In order to gain insight into the mechanism for the thermal ring-opening polymerization of strained dicarba [2]ferrocenophanes, the thermal reactivity of selected examples of these species with different substitution patterns has been explored. When heated at 300 C dicarba[2]ferrocenophanes meso/rac-[Fe(h 5 -C 5 H 4 ) 2 (CHPh) 2 ] (meso/rac-7) and meso-[Fe(h 5 -C 5 H 4 ) 2 (CHCy) 2 ] (meso-13) were found to isomerize or to undergo disproportionation, respectively. These processes are apparently general for dicarba [2]ferrocenophanes with one or more non-hydrogen substituents at each carbon atom in the dicarba bridge and both appear to involve homolytic cleavage of the C-C bond in the bridge as a key step. In striking contrast, derivatives containing either one or no non-hydrogen substituents on the bridge such as {Fe 17) undergo thermal ringopening polymerization (ROP) under similar conditions (300 C, 1 h). Thus, thermolysis of 15 yielded polyferrocenylethylene {Fe[h 5 -C 5 H 4 ] 2 [CH(Ph)CH 2 ]} n (16a) with a broad molecular weight distribution (M w ¼ 13,760, PDI ¼ 3.27). Analysis of 16a by MALDI-TOF mass spectrometry suggested that the material was macrocyclic. Thermal treatment of linear polyferrocenylethylenes {Fe[h 5 -C 5 H 4 ] 2 [CH(Ph) CH 2 ]} n with narrow molecular weight distributions (prepared by photocontrolled ROP) at 300 C confirmed that the macrocycles detected form directly, and not as a result of depolymerization.Copolymerizations of 15 with 17 and of 15 with the deuterated species [Fe(h 5 -C 5 H 4 ) 2 (CD 2 ) 2 ] (d 4 -17) were conducted in order to probe the bond cleavage mechanism. Comparative NMR spectroscopic analysis of the resulting copolymers 18 and d 4 -18, respectively, and of homopolymer 16a, indicated that thermal ROP does not occur via a homolytic C-C bridge cleavage mechanism. A series of thermolysis experiments were conducted with MgCp 2 (Cp ¼ h 5 -C 5 H 5 ) at 300 C, which resulted in the isolation of ring-opened species formed from 15 and 17, and indicated that the Fe-Cp bonds can be cleaved under the thermal ROP conditions employed. The studies indicated that a chain growth process that involves heterolytic Fe-Cp bond cleavage in the monomers is the most probable mechanism for the thermal ROP of dicarba[2]ferrocenophanes.
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