Conformational Analysis of Methyl−Phenyl−Siloxane Chains

Abstract

The conformational energy of chain fragments of poly(methylphenylsiloxane) is studied, considering contributions from bond stretching, bond angle bending, bond torsion, and van der Waals interactions. Energy is minimized without constraints, so that minima correspond to fully relaxed conformations. The energy minima of this phenyl-substituted polymer differ notably from those of the parent poly(dimethylsiloxane), due to the importance of the attractive interactions between pairs of phenyl groups. For two repeat units (diad), the stable conformations occur when the pair of phenyl rings is coupled parallel, face-to-face. For three repeat units (triad), the stable conformations occur also when a pair of phenyl rings is coupled parallel, face-to-face, but not necessarily those in consecutive units. Instead, the chain can coil to couple together the phenyl rings of alternate units, leaving uncoupled the phenyl ring in between. This coiling of the chain to yield a stable conformation is a genuine triad effect that cannot be predicted from the diad energies. In longer chain sequences, the contribution of this coiled triad conformation is significant.

Synthesis of Isotactic Rich Poly(methylphenylsiloxane) by Living Anionic Ring-Opening Polymerization

“…In this context it should be noted that the influence of phenyl-phenyl stacking upon the conformational properties of PMPS has been described from a theoretical perspective by Mark [18]. Horta and coworkers also studied the conformational analysis of the methylphenylsiloxane chain (by computer simulation) and found that a stable conformation occurs when a pair of the phenyl rings are facing each other but are not perfectly parallel [19,20]. Hence, it is proposed that the formation of phenyl-phenyl interactions in the system described here may impede the backbiting reaction in PMPS.…”

“…The products were carefully analyzed by 19 F-NMR [13,14]. After being crosslinked using covalent bonds, it was shown that the resulting stereoregular PMTFPS elastomeric networks exhibited strain-induced crystallization as shown by WAXS 10 and also by upturns in modulus at high elongations in their equilibrium stress-strain properties.…”

Dynamics of siloxane chains bearing phenyl chromophores

“…The force ®eld was the same as described before when reporting conformational energy maps. 5 The dielectric constant was 1.7, for the reasons given there. 5 The temperature for molecular dynamics was 300 K (the SiÐO backbone is very¯exible, so enough transitions occur without the need to go to a higher T).…”

“…5 The dielectric constant was 1.7, for the reasons given there. 5 The temperature for molecular dynamics was 300 K (the SiÐO backbone is very¯exible, so enough transitions occur without the need to go to a higher T). The molecular structure is ®rst optimized until minimum energy (with a criterion of 0.001 for maximum derivative).…”

“…Which value of d corresponds to optimum overlap between rings? From the study of conformational energy (CE) maps for these chains, 5 it is known that there is a correlation between CE and ICD, the states of lower CE being those of shortest ICD. At the minima of CE, the value of d is 0.34 nm.…”