A series of novel [2]ferrocenophanes with unsymmetrical C-E bridges has been prepared in which the covalent radius of the second-row element, E, and hence the ring strain present is varied. Species [Fe(eta -C5Me4)(eta -C5H4)CH2ERx] (7, ERx = SiMe2; 8a, ERx = PPh; 8b, ERx = PMes: 9; ERx= S) were synthesized via reaction of the PMDETA (N,N,K,N',N " -pentamethyldiethylenetriamine) adduct of [(eta -C5H4Li)Fe(eta -C5Me4)CH2Li] with Cl2ERx (E = Si or P) or S(SO2Ph)(2). Studies of 7-9 by single-crystal X-ray diffraction confirmed the presence of ring-tilted structures: for 7, alpha (angle between the planes of the Cp rings) = 11.8(1)degrees; for 8a. alpha (average) = 14.9(3)degrees; for 8b, alpha (average) = 18.2(2)degrees; and for 9, alpha = 18.5(1)degrees. The least tilted compound, 7, was found to be resistant to thermal, anionic, and transition metal catalyzed ROP. In contrast, the significantly more tilted compounds 8a, 8b, and 9 were all found to polymerize thermally with small negative values of DeltaH(ROP) Of ca. 10-20 kJ . mol(-1) determined by DSC. Whereas thermal ROP of 8a yielded the soluble high molecular weight polycarbophosphaferrocene [(r-C5Me4)Fe(eta -C5H4)CH2PPh](n) (11), species 9 formed the insoluble polycarbothiaferrocene [(eta -C5Me4)Fe(eta -C5H4)CH2S](n) (14). Attempted anionic ROP of 8a and 9 with (BuLi)-Bu-n was unsuccessful and treatment of 8a with CF3SO3Me resulted in the formation of the novel phosphonium salt [(eta -C5Me4)Fe(eta -C5H4)CH2PMePh][CF3SO3] (13). which was found to be resistant to thermal ROP as a result of its less strained structure (for 13, alpha = 11.4(7)degrees). Treatment of 9 with CF3SO3Me or BF3. Et2O resulted in the first example of cationic ROP for a transition metal-containing heterocycle to yield polycarbothiaferrocene 14. In the presence of excess 2,6-di-tert-butylpyridine as a selective proton trap, ROP of 9 was only observed with CF3SO3Me, and not BF3. Et2O, which indicated that Me+ and H+ are the probable cationic initiators, respectively. Thermal copolymerization of 9 with trimethylene sulfide resulted in the isolation of the soluble, high molecular weight, random copolymer [(eta -C5Me4)Fe(eta -C5H4)CH2S](n)(CH2)(3)S](m), 15.