Addition polymerization of 1,1-dimesitylneopentylgermene: Synthesis of a polygermene

Article abstract of DOI:10.1039/b801762j

A new polymer with an alternating germanium-carbon backbone has been synthesized from 1,1-dimesitylneopentylgermene via addition polymerization using an anionic initiator. The Royal Society of Chemistry.

Full text of DOI:10.1039/b801762j

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Addition polymerization of 1,1-dimesitylneopentylgermene: synthesis of a
polygermenew
Laura C. Pavelka,a Simon J. Holderb and Kim M. Baines*a
Received (in Berkeley, CA, USA) 31st January 2008, Accepted 11th March 2008
First published as an Advance Article on the web 17th April 2008
DOI: 10.1039/b801762j
A new polymer with an alternating germanium-carbon backbone
has been synthesized from 1,1-dimesitylneopentylgermene via
addition polymerization using an anionic initiator.
and the relatively scarce information available on the pyrolysis
of polycarbogermanes,7 we have now investigated the addition
polymerization of a solution stable germene as a new route to
polymers with a [GeC]n backbone.
A pale yellow pentane solution of 1,1-dimesitylneopentyl-
germene ( 1) was prepared from the addition of t-butyllithium
(1 equiv.) to a solution of fluorovinylgermane 2.8 Germene 1
was stable in solution for several hours; however, upon addi-
tion of t-butyllithium (0.1 equiv.) to the solution (Scheme 1),
the colour of the solution changed from pale to bright yellow.
After 30 min, the reaction was quenched with methanol and
the bright yellow colour dissipated immediately. The solvents
were removed and the residue was dissolved in CH2Cl2. A
white solid precipitated from the CH2Cl2 solution upon the
addition of methanol; the solid was purified by re-precipita-
tion. The air stable material (3) was isolated in 45% yield. The
1H NMR spectrum of the solid displayed broad resonances
consistent with a polymeric substance. No resonances attribu-
table to the methanol adduct of germene 18 were observed in
the residue or the precipitated material, indicating that all of
the germene had been consumed prior to the addition of
methanol. The rapid polymerization of 1 is in contrast to the
much slower polymerization of phosphaalkenes.2,3 The mole-
cular weight of the polymer was determined by GPC in THF;
two distinct fractions were observed indicating a bimodal
molecular weight distribution. The number-average molecular
weights (Mn) were estimated to be 36 000 g molꢀ1 and 7 100 g
molꢀ1 (vs. polystyrene) with polydispersity indices (PDI =
Mw/Mn) of 1.5 and 1.1, respectively. The bimodal molecular
weight distribution was reproducible; similar Mn values were
Addition polymerization of vinylic monomers, a standard
protocol for the preparation of organic polymers, has only
recently been applied to the synthesis of inorganic polymers.
The development of this important method for inorganic-
based polymers was undoubtedly delayed by the lack of or
difficulty in the synthesis of suitable monomers. In fact, much
of the research on unsaturated inorganic, primarily main
group, compounds over the last 30 years has focused on
understanding how to prevent oligomerization reactions. One
key strategy involves the use of bulky substituents to kineti-
cally stabilize the doubly-bonded species;1 it follows that such
compounds would not be suitable as monomers for polymer
synthesis. However, in a landmark series of papers, Gates et al.
have shown that readily accessible stable phosphaalkenes
(PQC) with relatively bulky substituents can indeed undergo
addition polymerization using radical or anionic initiators to
form poly(methylenephosphine)s, a new and interesting class
of polymers.2 Furthermore, the phosphaalkenes undergo living
anionic polymerization at ambient temperatures using orga-
nolithium reagents as initiators.3 This discovery has opened up
numerous exciting possibilities for the synthesis of novel
copolymers containing the functional poly(methylenephos-
phine) block. Block copolymers, in general, and particularly
those with inorganic segments, have been the subject of intense
research of late due to the spontaneous self assembly of the
polymers into varied and interesting nanostructures.4
We have long been interested in the chemistry of multiply-
bonded germanium derivatives and were intrigued by the
possibility that germenes (GeQC) may also be able to undergo
addition polymerization to provide an entry into a hitherto
unknown [GeC]n polycarbogermane system. Given the exten-
sive interest in poly(silylenemethylene) polymers, [SiC]n,5 par-
ticularly in their pyrolysis chemistry to give silicon carbide,6
a Department of Chemistry, University of Western Ontario, 1151
Richmond Street, London, Ontario, Canada N6A 5B7. E-mail:
kbaines2@uwo.ca; Fax: +1 (519) 661-3022; Tel: +1 (519) 661-
2111
b Functional Materials Group, School of Physical Sciences, University
of Kent, Canterbury, UK CT2 7NH
w Electronic supplementary information (ESI) available: Experimental
procedures, 1H and 13C NMR spectra, TGA data, DSC data, GPC
data and crystallographic data for CCDC 676604 in CIF format. See
DOI: 10.1039/b801762j
Scheme 1 Synthesis of polygermene 3.
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2346 | Chem. Commun., 2008, 2346–2348

