Effect of the Alkyl Side Chain on the Helical Conformation of Dialkylpolysilane Copolymers†

doi:10.7503/cjcu20140577

Abstract

Abstract:

Silane copolymers [P(Si1-co-Si2)] were synthesized using (S)-2-methylbutyl-n-decyldichlorosilane(Si1) and 2-ethylbutyl-n-decyldichlorosilane(Si2) monomers. A series of P(Si1-co-Si2) was synthesized by varying the ratio of Si1 and Si2. The random copolymer was confirmed by nuclear magnetic resonance. The chirality transfer behaviour of polysilane samples was characterized by circular dichroism spectrum(CD) and ultraviolet absorption spectrum(UV). It was found that homopolymer of achiral monomer Si2 exhibited no CD signal, but intense and sharp UV absorption at 324 nm. When the chiral monomer Si1 was added into the main chain of polymer molecules, the obtained copolymer molecules were induced with a marked CD signal. When the mole fraction of Si1 increased, the intensity of CD signal became higher rapidly, even as little as 2.0%(molar fraction) of enantiopure Si1 could induce a marked single-handed helical conformation in these copolymers. After that, the intensity of CD signal increased slightly and then decreased slightly. Moreover, the λ_max value tended to blueshift progressively with increasing the molar fraction of the chiral moiety in the copolymers. For P(Si1-co-Si2) of 50%Si1(P50) with diffierent molecular weight, the λ_max tended to redshift with increasing molecular weight, and the lower molecular weight P50 possessed a stronger optical activity. The result indicated that the handedness of the copolymers was controlled by the chiral alkyl chain and the odd-even effect of the achiral alkyl chain was not reflected.

Key words: Optically active polymer, Polysilane, Helical conformation, Chiral amplification, “Sergeants and Soldiers”, principle

CLC Number:

O631

TrendMD:

TANG Xianhui, JI Xingxing, QIAN Kai, LI Yi, YANG Yonggang. Effect of the Alkyl Side Chain on the Helical Conformation of Dialkylpolysilane Copolymers^†[J]. Chem. J. Chinese Universities, 2014, 35(11): 2481.

Figures/Tables 6

Fig.1 Molecular structures of the silane monomers and polymers

Fig.2 29Si NMR spectra of P0(a), P50(b) and P100(c)

Table 1 Selected samples for UV and CD measurements

Sample	n	m	10^-4M_w	M_w/M_n
P0	0	1	2.5	1.24
P02	0.02	0.98	2.5	1.12
P05	0.05	0.95	2.1	1.05
P30	0.3	0.7	2.4	1.10
P50	0.5	0.5	2.5	1.12
P70	0.7	0.3	2.2	1.06
P90	0.9	0.1	2.5	1.09
P100	1	0	2.3	1.08

Fig.3 UV(A) and CD(B) spectra of polysilane copolymers containing(S)-2-methylbutyl and 2-ethylbutyl side groups in THF at 25 ℃

Fig.4 CD signal of different copolymers with increase of chiral substituents

Fig.5 UV(A) and CD(B) spectra of various molecular weights of copolymer with 50%(molar fraction) of Si1

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