Polymers with Metallocene Side Groups

Many examples of inorganic polymers with metallocene-containing side groups also exist. Most of the polymers prepared have been based on main chains of po-lyphosphazenes, polysflanes, polysfloxanes, and polycarbosilanes. These materials are surveyed in the following sections. 

1 Polyphosphazenes with Ferrocene- or Ruthenocene-Containing Side Croups 

The earliest and most well-developed route to polyphosphazenes involves the thermal ring-opening polymerization of cyclic phosphazenes bearing halogen substituents at phosphorus. Use of this method and subsequent halogen replacement with alkoxides has led to the synthesis of ferrocene- and ruthenocene-containing polyphosphazenes (2.20 and 2.21) with molecular weights (M ) in excess of 2 x 10 [49, 50]. 

Polymers 2.20 and 2.23 (as well as copolymers bearing both ferrocene and ruthenocene moieties) have been partially oxidized with iodine, resulting in weakly semiconducting materials. These materials have also been deposited on electrode surfaces, where the polymers act as electrode mediator coatings that aid electron transfer between the electrode and redox-active species in solution [54]. 

Polyphosphazenes with ferrocenyl substituents (2.24) have also been synthesized by the functionalization of poly(methylphenylphosphazene) and related polymers by means of a deprotonation-electrophilic addition strategy (see, e.g., Eq. 2.9) [55, 56]. This versatile reaction sequence has yielded materials with, for example, degrees of substitution of 45% and 36% for polymers 2.24 (R=H) and 2.24 (R=Me), respectively. The molecular weights of the polymers were Mw=2.0xl0 and 1.5x10 for 2.24 (R=H) and 2.24 (R=Me), respectively (with PDI values of 1.4—2.0). The glass transition temperatures increased in comparison with the unsubstituted polymer (Tg=37°C) with values of 92 °C for 2.24 (R=H) and 87 °C for 2.24 (R=Me), respectively. 

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Organic polymers with metallocene side-groups... 

A variety of other organic polymers with metallocene-containing side-groups have been synthesized. However, very few of the materials have been as well studied as PVEc. Crystalline organometallic complexes containing ferrocene... 

Third, metallocene units, such as ferrocene or ruthenocene, have been linked to phosphazene cyclic trimers or tetramers and these were polymerized and substituted to give polymers of the type mentioned previously (41). Polyphosphazenes with ferrocenyl groups can be doped with iodine to form weak semiconductors. Polymer chains that bear both ruthenocenyl and ferrocenyl side groups are prospective electrode mediator systems. 

Dlubek et al [49] studied a series of metallocene-catalyzed poly a-olefins) with progressively longer chains as the pendant side groups from polypropylene to poly-l-eicosene (20 carbons). Their results show an interesting relationship between the o-Ps lifetime and intensity in the amorphous phase for this series of polymers. They found that the average hole size and o-Ps intensity from the amorphous phase, decreased from polyethylene to polypropylene, followed by a slight increase to poly-1-butene. There was a rapid rise in hole size and intensity to poly-l-dodecene. 

The carrier polymers selected in accordance with the aforementioned requirements were random copolyamides of the type depicted schematically below, which comprises repeat units bearing unreactive but solubilizing side groups S, as well as repeat units equipped with spacers that are terminated by functional groups F suitable for metallocene binding, and x is. chosen to be equal to, or larger than, y. 

Busico et al., on the other hand, came to a conclusion [299] that the stereoregularity of polypropylene produced with C2-symmetric group 4 ansa-metallocene catalysts is a result of the interplay of two competing reactions. These are isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end. That makes this class of homogenous catalysts different from the typical Ziegler-Natta catalyst, because with these catalysts enantioselectivity and stereoselectivity are not necessarily coincidental [96]. 

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