Communications
reflected in a characteristic downfield shift (Dd ꢀ 20 ppm) of
loadings. Since the polymers 6a–c contain uncommon repeat
units, only the relative Mn and PDI could be determined and
are calibrated to polystyrene standards.[7] Polymer 6a is
insoluble in the reaction mixture, thus its molecular weight
rarely exceeds 5.0 ꢁ 104, and a PDI greater than 2.0. The
ROMP of the halogenated dibenzo[8]annulenes 1b and 1c
shows a linear relationship between the monomer/catalyst
(M/C) loading and the attained molecular weights. The
polydispersity of 6b and 6c is also high, between 1.4 and
2.7. At equimolar amounts of monomer to catalyst, we
observe by 1H NMR spectroscopy that the monomer is
consumed before all of the catalyst has initiated. Time-
resolved GPC analysis reveals a fast initial linear phase (t <
15 s) for the polymerization of the strained cyclooctyne
leading to a polymer with low PDI (extrapolated to ca. 1.1)
and high molecular weight (Figure 2). Our efforts to isolate
the alkyne carbon resonances in the 13C NMR spectrum with
respect to unstrained diphenylacetylene.
Figure 1 depicts the DFT (B3LYP 6-311G**) calculated
frontier orbitals of 1a.[6] The HOMO on the strained alkyne is
localized in the p system conjugated to the adjacent phenyl
rings. The p orbital distorted by the ring strain (HOMOÀ1)
lies in the plane of the aromatic rings and is efficiently
shielded by two neighboring protons. We conclude that an
electron-deficient metathesis catalyst is likely to approach the
strained alkyne perpendicular to the extended aromatic p
system, thus activating the electronically as well as sterically
most favorable p bond of the alkyne (HOMO).
We initially studied the alkyne ROMP of 1a–c using the
commercially available tungsten-based Schrockꢀs metathesis
ꢁ
catalyst. Adding [(tBuO)3W CtBu] to a solution of 1a–c in
toluene at 248C leads to an instantaneous polymerization to
give 6a–c (Scheme 2) in essentially quantitative yields. The
Figure 2. Time-resolved GPC data for the polymerization of 1c with
ꢁ
Scheme 2. Reaction conditions: a) [(tBuO)3W CtBu], toluene, 248C, or
ꢁ
[(tBuO)3W CtBu]. Samples were quenched at 15, 45, 75, and 600 s.
ꢁ
[(N(tBu)Ar)3Mo CCH2CH3] (Ar=3,5-dimethylbenzene), alcohol/
*
^
.
Mn: ; PDI:
phenol, toluene, 248C.
this primary polymerization product have failed due to
unpredictable variations in the initiation rates of individual
catalyst batches. Once all starting material is consumed (t >
15 s), residual uninitiated catalyst or terminal groups on the
growing polymer are active enough to undergo competing
cross-metathesis reactions with internal alkynes in the
polymer backbone leading to a progressive broadening of
the molecular weight distribution and an increased PDI. The
low selectivity of Schrockꢀs alkyne metathesis catalyst—ring-
strained monomer over internal alkyne—led us to explore the
ROMP activity of molybdenum-based catalyst systems.
Recent advances in alkyne metathesis catalyst design
based on the formation of a trialkoxymolybdenum(VI)
alkylidyne complex from readily available [(N-
monomeric alkyne precursor is consumed within less than 10 s
as judged by TLC and GC analysis of aliquots taken from the
reaction mixture. Diagnostic for the ROMP reaction is the
upfield shift of the NMR resonances of the acetylene 13C
atoms from ca. 110 ppm in 1a–c to ca. 90 ppm in the polymers
(Figures 2SI to 7SI). The polymerization is accompanied by a
ca. 8 nm red-shift of the highest absorption peak in the UV/
Vis spectrum (Figure 8SI).
Table 1 summarizes the GPC results obtained for the
polymerization reactions of 1a–c at various monomer/catalyst
Table 1: Number-average molecular weight (Mn) and polydispersity
index (PDI) for 6a–c at different monomer/catalyst (M/C) loadings.
ꢁ
(tBu)Ar)3Mo CCH2CH3] (Ar= 3,5-dimethylbenzene) and
an activating alcohol/phenol give access to a variety of
molecularly well-defined metathesis catalysts.[8] These sys-
tems have been extensively applied to acyclic diyne meta-
thesis (ADIMET) and ring-closing-metathesis (RCM) reac-
tions both in the synthesis of natural products and functional
material systems.[9] The great majority of these reactions are
run under dynamic equilibrium conditions, and are relatively
unselective. The key requirement to prevent chain transfer
and the resulting increase in the polydispersity is for the
polymerization catalyst to discriminate between strained
alkynes and unactivated triple bonds.
Entry
Polymer[a]
M/C
Mn
PDI[b]
1
2
3
4
5
6
7
6a
6b
6b
6b
6c
6c
6c
10
10
5
2
10
5
54000
65100
19600
13300
32000
19000
15400
2.0
2.6
1.6
1.4
1.8
1.9
1.5
2
ꢁ
[a] Prepared by adding Schrock’s tungsten catalyst [(tBuO)3W CtBu] to a
solution of 1a–c in toluene at 248C. The polymer was precipitated with
MeOH after 30 min. [b] PDI=Mn/Mw.
7258
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 7257 –7260