Poly(ferrocenylmethylsilane): An Unsymmetrically Substituted, Atactic

Article pubs.acs.org/Macromolecules

Poly(ferrocenylmethylsilane): An Unsymmetrically Substituted, Atactic, but Semicrystalline Polymetallocene Van An Du and Ian Manners* School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. S Supporting Information *

ABSTRACT: Polyferrocenylsilanes (PFSs) [Fe(μ-C5H4)2SiRR′]n are generally atactic and amorphous when unsymmetrically substituted at silicon (R ≠ R′) but are often able to crystallize if the substitution is symmetrical (R = R′). In this paper we report detailed studies of the ring-opening polymerization (ROP) of [1]methylsilaferrocenophane Fe(μ-C5H4)2SiMeH (1) by thermal, anionic and photolytic methods to yield an unsymmetrically substituted yet crystallizable poly(ferrocenylmethylsilane) (PFMS) (R = Me, R′ = H) with Me and H substituents at silicon (designated PFMST, PFMSA, and PFMSP, respectively). The structures of the resulting polymers were shown to possess significant differences as revealed by MALDI−TOF mass spectroscopy experiments. For example, PFMSA prepared using n-BuLi as an initiator was shown to contain cyclic contaminants whose formation indicated the existence of backbiting reactions during polymer chain growth. On the other hand, photolytic ROP of 1 using Na[C5H5] as an initiator led only to the formation of linear material but was not a living process due to side reactions between the initiator (and presumably the propagating polymeric anions) and the Si−H groups in the monomer 1. Transition metal-catalyzed ROP of 1 was also explored and, in contrast, was found to afford a hyperbranched and amorphous low molar mass polyferrocenylsilane (4), presumably also as a result of side reactions involving the Si−H groups in the monomer. High resolution 1H and 13C NMR spectroscopic studies revealed that PFMST, PFMSA, and PFMSP were all atactic, irrespective of the polymerization route utilized. The crystallization of the samples was investigated by wide-angle X-ray scattering (WAXS), which showed a reflection corresponding to a d-spacing of 6.32 Å, by differential scanning calorimetry (DSC), which revealed melting endotherms in the range 106−139 °C, and by polarizing optical microscopy (POM).

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polymer.49 Other symmetrically substituted PFSs (R = R′ = nPr, n-Bu, n-Pentyl, H, or Cl)11,50,51 have not been studied in depth but have also been identified as semicrystalline materials based on the presence of melting transitions in DSC studies and, in some cases, sharp peaks in WAXS traces. While symmetrically substituted PFS materials are usually able to crystallize, unsymmetrically substituted analogues are generally amorphous. An apparent exception is poly(ferrocenylmethylsilane) (PFMS) with Me and H substituents at silicon, for which melting transitions at 87 and 102 °C were briefly noted.42 In general, in the absence of significant interchain interactions such as hydrogen-bonding, unsymmetrically substituted polymers are only able to crystallize if they are stereoregular (e.g., polypropylene),52 although even some atactic polymers with sterically undemanding side groups (e.g., polyvinyl chloride) are semicrystalline.53 Until now, only thermal polymerization has been performed to obtain high molecular weight PFMS and no structural investigations have been carried out on this interesting material.42 In this contribution, different polymerization methods were employed

INTRODUCTION Metal-containing polymers are attracting growing attention because of their novel physical properties and have emerging applications in gas sensing, photovoltaic devices, nanopatterning, and as stimuli-responsive gels, synthetic metalloenzymes, macromolecular catalysts and electrocatalysts.1−10 High molecular weight PFSs, which feature iron centers in the polymer main chain together with organosilicon spacers, have been wellstudied due to their synthetic accessibility and interesting properties.4,11−15 These materials have attracted particular attention as redox-responsive polymers,16−31 as precursors to nanostructured iron-containing ceramics,32−36 or as etch resists for plasmas.2,37,38 A variety of synthetic methods has been developed for PFS materials, including thermal, transition metal-catalyzed, anionic and photolytic ROP routes.39−45 Symmetrically substituted PFS homopolymers (R = R′ = Me, Et, n-Pr) are known to be semicrystalline and have been previously studied in detail. In particular, the structure of poly(ferrocenyldimethylsilane) (PFDMS) and its oligomers has been examined by fiber diffraction and single-crystal X-ray methods.46−48 Comparison of the powder X-ray diffraction patterns of the oligomers with that of the higher molecular weight polymer showed that the oligomer crystal structure provides an excellent model for the crystalline domains in the © XXXX American Chemical Society

Received: April 26, 2013 Revised: May 29, 2013

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dx.doi.org/10.1021/ma400866u | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Polyferrocenylsilanes