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achieved. Polymer 3 was determined to be amorphous in
nature based on the DSC results where only a glass transition
was observed (Tg = 144 1C). The thermal stability of polymer
3 was determined using TGA. The material was stable to
weight loss until 290 1C, at which point 87% of the mass was
lost.
Fig. 1 Cyclic dimers (4a,b) and isomeric germane (5) of germene 1.
Polymer 3 can also be synthesized in one step from the
addition of excess t-butyllithium (1.1 equiv.) to fluorovinyl-
germane 2 in pentane at ꢀ78 1C. Upon warming, germene 1
formed quantitatively as noted by the pale yellow colour of the
solution. When the solution reached room temperature, ger-
mene 1 then reacted with the remainder of the t-butyllithium,
as indicated when the colour of the solution changed from pale
to bright yellow. Quantitative formation of germene 1 under
these conditions was verified in a separate experiment in which
methanol was added to the solution immediately upon reach-
ing room temperature. The methanol adduct of 1 was the only
product observed.
of 0.1 equiv. t-BuLi to the germene is indicative of more than
one polymerization mechanism. Other possible concurrent
mechanisms may include radical polymerization by
electron transfer to 1 or even ring-opening polymerization
involving the intermediate formation of germene dimers
(4a and 4b, vide infra).z
Germene 1 slowly reacted to give head-to-tail cyclic dimers
4a and b, an isomeric germane (5), and polymeric material
(Fig. 1) when left in solution (C6D6) for several days. The
head-to-head cyclic dimer was not observed. The two head-to-
tail cyclic dimers (4a,b) were consistently formed in a 1 : 1
ratio. However, the relative ratio of 4a and b to germane 5,
presumably formed via a hydride shift, varied depending on
the concentration of the germene. Germane 5 was favoured
when the initial solution was dilute, whereas in concentrated
solutions the cyclic dimers (4a,b) were formed almost exclu-
sively. The trans dimer (4a) selectively crystallized from a
concentrated C6D6 solution. The molecular structure of 4a
was determined by X-ray crystallography (Fig. 2). The 1,3-
digermacyclobutane ring lies in a plane. The intracyclic bond
angles are close to 901 and the intracyclic Ge–C bond lengths,
2.016(2) and 2.041(2) A, are long but still well within the
normal Ge–C single bond length range (1.90–2.05 A).9 The
metrics of 4a are comparable to those of other crystallogra-
phically characterized 1,3-digermacyclobutanes.10 DSC analy-
sis was performed on 4a and on a mixture of 4a–4b. An
irreversible endotherm was observed in each sample with an
onset temperature of 290.4 and 248.7 1C, respectively. Despite
After precipitation, the polymer was obtained in 62% yield.
The number-average molecular weight (Mn) of the polymer
prepared by this method was higher than that prepared
previously (Mn = 39 000 g molꢀ1, PDI = 2.0). The broad
PDI is possibly a result of initiation over an extended time
period during the start of the polymerization resulting from
the gradual warming of the solution. The addition of one
portion of t-butyllithium is the preferred method for the
formation of polymer 3 since monomodal higher molecular
weight polymer was isolated in greater yield with fewer
experimental manipulations.
The polymeric material (3) was characterized by one- and
two-dimensional NMR spectroscopy. The 13C NMR
spectrum of
3 was particularly useful in elucidating
the structure of the polymer. The signals in the 13C NMR
spectrum of 3 were readily assigned by comparison of
the chemical shifts with those of the model compound,
Mes2Ge(tBu)CH2CH2tBu.8 For example, the GeCH and
ꢀ
GeCHCH2 carbons in polymer 3 were observed to resonate
ꢀ
at 16.3 and 38.7 ppm, respectively, compared to 14.4 and 40.5
ppm in the model compound. Similarly, signals attributable to
the (C(CH3)3) moiety, the ortho- and para-methyls and the
aromatic carbons of the mesityl groups could be assigned.
However, in the polymer, two sets of mesityl signals are
