Journal of Organometallic Chemistry 636 (2001) 164 – 171
www.elsevier.com/locate/jorganchem
UV and Raman study of thermochromic phase transition in
poly(di-n -hexylgermane)
Sergey S. Bukalov
a
, Larissa A. Leites
a
, Irina V. Krylova
b
, Mikhail P. Egorov
b,
*
a
A.N. Nesmeyano Institute of Organoelement Compounds, Scientific and Technical Center on Raman Spectroscopy, Russian Academy of Sciences,
28 Vailoa Street, 119991 Moscow, Russia
b
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prosp., 117913 Moscow, Russia
Received 23 April 2001; accepted 27 June 2001
Dedicated to Professor Dr Oleg Nefedov on the occasion of his 70th birthday
Abstract
The thermochromic phase transition of poly(di-n -hexylgermane) (1) was reinvestgated using low-molecular-weight (lmw ) and
high-molecular-weight (hmw ) polymer samples. The equilibrium phase transition temperature T
c
of 1, determined by variable
temperature Raman and UV spectroscopy as well as by DSC, was found to lie not in the interval from +15 to -10°C, as was
reported previously, but in the region 45–55 °C, that is, very close to T
c
of the Si and Sn analogues. Thus, the phase behaviour
of the polymetallanes [
n
Hex
2
M]
n
,M=Si, Ge, Sn is governed not by the nature of the main chain but by intermolecular
interactions of the substituent groups. The nature of the order – disorder phase transition of 1 depends on the polymer molecular
weight and the sample thermal history. Above T
c
, hmw polymer 1 exists as hexagonal columnar mesophase while lmw 1 is in
amorphous state, both with disordered backbone. On cooling to room temperature, hmw polymer transforms to a crystalline
modification with an all-anti conformation of the backbone and ordered hexyl groups, whereas the process of ordering of the main
chain for lmw sample is more complicated. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Polygermane; Phase transition; Raman; UV; DSC
1. Introduction
Chemistry and physical chemistry of polydiorganyl-
metallanes, whose macromolecules contain catenated
homoatomic chains of Group 14 elements — silicon,
germanium, and tin, are rapidly and dynamically devel-
oping areas of modern organometallic chemistry. In
this paper, it is pertinent to emphasise a fundamental
contribution to this field made by pioneering papers of
Nefedov and his co-workers in the early 1960s on the
synthesis and properties of polysilanes and first
polygermanes [1,2].
In 1964, Gilman et al. [3] reported the family of
permethylated straight-chain polysilanes Me(SiMe
2
)
n
-
Me with n =2–8 along with their UV spectra. Surpris-
ingly, the position of the absorption band,
max
, and the
molar absorptivity were found to increase regularly
with increasing chain length, in a manner typical for
-conjugated systems. In 1966 Shorygin, Nefedov et al.
[4] published analogous UV data for linear ho-
moatomic chains consisting of SiR
2
, GeR
2
and SnR
2
units. They also observed a remarkable red shift of the
lowest-energy absorption band with the chain lengthen-
ing, the transition energy for a given chain length
decreasing in the order Si, Ge, Sn. Moreover, these
authors made an important suggestion that there is no
principal difference between interaction of -bonds in
these chain compounds and of -bonds in linear conju-
gated polyenes, the degree of interaction between the
chain units just increasing in the series
CR
2
SiR
2
GeR
2
SnR
2
CHCH
These first findings, along with some others published
later (see, e.g. [5,6]), laid the foundation for the concept
of – conjugation, first reviewed by Pitt [7]. Corre-
* Corresponding author. Tel.: +7-095-135-5309; fax: +7-095-135-
5308.
E-mail address: mpe@cacr.ioc.ac.ru (M.P. Egorov).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
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