Scheme 2. Different Polymerization Routes Used in This Study

to synthesize PFMS and detailed structural and morphological studies on the homopolymer were carried out. In-depth knowledge about controlled polymerization approaches and morphology is a prerequisite for the development of PFMS based block copolymers for self-assembly studies. Functionalization of Si−H bonds via hydrosilylation is also possible to create materials with other features of interest, e.g., liquid crystalline PFSs.54,55 This will offer the option to chemically functionalize materials obtained from PFMS based multiblock copolymers

This mild ROP method has opened the door to a whole range of new materials.60,61 1. Synthesis and Characterization of PFMS. In this work, monomer 1 was polymerized by different methods and the resulting polymers were compared regarding their structure and morphology (see Scheme 2). (a). Thermal ROP of 1. Silicon-bridged ferrocenophanes undergo thermally induced ROP when heated at elevated temperatures and yield high molecular weight PFS. This procedure generally allows no control over molecular weight and the molecular weight distribution is fairly broad,42,50,62 which was confirmed in this work on thermal ROP of 1 which afforded PFMST1 (150 °C, 1 h, 78% yield). GPC measurements of PFMST1 showed the material to be of high molecular weight and to possess a large PDI value (Mn = 74 030 g/mol, PDI = 3.26). To exclude the formation of hyperbranched species in appreciable amounts, a 29Si NMR spectrum was recorded and this resulted in only one detectable resonance at −20.8 ppm. 1 H and 13C NMR spectra for PFMST1 were also consistent with data from the literature.42 Polymer PFMST1 with high molecular weight was not ionized during MALDI−TOF experiments, as expected from the literature: samples of PFDMS with molecular weights greater than ∼10 000 g/mol were not ionized under similar experimental conditions.63 (b). Anionic ROP of 1. Living anionic ROP of [1]dimethylsilaferrocenophane with n-BuLi as an initiator is well established as a method to achieve control over the molecular

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RESULTS AND DISCUSSION The first and most general route to obtain PFS materials is by thermal ROP, although no control over molecular weight and molecular weight distribution is possible.39,40,42,56 In contrast, transition metal-catalyzed ROP of silicon-bridged [1]ferrocenophanes, which operates at ambient temperature, is a feasible route to obtain control over molecular weight in the presence of Et3SiH but polydispersities are typically about 2.43,57−59 These deficiencies led to the exploration of alternative ROP routes, such as living anionic ROP initiated by different organolithium reagents, which yields well-defined PFDMS with molecular weight control and low PDI values and also multiblock copolymers.44 In addition, synthesis of monodispersed PFDMS by living photolytic ROP was developed, where initiation of the polymerization by Na[C5H5] was enabled by photoexcitation of [1]dimethylsilaferrocenophane.45 B


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Table 1. Anionic ROP of 1 Initiated by n-BuLi PFMSA1 PFMSA2 a

M:Ia

monomer 1 (mg)

initiator (μL)

Mn (g/mol) calculatedb

Mn (g/mol) foundc

PDI

yield (%)

34:1 21:1

100 100

8 13

7800 4800

8085 5060

2.40 2.37

55 60

Monomer:initiator ratio. bOn the basis of monomer:initiator ratio. cAbsolute value as determined by GPC analysis using a triple detector system.

weight and low PDI values (2.37) for the resulting PFMSA samples and also the inability to prepare higher molecular weight samples (Mn > 8000 g/mol). Nevertheless, the fact that the molecular weight obtained using triple detection (Mn = 5060 g/mol, PDI = 2.37) was close to the theoretical value (Mn(calc) = 4800 g/mol) and reasonably similar to the value determined by 1H NMR end group−main chain integration (Mn = 4330 g/mol), which assumes the material is linear,66,67 suggests that the sample of PFMSA2 consists mainly of linear polymer with only a relatively small amounts of cyclic contaminant. The presence of the latter may be amplified in the MALDI−TOF spectra because of their relatively high volatility. (c). Photolytic ROP of 1. Photolytic living ROP is a powerful tool to create well-defined metal-containing polymers from D


