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Chapter 12
Stimuli-Responsive Gels Based on Ring-Opened Polyferrocenes: Synthesis, Characterization, and Electrochemical Studies of Swellable, Thermally Cross-Linked Polyferrocenylsilanes Kevin Kulbaba, Mark J. MacLachlan, Christopher E. B. Evans, and Ian Manners *
Department of Chemistry, University of
Toronto,
80 St. G e o r g e Street,
Toronto M5S 3H6, Canada
A series of crosslinked polyferrocenylsilane networks have been prepared via the copolymerization of the silicon-bridged [1]ferrocenophane, fcSiMe (fc = Fe(η -C H ) ) with controlled amounts of a spirocyclic [1]ferrocenophane fcSi(CH ) . Thermal analysis revealed that the crosslinked polyferrocenylsilanes have improved thermal stability relative to their linear counterparts. Swellability was investigated as a function of temperature, solvent and crosslink density. As expected, the degree of crosslinking had a dramatic effect on the swelling in various media. From swellability measurements in various solvents, it was determined that the best solvents for poly(ferrocenyldimethylsilane) are THF, chloroform, and dichloromethane. The solubility parameter (δ) for the homopolymer was found to be 18.7(7) MPa . The redox and spectroscopic properties of the gels were investigated using spectroelectrochemistry in an optically transparent thin-layer electrochemistry (OTTLE) cell. Significant oxidation of the gel was evident at oxidation potentials greater than 450 mV (vs Ag/AgCl), consistent with thefirstoxidation potential of the linear homopolymer. 2
2
5
4
2
3
1/2
© 2003 American Chemical Society
175
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176 Crosslinked polymers can form gels that swell, but do not dissolve, in a liquid medium. A polymer network changes its volume in response to changes in its environment such as temperature, solvent composition, mechanical strain, electric field, exposure to light etc. (la). "Smart gels" are crosslinked networks capable of responding to such a stimulus with a rapid swelling or contraction. As a consequence of this fast dimensional change, these materials have potential for applications in drug delivery, chemical sensing, shape memory, and molecular separation devices (7). The incorporation of transition metals into polymer networks represents an appealing route to gels with electrochemically controllable properties. For example, pendant ferrocene moieties have been incorporated into networks via copolymerization of vinylferrocene with acrylamide and N,N'methylenebisacrylamide. The resulting random copolymer gels have been investigated for potential use as glucose biosensors (2). An interesting redoxactive self-oscillating gel has been synthesized by Yoshida and coworkers by inducing the Belousov-Zhabotinsky reaction within a copolymer gel of Nisopropylacrylamide and tris(2,2'-bypyridine) ruthenium(II). The gel was found to swell and contract at the oxidized and reduced states of [Ru(bpy) ] , respectively. The reaction produces periodic redox changes within the gel, which autonomously pulsates like a beating heart (3a). Tatsuma and coworkers have synthesized a redox-active poly-N-isopropylacrylamide-co-vinylferrocene gel and that displays electrochemically and thermally controllable phase transitions (3b). As a consequence of its high stability and interesting physical properties, ferrocene is an attractive moiety to incorporate into polymeric structures (4). Ring-opening polymerization (ROP) of strained, ring-tilted [l]ferrocenophanes (e.g. 1) provides a well-established route to high molecular weight, soluble polyferrocenes (e.g. 2) (5). For such polymers, metal-metal interactions occur along the polymer backbone, as is illustrated by the cyclic voltammetry of 2 which shows the presence of two reversible oxidation waves which occur at 460 mV and 710 mV respectively vs a Ag/AgCl reference electrode (6). 3+/2+
3
-
1
2
1
n
177
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Recently, we reported the first examples of well-characterized, crosslinked, swellable polyferrocenylsilanes (7). Thermal copolymerization of ferrocenophane 1 with the spirocyclic [l]ferrocenophane 3 allows access to material with controlled crosslink densities. Because of the interesting properties of polyferrocenylsilanes, we identified crosslinked examples as possible candidates for stimuli-responsive gels.
