chapter 30. inorganic and organometallic polymers
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30 Inorganic and organometallic polymers
By IAN MANNERSDepartment of Chemistry, University of Toronto, 80 St. George St., Toronto M5S 3H6,
Polymeric materials based on inorganic elements continue to attract attention as aresult of their interesting and unusual properties and applications as speciality ma-terials.14 This review focuses on developments in inorganic and organometallicpolymer science published in 1997 and has a similar format as and follows on from thethree previous articles in the series which cover the years 19911996.510 The firstsections of the review cover new developments concerning the inorganic polymersystems based on main group elements including the well established polysiloxanes,polyphosphazenes and polysilanes.13 A brief introduction to each of these classes ofinorganic polymer systems was included in the appropriate sections of the first articleof this series.5 Following this section, recent developments concerning polymers basedon transitionmetals are discussed.4 As with previous articles in this series,510 the mainemphasis is placed on polymers with inorganic elements within the main chain ratherthan in the side group structure. A review of inorganic polymer science, which focusesmainly on the new polymer systems prepared recently was published in 1996 and mayalso be of interest to readers.11
2 Polysiloxanes (silicones), polysilanes and other silicon-containingpolymers
Molenberg and Moller have reported detailed studies of the structure and phasetransitions in poly(diethylsiloxanes) using differential scanning calorimetry (DSC) andtransmission electron microscopy (TEM) studies on replicas of freeze-fracturedsamples.12 The diethyl-substituted polymer is the first member of the series ofpoly(dialkylsiloxanes) which is capable of forming a mesophase and forms differentcrystalline polymorphs. Narrow molecular weight samples were prepared via anionicring-opening polymerization (ROP) of hexaethylcyclotrisiloxane (Scheme 1). Usingthis system, above M
n[ 105 the polydispersity broadens to above 1.3. A number of
interesting observations were apparent from studies of samples with different molecu-lar weights. Thus, the isotropization temperature was found to strongly depend onmolecular weight. In addition, no mesophase was formed below a critical value ofM
n\ 28 000.
Royal Society of Chemistry Annual Reports Book A
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BusEt2SiOLicryptand 211 Me3SiCl
1 or 2 Si(O) CH2n
CH2 CH MePhSiBus Hn m
BusLi i Si4Ph4Me4
In other areas of polysiloxane chemistry, research has focused on ferroelectric liquidcrystalline materials based on biphenylcarboxylate mesogenic groups andoligooxyethylene spacers.13 In addition, work on the ethanol permselectivity ofpoly(dimethylsiloxane) membranes and the control by surface modification by addi-tives has been described.14 Research targeting silicon oxycarbide ceramics via thepyrolysis of polycarbosilane/polysiloxane hybrid polymers has also been reported.The hybrid materials were prepared via solgel processing of cyclic carbosilanes 1 andpolycarbosilanes 2 containing ethoxysilane moieties (Scheme 2).15
Moller and co-workers have reported studies of a polystyrenepolysilane blockcopolymer 3 with a polystyrene block with M
n\ 18 700 and poly(methylphenylsilane)
block with Mn\ 9000.16 This material was prepared via the anionic ROP of cyclo-
silane using living polystyrene anions (Scheme 3). Microphase separation was ob-served and for the aforementioned material by TEM of a thin section of a film whichwas cast from THF, which is slightly selective for the polystyrene block. Poorly definedwormlike domains of polysilane were observed in a polystyrene matrix. After exposureto UV light which photodegrades the polysilane blocks, a texture consistent with theTEM results was detected by scanning force microscopy.
A remarkable thermo- and ion-responsive non-ionic water soluble polysilane 4 hasalso been reported.17 This polymer shows a j
.!9\ 281nm which is blue shifted from
the usual value when dissolved in water. This was attributed to the reduction in thedegree of conjugation due to the presence of a more distorted polymer backbone as a
604 I. Manners
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consequence of strong solutesolvent interactions. Above the lower critical solutiontemperature at 46 C (concentration\ 0.40m) the solution turns opaque due to theabrupt onset of light scattering due to association. In addition, an instantaneousbathochromic shift of j
.!9from 281 to 320nm is observed suggesting increasing
p-delocalization. Interestingly, the LCST can be tuned via the addition of inorganicsalts.
Studies of the photodegradation of poly(phenylmethylsilane) using GPC/light scat-tering analysis have also been reported.18 In addition, adjacent reentry of foldedpoly(dimethylsilane) polymer chains has been established using atomic force micro-scopy.19 Molecular scale resolution of poly(dimethylsilane) single crystals using AFMrevealed rows of rod-like features which were much longer than the SiSi bond lengthwhich were assigned to chain folds at the single crystal surface, as expected for theregular adjacent reentry model.
Anionic polymerization of 3-methylenesilacyclobutanes has been reported byYamaoka and co-workers.20 Reaction of the silacyclobutane 5 wih BuLi in THF at[78 C followed by treatment with methanol yielded the novel polycarbosilane 6.This material could be hydroborated with BH
3THF and after alkaline hydrolysis a
hydroxyl functionalized material could be isolated (Scheme 4). Cyclopropanation wasalso attempted but side reactions were also observed.
Yokoyma and co-workers have described the formation of gold colloids inAu/poly(methylphenylsilane) layered films by heat treatment which depends on UV-light pre-exposure (Fig. 1).21 Colour variations were detected only in the areas of thepolysilane films which were exposed to UV light prior to Au film deposition. Studiesindicated that the thermally induced Au colloid formation is strictly related to thedegree of photodecomposition of the polysilane surface. The Au/polysilane layeredfilms were used as novel materials for write-once laser optical disc memory. To achievethis a Ti phthalocyanine layer was vacuum deposited under the polysilane layer.Optical recording was performed using a laser disc head equipped with a diode laser(830nm) focused on a tiny spot in the TiOpc layer. The recording process of pitregistration was monitored by the reflection of a stationary low power laser. A distinctdecrease in the reflectance of the monitor light intensity from the Au surface wasobserved on Au colloid formation. The recording contrast (R
2) where R
2are the reflected intensity before and after laser recording was monitored as a functionof laser power.
605Inorganic and organometallic polymers
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Photoconductivity of poly(disilyleneoligothienylene)s 7 with SiEt2SiEt
thiophene groups in the polymer main chain has been studied.22 These materialspossess photocarrier generation maxima in accordance with their optical absorptionspectra. The polymer with four thiophene groups per repeat unit was photoconductingwhen irradiated with visible light and the quantum efficiency for photocarrier gener-ation was 2% at 480nm (electric field strength\ 6] 105V cm~1). The hole mobilitieswere found to be ca. 12] 10~4 cm2V~1 s~1 at room temperature at fields of26] 105V cm~1. Addition of C
60enhanced the photoconductivity quantum effi-
ciency effectively to 85% at 470nm (field strength\ 3] 105Vcm~1) via a photoin-duced charge transfer mechanism.
7 x = 24
3 Polyphosphazenes and other polymers based on main groupelements
Further developments in the novel ambient temperature synthesis of polyphos-phazenes reported in 1995 have been described.23 Polyphosphazene block copolymersare avai