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attributed to a −Si(MeH)OMe group, which should appear at around 20 ppm,69 could not be detected. No cyclic polymer could be detected by MALDI−TOF experiments. Any backbiting reaction products were therefore below the detection limit and were therefore much less significant compared to the case of anionic ROP using n-BuLi as an initiator. The MALDI−TOF spectrum of the sample of PFDMSP1 (Mn = 2200 g/mol, PDI = 1.05) obtained by photolytic ROP terminated by MeOH was recorded for comparative purposes and also showed only linear polymeric species (see Figure S7, Supporting Information). The resulting larger than expected molar masses could be caused by consumption of the initiator by side reactions. To explore this hypothesis, 1 equiv of Na[C5H5] was added to PFMSP1 and the mixture was irradiated at 5 °C for 16 h, conditions identical to the polymerization experiment. In addition, another experiment at 25 °C without UV-light exposure was carried out. Neither procedure resulted in any detectable attack on the Si−H bonds. Significantly, however, reaction of 1 equiv of Na[C5H5] with monomer 1 after 16 h at 25 °C without exposure to UV-light led to partial nucleophilic substitution of the Si−H group with cyclopentadienyl anion (to give 2) and the ring opened species (3) (see Scheme 5), which was identified by 1H and 13C NMR experiments (see Figures S8 and S9, Supporting Information). Similar polymerization results were observed with Li[C5Me4H] as an initiator: the molecular weight obtained (Mn = 1810 g/mol, PDI = 1.93, as measured by GPC) was almost twice as high as the initially targeted molecular weight (Mn(calc) = 1200 g/mol). Reaction of 1 equiv. Li[C5Me4H] with monomer 1 after 16 h at 25 °C without exposure to UV-light also resulted in products and yields similar to those obtained in the reaction with Na[C5H5] (20% of 2# and 14% of 3#, see Figure S10, Supporting Information). Therefore, the bulkier nature of the [C5Me4H] anion did not appear to prevent nucleophilic substitution at the Si−H moieties. Presumably, analogous reactions for propagating polymeric anions and 1 may be responsible for the large PDI values determined for the PFMSP samples formed using Li[C5R4H] (R = H or Me) initiators. (d). Transition Metal-Catalyzed ROP of 1. Transition metalcatalyzed ROP (PtCl2, toluene, 3 h, 70 °C) of 1 resulted in hyperbranched polymer 4 (see Scheme 6) which was confirmed by NMR analysis. The 1H NMR spectrum showed signals at 3.96−4.36 ppm corresponding to Cp protons and peaks of −SiCH3 groups were observed at 0.34−0.77 ppm (see Figure S12, Supporting Information). The 29Si NMR spectrum revealed a signal at −10.2 ppm for the SiMeCp3 group (see Figure S13, Supporting Information), which is shifted to lower field compared to the -Si(MeH)- group in PFMST1/A2/P1 (around −20 ppm). Tang et al. previously synthesized this hyperbranched polymer by a condensation reaction of 1,1′dilithioferrocene with methyltrichlorosilane.70 These workers

was Mn = 1900 g/mol (PDI = 1.18), which was almost twice as high as the initially targeted molecular weight (see Table 2). The recorded MALDI−TOF spectrum of PFMSP3 showed two distributions (see Figure 3). One species corresponded to

Figure 3. MALDI−TOF spectrum (linear mode) of sample PFMSP3 (Mn = 1900 g/mol, PDI = 1.18).

linear PFMS with a Cp end group, while the other species had a repeat unit of 228 g/mol conforming to the PFMS repeat unit and an -OMe end group, which was also visible in the 1H NMR spectrum (see Figure S6, Supporting Information). This end group can arise from the treatment with MeOH, the quenching agent used in the reaction.60 A closer inspection of the 29Si NMR spectrum of PFMSP3 revealed two signals, one peak at −20.6 pm for the −Si(MeH)− repeat unit and another peak at −20.7 ppm, assigned to the −Si(MeH)Cp end group (see Figure 4). A peak

Figure 4. PFMSP3.

29

Si NMR (CDCl3, 500 MHz) spectrum of polymer

Scheme 5. Reaction Products after Treatment of 1 with Na[C5H5]

E


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3.26), PFMSA2 (Mn = 5060 g/mol, PDI = 2.37) and PFMSP1 (Mn = 72 100 g/mol, PDI = 2.14) were carried out (see Figure 5). In the case of PFMSA2, the repeat units associated with the cyclic and linear polymer components would be expected to be indiscernible by NMR spectroscopy based on previous work63 with cyclic and linear PFDMS. This indicated that the cyclic contaminants present in PFMSA2 were not expected to complicate the high resolution NMR spectra and the tacticity analysis. The 29Si NMR measurements performed on PFMST1, PFMSA2, and PFMSP1 revealed shifts at around −20 ppm for the −Si(MeH)− groups in the main chain. For PFMSA2, a small peak at −15.1 ppm was assigned to the n-Bu end group (see Figure 2). The absence of other silicon peaks showed that the amount of hyperbranched PFMST1, PFMSA2, and PFMSP1 was below the detection limit. The observed resonances for the Si-bound methyl group in the 1H NMR and 13C NMR spectra indicated an atactic polymer and could be assigned to mm (or rr), mr/rm, and rr (or mm) triads (Scheme 7).71,72 The central peak of the methyl resonances in 1H and 13C NMR stemmed from the mr and rm mirror images configuration, which cannot be distinguished by NMR. The Si−H resonance in the 1H NMR spectra of each polymer obtained by a different polymerization method consisted of six peaks (see Figure S14, Supporting Information). Integration led to a 1:5:10:10:5:1 ratio and could be ascribed to four overlapping quartets with a ratio of 1:2:1. As far as PFMSA2 is concerned, cyclic and linear species are expected to be indistinguishable by 1H and 13C NMR. All of the polymerization methods employed resulted in atactic polymers, which indicated that steric effects that would be expected to favor the formation of a syndiotactic

