The Nature of the Active Catalyst in Late Transition Metal-Mediated Ring-Opening Polymerization (ROP) Reactions:  Mechanistic Studies of the Platinum-Catalyzed ROP of Silicon-Bridged [1]Ferrocenophanes

Abstract

Current mechanisms for the transition metal-catalyzed ring-opening polymerization (ROP) of [1]-ferrocenophanes and related strained species such as cyclic carbosilanes invoke a homogeneous mechanism. In this paper we describe experiments which indicate that the Pt(0)-catalyzed ROP of silicon-bridged [1]-ferrocenophanes proceeds mainly by a heterogeneous mechanism with colloidal platinum as the catalyst. Mechanistic studies were initiated to elucidate the fate of the ferrocenylsilane component of the precatalyst, fcPt(1,5-cod)SiMe 2 (4) (fc ) Fe(η 5 -C 5 H 4 ) 2 ), a proposed intermediate in the catalytic cycle for fcSiMe 2 (1a) with Pt(1,5-cod) 2 as the initiator. The ring-opened addition product Et 3 SifcSiMe 2 H (6) was isolated in high yield from the Pt(0)-catalyzed ROP of 1a in the presence of 35-fold excess of Et 3 SiH. Species 6 and Et 3 SiH were subsequently employed as capping agents in the Pt(0)-catalyzed ROP of fcSiMePh (1b) to generate model oligomers, Et 3 SifcSiMe 2 (fcSiMePh) n H (10, n ≈ 20) and Et 3 Si(fcSiMePh) n H (8, n ≈ 7), respectively. Copolymerization of a mixture of 1a and 1b using Pt(0) catalyst afforded a random copolymer (fcSiMe 2 -rfcSiMePh) n (9). Comparative end-group analysis of 8, 9, and 10 was performed with the oligoferrocenylsilane synthesized via the ROP of 1b in the presence of Et 3 SiH initiated by the precatalyst 4. Significantly, this revealed that the ferrocenyldimethylsilane component of 4 is not incorporated into the resultant polymer backbone which possessed the structure Et 3 Si(fcSiMePh) n H (8). Similarly, the ROP of 1a in the presence of Et 3 SiH initiated by the precatalyst fcPt(1,5-cod)Sn( t Bu) 2 (11) gave end-capped oligomers Et 3 Si(fcSiMe 2 ) n H (5), devoid of Sn( t Bu) 2 groups. The observations that the ferrocenylsilane and ferrocenylstannane components of 4 and 11 are not incorporated into the polyferrocene products and that, in addition, mercury was found to significantly retard ROP, indicates that the previously proposed homogeneous ROP mechanism is incorrect and that colloidal platinum is the main catalyst. A new heterogeneous mechanism for the platinum-mediated ROP of siliconbridged [1]ferrocenophanes is proposed which is likely to have important implications for the metal-catalyzed ROP mechanisms for related species such as silacyclobutanes.

Synthesis and Solution Self‐Assembly of Polyisoprene‐block‐poly(ferrocenylmethylsilane): A Diblock Copolymer with an Atactic but Semicrystalline Core‐Forming Metalloblock

“…36 In the case of Pt, it has been reported that heterogeneous metal colloids form as the active catalysts, and postulated that oxidative-addition and reductive-elimination reactions at the metal surface resulted in formation of the polymer. 37 For silicon-bridged ferrocenophanes in which one Cp ring is methylated, Pt II -catalysed ROP proceeds solely by selective cleavage of the Si-Cp H (Cp H = C5H4)bond to yield a regioregular, crystalline PFS, unlike thermal ROP which affords a regioirregular amorphous material. 38 Transition metal-catalysed ROP is advantageous in that it does not require high monomer and solvent purity unlike anionic ROP (Section 2.2.4.1), or the high temperatures used in thermal ROP.…”

“…The mechanism is believed to involve competition between the Si-H and strained Si-C bonds for oxidative-addition at the metal centre. 37,40 Use of such materials with Si-H groups has allowed the preparation of regioregular, comb-shaped graft copolymers, star-shaped polymers, and block copolymer architectures. 38 Chlorosilanes such as ClMePhSiH are more effective terminating agents than Et3SiH due to steric considerations, and also introduce a Si-Cl end functionality on telechelic polyferrocenylsilanes.…”

“…32,33,37,38,210 The addition of termination agents bearing a Si-H bond (e.g. Et3SiH) can be used to control the molecular weight of the polymer by controlling the initial monomer-to-silane ratio.…”

“…40 Initially, a terminal Si-H functionality was incorporated into the polymer by a been prepared via this method by using a difunctional PDMS polymer with terminal Si-H bonds at either end of the polymer chain. 212 As transition metal-catalysed ROP of ferrocenophanes is characterised by random chain termination events involving reductive elimination at the metal centre 37 it is not a living process and the PFS segment in the resulting BCPs therefore have relatively high polydispersities (PDI > 1.5).…”

Polymer science with transition metals and main group elements: Towards functional, supramolecular inorganic polymeric materials

“…21 42,43 Recent detailed mechanistic work has indicated that the major catalyst in the Pt-promoted ROP reactions is colloidal metal. 44 Transition-metal-catalyzed ROP is a mild method that does not require monomer with an exceptional degree of purity and has now been developed to the stage at which considerable control over polymetallocene architectures is possible. 45 For example, [1]silaferrocenophanes with different cyclopentadienyl rings can undergo regioregular transition-metal-catalyzed ROP, whereas thermal ROP affords a regioirregular PFS material.…”