Polymorphism of Poly(di-n-pentylsilane)

Abstract

Poly(di-n-pentylsilane) has been previously reported to adopt a 7/3 helical conformation at room temperature. We have studied the solid state structure of a low molecular weight (Mv = 46 000) sample and its dependence on temperature using wide angle X-ray diffraction, differential scanning calorimetry, and Raman spectroscopy. The results suggest that domains of two distinct structures are present, one having an all-trans conformation and the other a 7/3 helical conformation. The all-trans conformation was observed (even at room temperature) in samples cooled previously to -15 °C. Characteristic resonance enhancement of certain Raman vibrations is observed for the planar zigzag polymorph. The all-trans material converts to the 7/3 helical structure at 35 °C and subsequently undergoes an order/disorder transformation at about 56 °C.

Regularities and Peculiarities of Solid Polydialkylsilane Order-Disorder Transitions as Studied by Optical (UV, Raman and IR) Spectroscopy

“…2), all these modifications were previously treated as having all-A (trans) backbone conformations. However, polysilanes with longer side alkyl chains, when forming crystalline modifications with a planar all-A conformation of the backbone, commonly exhibit in the UV spectra more red-shifted bands at 365−375 nm [26,33]. Now the UV bands at 340−350 nm are sometimes regarded as belonging to a near-planar transoid conformation [1b,c], especially since recent X-ray data for model methylated oligosilanes have shown SiSiSiSi dihedral angles of~160° [ 34].…”

Silicon-Containing Polymers

“…Although the anion is the reactive centre, the role of the cation is very important. The ion-ion interaction is so effective that higher Ion aggregates are commonly formed in fast and reversible reaction, Equation 36 [8,107,[122][123][124]. In most systems, the naked silanolate ion does not appear in kinetically significant concentration and cannot be detected by reliable conductance measurements [118,120,121].…”

“…Similarly, heteropolybearing a (61 ) Another example of the use of hydrolytic copolycondensation is the synthesis of random copolymers bearing carboxylate groups that initiate the anionic ring-opening polymerisation of pivaiolactone, thus producing polyesters grafted onto polysiloxane [ 122].…”

“…Among the various reasons for such filler incorporation is the fact that the low melting temperature of PDMS [84,113] makes it difficult to utilise self-reinforcing effects such as strain-induced crystallisation which is seen for many of the commercial organic elastomers having a regular structure. In particular, the isolation of the cis-stereoisomeric trimer and subsequent polymerisation under kinetic conditions leads to polymers which crystallise above room temperature for the R = 3,3,3-trifluoropropyl- [114,115] and phenyl- [116][117][118][119][120][121][122] substituted systems. In particular, the isolation of the cis-stereoisomeric trimer and subsequent polymerisation under kinetic conditions leads to polymers which crystallise above room temperature for the R = 3,3,3-trifluoropropyl- [114,115] and phenyl- [116][117][118][119][120][121][122] substituted systems.…”

“…An exciting development in this area is the stereochemical control of the reactions used in the synthesis ofthe asymmetrically substituted polysiloxanes -[(CH3)RSiO]y -. Subsequent cross-linking of these stereoregular polymers then leads to unfilled siloxane elastomers with properties far superior to their atactic analogs [115,122]. Subsequent cross-linking of these stereoregular polymers then leads to unfilled siloxane elastomers with properties far superior to their atactic analogs [115,122].…”