer chemistry. Strained cyclic olefins such as cyclobutene, cyclopentene, norbornene (NB), and their substituted compounds can be used as monomers and comonomers in a wide variety of polymers." Much interest is focused on norbornene homo- and copolymers because of the easy availability of norbornene and the special properties of their polymers. Norbornene can be polymerized by ring opening metathesis polymerization (ROMP), giving elastomeric materials, or by double bond opening (addition polymerization). Homopolymerization of norbornene by double bond opening can be achieved by early and late transition metal catalysts, namely Ti, Zr, Hf, Ni, - ° and Pd (Scheme 16.1). 

Wu, Z., Wheeler, D.R., and Grubbs, R.H. (1992) Living ring-opening metathesis polymerization of cyclobutene The thermodynamic effect of a reversibly binding ligand. Journal of the American Chemical Society, 114,146-151. 

Polyacetylene itself is not processible. A precursor route (the Durham route) overcomes this drawback. Ring-opening metathesis polymerization of cyclobutene... 

Durham" polyacetylene and involves the ring-opening metathesis polymerization of cyclobutene derivatives followed by a thermal retro-Diels Alder reaction to perform the... 

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

As can be seen in the two examples (cyclopentene and norbomene) polymerization of cyclic olefins leads to polymers containing double bonds at periodic intervals in their backbone. An interesting application of the metathesis polymerization is the preparation of tra 5-polyacetylene [1]. Metathesis of the tricylic monomer leads to the ring-opening of the cyclobutene ring to afford a polymeric derivative. Heating this polymer eliminates bis-trifluoromethylbenzene to leave behind traiis-polyacetylene (see Eq. 2.36). 

Ci6j C24/C16, C28/C16 and C32/C16 are essentially constant throughout the ring opening polymerization. The constancy of the C12/C16 ratio is especially significant in this respect, since the pairwise mechanism would have required cross metathesis of the cyclooctadiene and cyclobutene, which is unUkely [33]. 

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