observed resulting from meso and racemic replacements be-
tween the stereogenic carbon centres. The assignments were
confirmed by both 2D 13C–1H correlation spectroscopy and
DEPT spectroscopy. There were no remaining, unassigned
signals in the 13C NMR spectrum of 3, indicating a regular
alternating [GeC]n backbone.
The regioselective addition of t-butyllithium to germene 1
dissolved in ether has been reported; upon quenching with
water, only germane, Mes2Ge(tBu)CH2CH2tBu, was isolated.8
Presumably, the polymerization of 1 is initiated by the regio-
selective addition of t-butyllithium to the germene to give
Mes2Ge(tBu)CH(Li)CH2tBu; in pentane, the carbanion ra-
pidly adds to the germanium centre of another germene (1) to
propagate the growth of the polymer. The decreased stability
of the germene, along with a less sterically hindered propagat-
ing carbanion, likely contribute to the high reactivity. The
bimodal distribution of polymer 3 synthesized by the addition
Fig. 2 Thermal ellipsoid plot (50% probability surface) of 4a. Two
molecules of 4a are present in the asymmetric unit; parameters are
given for one of the two molecules. Atoms labeled with ‘A’ are located
at equivalent positions (ꢀx, ꢀy, ꢀz). Hydrogen atoms are omitted for
clarity. Selected bond lengths (A) and angles (deg): Ge(1)–C(19) =
2.041(2), Ge(1)–C(19A)
90.11(10).
=
2.016(2), C(19)–Ge(1)–C(19A)
=
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the irreversibility, the endotherms appear to be melting transi-
tions. In the case of 4a, the endotherm coincides with
the observed melting point (280 1C), which was immediately
followed by decomposition. No exotherm indicative of a
thermal ring opening polymerization was observed.
Notes and references
z Any significant termination process in an ionic polymerization
typically leads to broader monomodal molecular weight distributions
rather than a bimodal molecular weight distribution.11
1 For reviews see: (a) E. Rivard and P. P. Power, Inorg. Chem., 2007,
46, 10047; (b) M. Driess and H. Grutzmacher, Angew. Chem., Int.
The polymerization of germene 1 in the absence of an
anionic initiator occured slowly; the polymer was formed
as a minor component of the product mixture (25 wt%) under
these conditions. The polymer was isolated from the mixture
by precipitation from CH2Cl2 with methanol. The 1H and
13C NMR spectra of the polymer obtained under these con-
ditions were identical to the spectroscopic data of 3. The
exclusive formation of head-to-tail cyclic dimers ( 4a,b)
under the same conditions was also taken as evidence for a
polymer with an alternating [GeC]n backbone. The molecular
weight of the polymer was lower than that obtained using
¨
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an anionic initiator and had
a broader polydispersity
(Mn = 21 000 g molꢀ1 vs. polystyrene; PDI = 2.5).
This polymerization is presumably a result of a radical
polymerization process, though further experiments are
needed to verify this.
In summary, we have reported the first addition polymer-
ization of a stable germene to yield a new inorganic polymer, a
polygermene. We are currently exploring whether the poly-
merization can be carried out in a living fashion with the goal
of synthesizing block copolymers containing this new inor-
ganic segment.
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We thank the Natural Sciences and Engineering Research
Council of Canada, the University of Western Ontario, the
Canadian Foundation for Innovation—Ontario Innovation
Trust, and the Ontario Photonics Consortium for financial
support. We also thank Teck Cominco Ltd. for a donation of
GeCl4, D. Hairsine for acquisition of mass spectral data,
Dr M. C. Jennings for acquisition of X-ray diffraction data,
and the ASPIRE program (UWO; A Special International
Research Experience) for the opportunity for L. C. P. to study
at the University of Kent under the supervision of Dr Simon
Holder.
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