Scheme 6. Reaction Scheme of Transition-Metal-Catalyzed ROP of 1 Leading to 4

reported that the protons of the Cp rings exhibited a broad 1H NMR resonance peak centered at 4.2 ppm, which is close to our measurement (3.96−4.36 ppm) and those of the methyl group resonated at 0.6 ppm compared to our peaks at 0.34− 0.77 ppm. The integration of the Cp and Me protons gave a ratio of 12:3 which is the same as for 4. The measured GPC trace of the hyperbranched structure showed low molecular weight and a high PDI (Mn = 1080 g/mol, PDI = 1.89). Activation of the Si−H bond in 1 during transition metalcatalyzed ROP by PtCl2 is possibly responsible for the formation of hyperbranched polymer.58,59 2. Tacticity studies on PFMST1, PFMSA2 and PFMSP1. To gain insight into the tacticity of the unsymmetrically substituted polymer obtained from different polymerization routes (see Scheme 2), high resolution 1H and 13C NMR investigations on samples PFMST1 (Mn = 74 030 g/mol, PDI =

Figure 5. 1H NMR (CDCl3, 500 MHz) spectra (top) and 13C NMR (CDCl3, 500 MHz) spectra (bottom) of PFMST1 (a, d), PFMSA2 (b, e), and PFMSP1 (c, f). F


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Scheme 7. Triad Configuration Sequences for an Atatic Polymer

with amorphous halo see Figure S15, Supporting Information); therefore, neither the polymerization method nor molar mass significantly influenced the morphology. A similar result was previously noted in a study on PFDMS.74 In comparison, previous WAXS studies on PFDMS obtained by thermal ROP showed a reflection corresponding to a d-spacing of 6.4 Å.42 In addition, no change in d-spacing was observed for polymer PFMSA2 consisting mainly of linear polymer but with cyclic contaminants. This suggested that the lattice parameters of all species are similar, in analogy to results obtained for linear and cyclic PFDMS.63 Previous investigations on linear oligo(ferrocenyldimethylsilanes) have assigned the strongest reflection (d = 6.29 Å) to the distance between (011) planes.75 For high molecular weight polymers, the value measured is slightly higher (d = 6.37 Å) than for PFMS (d = 6.32 Å). The reduced steric demand due to the hydrogen substituent at the silicon atom does not have a strong impact on the distance between the (011) planes. This may be a result of the atacticity of the polymer. In comparison, the d-spacing is 5.87 Å for poly(ferrocenyldihydrosilane),50 with two hydrogen substitutents at silicon. This is significantly lower than that reported for Me/Me or Me/H substituted PFSs.50 Sample preparation for TEM imaging of polymer PFMSP1 consisted of dissolution in THF and addition of n-hexane until precipitation occurred. The mixture was then heated at 60 °C for 3 h until the polymer redissolved and the solution was aged for 16 h at room temperature to induce crystal formation before TEM measurements. The TEM images obtained showed homopolymer arranged as twisted fibrillar structures with a typical width of about 500 nm (see Figure 7a). The resulting star-shaped assemblies are shown in Figure 7b. Similar structures were observed for PFDMS49 and it is noteworthy that Reiter, Hu et al. were able to clone polymer single crystals of PFDMS through a self-seeding procedure.76 Selected area electron diffraction (SAED) patterns were collected to confirm the crystalline nature of the fibrils, which was shown to consist of ordered homopolymer. The welldefined diffraction ring (see SAED pattern in Figure 7c) confirmed polycrystallinity. The ring indicated a d-spacing of 6.28 Å which is similar to the d-spacing obtained from the most intense reflection of the WAXS measurements (see Figure 6). In earlier work, electrospun nanofibers of PFDMS where shown to be crystalline by SAED measurements.47 (b). Thermal Transition Behavior of PFMST1, PFMSA2, and PFMSP1 by DSC Analysis. The thermal behavior of each polymer was studied by DSC experiments. Previous DSC investigations on unsymmetrically substituted PFMS showed