4
3
In this Chapter, the synthesis and characterization of polymer networks formed by copolymerization of monomers 1 (fcSiMe ) and 4 (fcSiPr ) with the spirocyclic [l]silaferrocenophane 3 are reported. The swelling response to changes in temperature, solvent, and substituents at silicon, as well as the effect of crosslinking on the thermal and mechanical properties of the material by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are reported. Lastly, the metal-metal interactions within the polymer network were investigated using spectroelectrochemistry. 2
2
Experimental Monomers 1, 3, and 4 were prepared according to literature procedures (6, 7) and purity was assessed by solution H NMR. A Perkin-Elmer DSC-7 differential scanning calorimeter equipped with a TAC 7 instrument controller was used to study the thermal behaviour of the gels. The thermograms were calibrated with the melting transitions of decane and indium and were obtained at a heating rate of 10°C min" under nitrogen. T values quoted in this study correspond to the inflection point of the heat capacity change. Thermogravimetric analyses were performed at a heating rate of 10°C min" under an atmosphere of prepurified N using a Perkin-Elmer T G A thermogravimetric analyser calibrated with the Curie points of Perkalloy and Nicoseal standards. Controlled potential electrolysis of 5b was effected using a spectroelectrochemistry cell of published design (8, 9). The electrodes consisted of a Pt mesh working electrode, a Pt wire counter electrode and a Ag/AgCl wire reference electrode. The electrolyte solution consisted of 0 . 1 M l
1
g
1
2
178 tetrabutylammonium hexafluorophosphate in freshly distilled CH C1 . Potentials were applied using an AMEL instruments Model 2049 general purpose potentiostat and UV-Vis-NIR spectra were collected with a PerkinElmer Lambda900 UV/Vis/NIR spectophotometer. Data were collected every 0.33 nm between 240 and 2200 nm using an integration time of 0.08 s at a scan rate of 247 nm/min. 2
2
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Synthesis of Poly(ferrocenyldimethylsilane)-Polycarbosilane Network 5a A solution of 10 mg (0.039 mmol) of fcSi(CH ) (3) and 476 mg (1.97 mmol) fcSiMe (1) in 5 mL CH C1 was added to a Pyrex polymerization tube (approx. 1 cm internal diameter and 10 cm length). Solvent was slowly removed under vacuum over a period of 1 h to leave a red powder. After drying under vacuum for an additional 15 min, the tube was sealed under vacuum. The polymerization tube containing the intimate mixture of monomer and crosslinking agent was heated at 140°C for 4 h then 180°C for 4 h. After cooling to room temperature, the contents of the tube appeared orange with some red polymer. The Pyrex tube was cut into 4 sections and stirred for 16 h under N in ca. 100 mL of dry THF, giving a pale yellow solution and a swollen orange gel. The product was isolated by filtration on a Buchner funnel and dried under vacuum for 24 h. Yield: 379 mg (78%) of orange powder. Crosslinked networks 5b-e were prepared using a method analogous to that for 5a. Table 1 gives the experimental data for 5a-e including amounts of starting materials used in the preparations, yields, swellability, and ceramic yield determined from thermal analysis. 2
2
2
3
2
2
Table 1. Experimental Details for the Preparation of Polymer Networks 5a-e. Sample
fcSifCHJs (3)
2
10 mg
476 mg
5b
0.039 mmol 28 mg
709 mg
5c 5d 5c
Mol%3
Yield
25 mg 0.098 mmol
1.91 mmol
51 mg 0.20 mmol
436 mg
76 mg
412 mg
0.30 mmol
1.70 mmol
Ceramic Yield b
(600°C)
2
379 mg 78%
260%
31%
4
383 mg
248%
--
1.97 mmol 2.93 mmol 462 mg
Swellability in THF
(1)
5a
0.11 mmol
a
fcSiMe
52% 5
178 mg 37%
200%
32%
11
275 mg 56%
170%
37%
344 mg
40%
41%
1.80 mmol 18
71%
T H F was chosen as a solvent for swellability measurements as the homopolymer 2 is soluble in
this solvent. Small pieces o f the polymer networks were weighed and immersed in T H F under N for 48 hours.
2
Surface solvent was removed from the pieces and they were reweighed. T h e
swellability given represents the % mass increase.
b
1
Determined by T G A under N at 10°C min" . 2
179 Synthesis of Poly(ferrocenyldi-n-propylsilane)-Polycarbosilane Network 6 A crosslinked network, 6 was prepared in a method analogous to 5a, using 16 mg (0.06 mmol) of fcSi(CH ) (3) and 400 mg (1.34 mmol) fcSi Pr (4). Yield: 103 mg (25 % yield) of an orange-red material 6 with 4 mol % of crosslinker 3. Samples were washed for a period of 4 weeks with fresh THF daily in an effort to remove unwanted oligomeric species from the gel network. Despite such efforts, discoloration of the THF solution was apparent upon standing for 12 hours. Such discoloration may have been due to the mechanical breakdown of the gel during washing. Additional errors associated with the loss of such material was deemed to be insignificant due to the large changes in weight between the gel in the dry and swollen state (a 2200 % mass increase). n
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2
3
2
Theory The Hildebrand solubility parameter is a fundamental thermodynamic property of polymers and is used extensively for the discussion of the miscibility of polymers in solvents and blends. The process of dissolving an amorphous polymer in a solvent is governed by the free energy of mixing, A G = A H -TAS m
m
[1]
m
where A G is the Gibbs free energy change of mixing, A H is the enthalpy change on mixing, T is the absolute temperature, and AS is the entropy change on mixing. Hildebrand and Scott and Scatchard (10) proposed that the free energy of mixing (AH ) can be related to the energy of vaporization (AE ) by: m
m
m
V
m
v
A H = V ((AE i / V 0
1/2
1/2
- (AEV V ) )
m
2
2
frfc
P]
V
where V is the volume of mixture (Vi + V ), AE i is the energy of vaporization of species i and
