Ring-opening polymerization (ROP) of strained, ring-tilted silicon-bridged [1]ferrocenophanes: Synthetic methods and mechanisms

The use of ROP, a chain-growth process, has allowed access to a range of high molecular-weight polymers with skeletal transition metal atoms with novel properties. Very interesting mechanistic questions arise for these polymerization reactions. To date, the most information is available for the ROP of silicon-bridged [1]ferrocenophanes. Although much systematic research is still needed, at this stage it appears that the mechanisms for the thermal, anionic and transition metal-catalysed ROP reactions of these species are quite similar to those for sila- and disilacyclobutanes. However, the ROP of other strained metallocenophanes such as hydrocarbon-bridged [2]metallocenophanes may well be very different. In addition, if an analogous mechanism to that for [1]silaferrocenophanes operates for the thermal ROP of sulfur-bridged [1]ferrocenophanes then a zwitterionic intermediate with a negative charge at carbon and a positive charge at sulfur would be involved. This would be against electronegatively-based chemical intuition, but is possible, especially if the intermediate is delocalized as may be the case for 25. The mechanism for the novel atom abstraction-induced ROP of trichalcogenido [3]metallocenophanes discovered by Rauchfuss and co-workers is also intriguing and is unclear at present.32 Clearly mechanistic studies of the ROP of other metallocenophanes will be an important area for research in the future. The knowledge obtained from such studies may well suggest new methods for the ROP of other strained rings containing transition metals, main group elements or even organic systems.