microstructure are insignificant. The formation of atactic material has already been shown for thermal ROP of different unsymmetrically substituted monomers.51 Syndiotactic poly(ferrocenylmethylphenylsilane) was obtained after γ-irradition induced polymerization of [1]methylphenylsilaferrocenophane monomer, where a specific orientation was enforced by the use of monomer crystals.73 A hyperbranched structure could be excluded for PFMST1, PFMSA2, and PFMSP1 as, in addition of the aforementioned 29Si NMR studies, the ratios of the integrated resonances for Cp and methyl groups in the 1H NMR spectra of polymers obtained via thermal or photolytic ROP were both 1:0.36 and for the anionic ROP 1:0.35 (theoretical value, 8:3 or 1:0.375). In addition, the ratio of the Cp integral to the Si−H region integral was 1:0.12 for polymer PFMST1 and PFMSP1, whereas a ratio of 1: 0.11 was observed for polymer PFMSA2 (theoretical value, 8:1 or 1:0.125). 3. Morphological Studies. (a). WAXS Measurements of PFMST1, PFMSA2, and PFMSP1. The atactic samples of PFMST1, PFMSA2, and PFMSP1 all showed a large, broad amorphous halo together with a significant reflection in the measured WAXS diffractogram at a 2θ angle of 14.00° (corresponding to a d-spacing of 6.32 Å) and a less intense reflection at 16.07° after removal of the amorphous halo (d = 5.50 Å). Identical reflections of similar intensity were observed for each polymer synthesized (see Figure 6, for WAXS traces

Figure 6. WAXS pattern for a film drop-cast from a THF solution of polymer PFMST1, PFMSA2 and PFMSP1 onto a silicon wafer. The asterisk denotes an artifact peak commonly observed for studies of this type using the instrument employed. The amorphous halo was removed for improved analysis of the diffraction peaks (WAXS patterns of polymers PFMST1, PFMSA2 and PFMSP1 with their amorphous halos are shown in Figure S15, Supporting Information). G


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Figure 7. TEM images (a, b) and SAED pattern (c) of homopolymer PFMSP1. A sample of PFMSP1 was dissolved in THF and n-hexane added until precipitation occurred. The mixture was then heated at 60 °C for 3 h until the polymer redissolved and the solution was aged for 16 h at room temperature to induce crystal formation. Scale bars correspond to 2000 nm.

only weak melt endotherms at 87 and 102 °C which disappeared in subsequent scans.42 The glass transition temperatures (Tg) obtained was found to vary with the polymer chain length. The material obtained via anionic ROP (PFMSA2, Mn = 5060 g/mol, PDI = 2.37) exhibited a weak Tg transition at 25 °C whereas the higher molar mass materials (PFMST1, Mn = 74 030 g/mol, PDI = 3.26; PFMSP1, Mn = 72 100 g/mol, PDI = 2.14) resulted in a weak transition at 35 °C. Flory et al. have developed an equation, where Tg depends on the number-average molar mass. This equation is appropriate for high molecular weight polymers (>5000 g/mol).77 O’Driscoll defined a law, which described an increasing Tg with an increasing Mn also for low molecular weight polymers.78 For polymer PFMSA2 two endotherms were observed at 98 and 118 °C which were shifted to higher temperatures with increasing molar mass for polymer PFMST1 and PFMSP1 (see Table 3). Multiple melting

Figure 8. DSC scan obtained at a scan rate of 10 K/min. Sample PFMSP1 was isothermally crystallized at the indicated temperatures for 16 h, respectively.

Table 3. Thermal Transition Temperatures of PFMST1 (Mn = 74 030 g/mol, PDI = 3.26), PFMSA2 (Mn = 5060 g/mol, PDI = 2.37) and PFMSP1 (Mn = 72 100 g/mol, PDI = 2.14) PFMST1 PFMSA2 PFMSP1

Tg (°C)

Tm (°C)

35 25 35

106/139 98/118 104/125

melting temperatures obtained after different isothermal crystallization temperatures is due to the thickening of lamellae formed at the crystallization temperature.84 Upon annealing at 120 °C, the melting temperature did not increase significantly, leading to the estimated Tm(equilibrium) = 131 °C for PFMSP1. A value of 145 °C has been previously reported74 for PFDMS although other studies85 have suggested the value is significantly higher. (c). Morphology Studies of Hyperbranched Polyferrocenylsilane 4. As discussed above, 1H and 29Si NMR measurements of material 4 (see Scheme 6) obtained by transition metal-catalyzed ROP of monomer 1 indicated the polymer to be hyperbranched. DSC characterization of 4 revealed two separated but very weak transitions at 74 and 105 °C, insufficiently clear to confirm semicrystallinity (see Figure S16, Supporting Information). Furthermore, the WAXS pattern measured showed an amorphous halo with no diffraction peaks, which is typical of amorphous materials (see Figure S17, Supporting Information). This confirmed that the hyperbranched material was amorphous compared to the semicrystalline PFMS obtained. (d). Studies of PFMSA2 by Polarizing Optical Microscopy. Polarizing optical microscopy is a useful technique for identification of birefringent materials (which polarize light due to their anisotropy). A film of polymer PFMSA2 was melted at 150 °C on a precleaned glass slide and appeared black under crossed polarizers at room temperature (Figure 9a). After

