PFS-PDMS

Abstract Polyferrocenyldimethyl-silane (PFS) diblock copolymers with polyisoprene (PFS-PI) or with polydimethylsiloxane (PFS-PDMS) self-assemble in simple alkane solvents to form what appear by TEM to be dense flexible cylinders (nanowires) or nanotube-like structures. Typical widths are on the order of 20 to 30 nm, with variable lengths often greater than 10 un. The structures that form, and the dimensions of the tube-like structures or wires, depend upon the composition of the polymers and the lengths of the blocks. Light scattering experiments show that the PFS-PDMS (block ratio 1 12) solutions aged... 

We have a specific interest in the self-assembled structures formed by poly(ferrocenylsilane) block copolymers, such as poly(ferrocenyldimethylsilane-Z -dimethyl-siloxane) (PFS-PDMS) and (ferrocenyldimethylsilane-Z>-isoprene) (PFS-PI). The PFS block contains an iron atom in the main chain repeat unit. These polymers are particularly promising for novel applications, since they can be used as charge-transport materials and, by pyrolysis, as precursors to ferromagnetic ceramics [4-6], Moreover, they can by synthesized with a very narrow molar mass distribution, with excellent control over chain length and composition [7], An important feature of PFS is that the polymers bearing two methyl groups on the silane unit are crystalline, whereas polymers with two different substituents on each silane (methyl, ethyl methyl, phenyl) are atactic and remain amorphous. This feature of the polymer composition has a strong influence on the type of self assembled structures that these poly-... 

Both PFS-PI and PFS-PDMS are synthesized by two-step anionic polymerization. The synthetic approach for the preparation of PFS-PDMS is shown in Scheme 1 [8,9] n-butyllithium was used to initiate the polymerization of the strained silicon-bridged ferrocenophane in THF solution, while the second block was built by the subsequent addition of hexamethyltrisiloxane (D3). The reaction was terminated with chlorotrimethylsilane. To obtain PFS-PI, the PI block was initiated with butyllithium, followed by the addition of the silicon-bridged ferrocenophane. 

Another intriguing feature emerged when we examined PFS-PDMS diblock copolymers having a longer soluble block as well as an increased block ratio (1 12 and 1 18). Samples of the two polymers were prepared by the same protocol, i.e., subjected to the same thermal history. Dilute solutions in decane were heated for 30 min at 61 °C, cooled to room temperature over two hours and allowed to age for 24 hours. We have already shown, in Fig. la, that under these conditions, PFS50-PDMS300 formed dense rod-like structures in hexane. However, as... 

PFS PDMS = 1 12) in hexane. (From Raez et al.35 Reproduced with permission.)... 

Cylindrical and tape-like morphologies have been identified in the case of PI-/ -PFS (PI = polyisoprene) where the PFS block crystallizes. " Water-soluble polyferrocenyldimethylsilane-/ -poly(aminoalkylmethacrylate) co-polymers of narrow polydispersity have also been prepared and cylindrical micelles have been identified. " Block co-polymers generated by transition metal-catalyzed ROP, such as PFS-/ -PDMS-/ -PFS triblock materials, have been shown to self-assemble in hexanes to yield a variety of remarkable architectures that include flower-like assemblies where the... 

Aside from the coil-coil diblocks and coil-coil-coil ABA triblocks, crystalline-coU poly(ferrocenyldimethylsilane)-foZock-poly(dimethyl silox-ane) or PFS-PDMS diblock copolymers were reported by Raez, Manners, and Winnik [42] to form nanotubes readily in block-selective solvents hexane and n-decane, which solubilized the rubbery PDMS blocks and not the crystalline PFS blocks. 

The fact that the PFS-PDMS diblocks at a coronal to core chain repeat unit number ratio of 12 formed elongated tubular nanoaggregates was surprising, because a coil-coil diblock at this composition would have formed spherical micelles. Such an abnormal behavior has been observed also for another PFS-PDMS polymer [43] with a PFS-to-PDMS repeat unit ratio of 1/18 and for... 

The self-assembly of crystalline-coil and rod-coil diblock copolymers in block-selective solvents presented quite some surprises. Crystalline-coil diblocks formed tubular nanoaggregates in block-selective solvents for the coil blocks at coil to crystalUne block repeat unit number ratios substantially larger than 1, e.g., 12 and 18 for the PFS-PDMS diblock copolymers. This made the block copolymer nanotubes much easier to access. It again remains to seen if such a trend can be generalized to other diblock copolymers. Thus, much remains to be done to establish the best experimental conditions for formation of self-assembled nanotubes. Theories need to be developed to understand the formation and property of self-assembled block copolymer nanotubes. 

Herein, we discuss the synthesis of the first PFS block copolymers with a water-soluble polyacrylate coblock (3) by using an anionic ROP followed by anionic polymerization of dimethylethyl methacrylate, as well as well-defined ABC triblock copolymers (4) conceptually derived from the PFS- -PDMS system where a short, amorphous atactic polyferrocenylphenylphosphine (PFP) block is also attached to the PFS segment. The self-assembly behavior of these two types of novel block copolymers were investigated either in water or in hexane. 

In conclusion, the well-defined organometallic amphiphilic AB block copolymer of PFS-PDMAEMA and ABC block polymers of PFP-i-PFS- -PDMS were synthesized via anionic ROP approach. ... 

As for ABC triblock copolymers of PFP-PFS-PDMS, the PFP block is amorphous because of its atacticity, whereas the PFS block is crystalline at room temperature. These triblock copolymers form micelles in hexane, a solvent selective for the PDMS block. When the PFP block is very short (DP=1 or 6) the polymer forms flexible wormUke micelles. A triblock copolymer with a longer PFP block (DP= 11) forms starUke spherical micelles. Films show a peak in the WAXS spectrum characteristic of a crystalline PFS block for the wormUke structures. There is no evidence... 

An interesting and as yet unanswered question for the self-assembly of PFP-PFS-PDMS ABC triblock copolymer is the location of the PFP component in these self-assembled micellar structures. The question is particularly important because the PFP segments present in these materials can coordinate to transition metals. Studies of metal coordination to PFP-Z)-PFS-Z)-PDMS, and the types of nanostructures they form, are ciurently in progress. 

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