transitions are frequently observed and can be explained by various theories: (i) melting and subsequent recrystallization of one initial crystal morphology,79,80 (ii) different melting behavior of two distinct morphologies,81,82 and (iii) physical aging, where changes in the morphology of the polymer occur over a certain time period.83 After a second scan the endotherms disappeared, however crystal growth could be induced by annealing the polymer below the melting temperature, which was also evident in polarizing optical microscope (POM) studies of the polymer films (see below). This has already been shown for PFDMS.74 Polymer PFMSP1 was subjected to isothermal crystallization at different temperatures. Before every annealing step (70, 85, 95, 105, 115, and 120 °C) the polymer was kept in the melt (at 200 °C) for 5 min to erase the thermal background. The melting temperatures obtained after each isothermal crystallization step are shown in Figure 8. The melting temperature was shifted to higher values with increasing annealing temperatures, which can be explained by the Hoffmann−Weeks theory: the difference between the H


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Figure 9. POM images under crossed polarizers of open PFMSA2 films at room temperature: (a) a film melted at 150 °C and cooled to room temperature and left for several hours and then (b) annealed at 95 °C for 2 d and (c) after 14 d at room temperature.

annealing the slide at 95 °C for 2 d under nitrogen, the material showed birefringence at room temperature (Figure 9b). The texture after 2 weeks under ambient conditions showed evidence for crystallites (Figure 9c). No change in the texture could be observed which confirmed its thermodynamic stability over this period. Clearly, the polymer sample needs to be annealed over a certain period to obtain anisotropy as determined by POM. Crystallization was thermally activated and nucleation and growth depended on annealing temperature and time as shown by DSC experiments (see above). Similar behavior was observed for the high molecular weight PFMST1 and PFMSP1 (see Figure S18, Supporting Information). The texture formed after annealing the films at 95 °C for 2 d were not as well-defined as for the low molecular weight PFMSA2 sample.

of PFDMS. The presence of two melting points which occur in the DSC heat-scan can be assigned to reorganization of the crystals during the heating process, as previously detected for PFDMS.74 POM images of melted and annealed polymer films support the necessity of annealing to form a crystalline polymer. Future work will target the design of block copolymers with a PFMS block, which are desirable for studies of the influence of unsymmetrically substituted, atactic but crystalline core-forming block segments on their solution selfassembly behavior.86

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EXPERIMENTAL SECTION

(a). Materials and Equipment. [1]Methylsilaferrocenophane (1) was synthesized according to the method previously described in the literature.42 The monomer was purified by repetitive sublimation and recrystallization from n-hexane. All chemicals were purchased from Aldrich and used as received unless otherwise noted. Dichloromethylsilane was distilled from CaH2 before use. Solvents were predried using anhydrous engineering double alumina and alumina/copper catalyst drying columns or by conventional methods. THF was distilled under reduced pressure from Na/benzophenone prior to each polymerization reaction. The photoirradiation experiments were carried out using Pyrex-glass-filtered emission (λ > 310 nm) from a 125 W high pressure Hg arc lamp (Photochemical Reactors Ltd.). The molecular weight and polydispersity indices (PDI) were obtained by triple-detection gel permeation chromatography (GPC) using a Viscotek VE 2001 triple-detector gel permeation chromatograph equipped with automatic sampler, pump, injector, inline degasser, column oven (30 °C), styrene/divinylbenzene columns with pore sizes of 500 Å and 100 000 Å, VE 3580 refractometer, fourcapillary differential viscometer, and 90° angle laser and low angle laser (7°) light scattering detector (VE 3210 and VE270). A 0.025% butylated hydroxytoluene (BHT) stabilized THF (Fisher) was used as the eluent, with a flow rate of 1.0 mL/min. The samples were dissolved in the eluent (ca. 2 mg/mL) and filtered (Acrodisc, PTFE membrane, 0.45 μm) before analysis. 1 H NMR, 13C NMR, and 29Si NMR spectra were recorded on a JEOL ECLIPSE 400 and a VARIAN NMR 500 MHz spectrometer. Each 29Si NMR spectrum was acquired over 14 h. MALDI−TOF mass spectra were collected on a 4700 Proteomics Analyzer (Applied Biosystems) equipped with a Nd: YAG laser, operating at 335 nm. Positive ion mass spectra were obtained in linear mode over a range of m/z values. Each spectrum was an accumulation of 10000 laser shots over 50 points on the sample (200 shots/point). Laser intensity was varied. Samples for analysis were prepared from a solution in THF (1 mg/mL) and a THF solution of dithranol (200 mg/mL) as a matrix (1:1 volume ratio) and drop-cast by micropipet into sample wells. For wide-angle X-ray scattering measurements, a film of the sample was drop cast from THF solution onto a silicon wafer and data were collected with Cu Kα radiation (λ = 1.5418 Å) on a Bruker D8 Advance powder diffractometer fitted with a 2 mm fixed divergence

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SUMMARY As expected, thermal ROP of monomer 1 led to PFMST1 with a broad molar mass distribution and a high but uncontrollable molecular weight. Anionic polymerization of 1 with organolithium initiators gave materials with relatively low molar masses and also large PDI values. The structure of PFMSA2 obtained by anionic ROP initiated by n-BuLi was investigated by MALDI−TOF mass spectroscopy and showed the presence of cyclic contaminants whose formation indicated the occurrence of backbiting reactions during polymer chain growth. Photolytic ROP of 1 led to PFMSP with higher than targeted molecular weights over a wide range and broad molecular weight distributions. We have shown that quenching of the initiator Na[C5H5] by nucleophilic substitution reactions at the Si−H moiety in monomer 1 which, presumably together with analogous reactions for propagating polymeric anions, appear to be responsible for these observations. MALDI−TOF mass spectra and 29Si NMR measurements of the lowest molecular weight PFMSP3 revealed the presence of linear PFMS only and confirmed that no significant backbiting reactions took place during photolytic ROP. Transition metalcatalyzed ROP of 1 resulted in a hyperbranched and amorphous low molar mass material (4). PFMS obtained by thermal, anionic and photolytic ROP was found to be atactic and formation of branched sites could not be detected by 1H and 29Si NMR measurements. Morphological studies revealed the semicrystalline character of PFMS and d-spacings calculated from WAXS measurements were independent of the thermal or anionic, or photolytic polymerization route or molar mass. Apparently, the reduced steric demand of the hydrogen substituents does not have a strong impact on the crystallization behavior compared to that I


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(e). Transition Metal-Catalyzed ROP of 1 To Yield Hyperbranched Polyferrocenylsilane 4. To a mixture of 1 (200 mg, 0.88 mmol) and degassed toluene (8 mL) 2.3 mg (0.009 mmol) of PtCl2 was added. The mixture was refluxed for 3 h at 70 °C and subsequently cooled to room temperature. The polymer solution was filtered and washed several times with THF over basic Al2O3 to remove the platinum. The filtrate was collected and the solvent was removed. After addition of 1 mL THF the polymer was precipitated in methanol. The material was dried under vacuum at 50 °C for 20 h. Yield: 60 mg (30%). 1H NMR, δ (ppm, CDCl3, 500 MHz): 0.34−0.77 (m, SiCH3), 3.45−4.36 (m, Cp). 13C NMR, δ (ppm, CDCl3, 500 MHz): −4.3−1.0 (SiCH3), 68.5, 68.6 (ipso-C, Cp), 70.8−73.8 (Cp). 29 Si NMR δ (ppm, CDCl3, 500 MHz): −10.2 (Si(MeCp3)). GPC: Mn (PDI) = 1080 g/mol (1.89).

slit, knife-edge collimator, and a LynxEye area detector. Data were collected between 5 and 50 degrees 2θ in θ/2θ mode with a step width of 0.05°. The samples for electron microscopy were prepared by drop casting 1 drop of suspension of the sample onto a carbon coated copper grid which was placed on a piece of filter paper to remove excess solvent. Bright field transmission electron microscopy (TEM) micrographs were obtained both on a JEOL1200EX TEM Mk1 and Mk2 microscope operating at 120 kV and equipped with an SIS MegaViewIII digital camera. Selected area electron diffraction (SAED) patterns were obtained with a JEOL 1200EX Mk1. The dspacing of SAED patterns was calibrated using a TlCl standard. DSC measurements were performed on a TA Instruments Q100 at a scan rate of 10 K/min under a 50 mL/min flow of prepurified nitrogen. Samples were placed in hermetic aluminum pans and weighed using a XT220A Precisa microbalance. DSC scans were analyzed using the software Universal Analysis 2000 version 4.5a build 4.5.0.5 from TA Instruments. The glass transition temperatures and melting temperatures were determined as the value at the intersection of two tangents at the start of the transition. Glass substrates (1 cm × 1 cm) were subject to a preliminary clean in an ultrasonic bath in methanol for 15 min and subsequently in acetone for 15 min. The slides were dried in a nitrogen stream and oxygen plasma etched using a Harrick Plasma Cleaner for 60 s to remove impurities and dust on the glass surface. Polarizing microscope investigations were carried out using a Leica Polarizing Microscope (DM750 P). Images were captured automatically with the Leica Application Suite MultiTime module. (b). Thermal ROP of 1 to yield PFMST1. A glass tube was loaded with 1 (250 mg, 1.10 mmol), evacuated and sealed. The sample was heated at 150 °C for 1 h. After cooling the tube to room temperature, the polymer was dissolved in THF and precipitated in methanol. The solid was filtrated and dissolved in THF for two additional precipitations in methanol. The polymer was dried under vacuum at 50 °C for 20 h and a yield of 194 mg (78%) was obtained. 1H NMR, δ (ppm, CDCl3, 400 MHz): 0.55−0.57 (d, SiCH3), 4.08−4.14 (m, Cp), 4.28−4.29 (m, Cp), 4.89−4.93 (q, SiH). 13C NMR, δ (ppm, CDCl3, 400 MHz): −4.3, −4.3, −4.3 (SiCH3), 67.7 (ipso-C, Cp), 71.9, 72.0, 73.8, 74.4 (Cp). 29Si NMR, δ (ppm, CDCl3, 500 MHz): −20.8 (-Si(MeH)−). GPC: Mn (PDI) = 74 030 g/mol (3.26). (c). Anionic ROP of 1 To Yield PFMSA2. In an Ar-filled glovebox, 13.0 μL of n-BuLi (1.6 M in hexanes, 20.10 μmol) was added to a stirred solution of 1 (100 mg, 0.44 mmol) in 2 mL of dry THF at 25 °C. After 5 min, the color of the solution changed from red to amber and the solution was stirred for further 55 min at room temperature. The reaction was quenched with NH4Cl and stirred for several minutes. The polymer was precipitated into methanol and redissolved in THF. This was repeated three times. After the last precipitation step the homopolymer was collected by filtration through a glass funnel and dried under vacuum at 50 °C for 20 h to give an amber solid in a yield of 60 mg (60%). 1H NMR, δ (ppm, CDCl3, 400 MHz): 0.56−0.57 (d, SiCH3), 4.08−4.15 (m, Cp), 4.28−4.30 (m, Cp), 4.89−4.93 (q, SiH). 13 C NMR, δ (ppm, CDCl3, 400 MHz): −4.3, −4.3, −4.3 (SiCH3), 67.7 (ipso-C, Cp), 71.9, 72.1, 73.9, 74.5 (Cp). 29Si NMR, δ (ppm, CDCl3, 500 MHz): −20.7 (−Si(MeH)−), −15.1 (-Si(MeH)n-Bu). GPC: Mn (PDI) -= 5060 g/mol (2.37). (d). Photolytic ROP of 1 To Yield PFMSP1. In the absence of light, 1 (300 mg, 1.32 mmol) was dissolved in 4 mL of dry THF in a Schlenk tube in an Ar-filled glovebox. After the addition of Na[C5H5] (2.0 M, 22 μL, 0.04 mmol), the mixture was photolyzed for 16 h at 5 °C. The reaction was quenched with a few drops of degassed methanol and the polymer was precipitated into methanol and dissolved in THF three times. The homopolymer was collected by filtration through a glass funnel and dried under vacuum at 50 °C for 20 h to give an amber solid in a yield of 254 mg (85%). 1H NMR δ (ppm, CDCl3, 400 MHz): 0.56−0.57 (d, SiCH3), 4.08−4.15 (m, Cp), 4.28−4.30 (m, Cp), 4.89−4.93 (q, SiH). 13C NMR δ (ppm, CDCl3, 400 MHz): −4.3, −4.3, −4.3 (SiCH3), 67.7 (ipso-C, Cp), 71.9, 72.1, 73.9, 74.5 (Cp). 29Si NMR δ (ppm, CDCl3, 500 MHz): −20.9 (-Si(MeH)−). GPC: Mn (PDI): 72 100 g/mol (2.14).

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ASSOCIATED CONTENT

* Supporting Information S

MALDI−TOF, ESI, and 1H, 13C, and 29Si NMR spectra, a GPC trace, WAXS patterns, a DSC scan, and POM images. This material is available free of charge via the Internet at http:// pubs.acs.org/

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. * Corresponding author: [email protected].

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ACKNOWLEDGMENTS V.A. Du thanks the Deutsche Forschungsgesellschaft (DFG) for a Postdoctoral Fellowship. I. Manners thanks the EU for an ERC Advanced Investigator Grant. The authors would like to thank the Bristol Centre for NanoScience and Quantum Information (NSQI) for providing the POM and F. Brömmel for POM measurements.

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REFERENCES

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Poly(ferrocenylmethylsilane): An Unsymmetrically Substituted, Atactic

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