Design of effective systems for controlled radical polymerization of ...

352 U. S. Schubert, G. Hochwimmer, Ch. E. Spindler, O. Nuyken
NH 4 PF 6 in CH 3 OH immediately a blue solid precipitated.
The solid was filtered off, washed with MeOH, H 2 O and
diethylether, dissolved in acetone and precipitated in diethyl
ether. After filtration the solid was dissolved in a small
amount of acetone. By slow diffusion of diethyl ether into
the solution blue crystals of 2 (93%) were obtained.
UV/VIS (CH 3 CN): k max /nm (e/(L N mol –1 N cm –1 )) = 285
(3270).
C 36 H 36 N 6 CuP 2 F 12 62H 2 O (942.2)
Calc. C 45.98 N 8.92 H 4.28
Found C 45.97 N 8.81 H 4.21
[Cu(II)(5,59-dimethyl-2,29-bipyridine) 3 ](PF 6 ) 2 (4): 5,59-Dimethyl-2,29-bipyridine
(3) was dissolved in CH 3 OH under
nitrogen. A solution of Cu(CH 3 COO) 2 6H 2 O in H 2 O was
added and the blue solution was refluxed for 5 h. After addition
of an excess of a saturated solution of NH 4 PF 6 in
CH 3 OH immediately a blue colored solid appeared. The mixture
was refluxed for another 5 min and was then allowed to
cool to room temperature. The remaining solid was filtered
off, washed with MeOH, H 2 O and diethyl ether, dissolved in
acetone and precipitated in diethyl ether. After filtration the
solid was dissolved in a small amount of acetone and crystallized
in diethyl ether to yield 84% of 4 as blue crystals.
UV/VIS (CH 3 CN): k max /nm (e/(L N mol –1 N cm –1 )) = 251
(47830), 299 (39420).
C 36 H 36 N 6 CuP 2 F 12 (906.2)
Calc. C 47.72 N 9.27 H 4.00
Found C 47.68 N 9.29 H 3.96
Polymerizations
A typical procedure was as follows: [(CH 3 ) 2 CHO] 3 Al
(59.64 mg, 0.292 mmol) and styrene (5.32 g, 51 mmol) were
added to a solution of the copper bipyridine complex
(0.073 mmol, 2: 68.78 mg, 4: 66.15 mg) in CH 3 CN (1.6 mL)
under argon. After addition of (1-bromoethyl)benzene
(13.44 mg, 0.073 mmol) the reaction mixture was degassed
by three freeze/pump cycles. Then the mixture was heated to
758C (oil bath) and kept at this temperature during polymerization.
Samples (0.1 mL) were taken in distinct time intervals
and quenched with 1,4-benzoquinone in CH 2 Cl 2 (2 mL,
27 g/L, 0.25 mol/L). The polymer samples were precipitated
in methanol and dried in vacuo at 508C. The conversion was
determined gravimetrically.
Results and discussion
The main focus of our approach in controlled radical
polymerization is the design of new effective systems
which circumvent some of the drawbacks of established
systems. In typical procedures a ratio of 2:1:1:100
(bipyridine:copper(I) salt:alkyl halide:monomer) is
used 9–11) (also a 3:1:1:100 ratio is often applied, see e.g.
ref. 21) ). Copper(I) chloride or copper(I) bromide is dispersed
in monomer and a bipyridine is added separately.
It is assumed that a bipyridine complex is formed in situ.
Fig. 1. Wireframe models of the complexes: left: [Cu(II)(4,49-
dimethyl-2,29-bipyridine) 3 ](PF 6 ) 2 (2); right: [Cu(II)(5,59-
dimethyl-2,29-bipyridine) 3 ](PF 6 ) 2 (4) (MAC Spartan Plus, level
MM2, PF 6 counter ions are omitted)
The resulting polymers are quite often colored, due to a
rather high residual metal concentration in the resulting
polymer 22) . Therefore, it was worthwhile to test welldefined,
already preformed bipyridine metal complexes
for this polymerization. We see an advantage over its in
situ formation by mixing together the components. In our
investigations we have further studied the influence of
the bipyridine substitution pattern (4-, 5- and preliminary
the 6-positions) for the polymerization process.
We first choose two different types of ligands. 4,49-
Dimethyl-2,29-bipyridine (1) is commercially available,
whereas the corresponding 5,59-dimethyl-2,29-bipyridine
(3) was synthesized in high yield using Stille type carbon-carbon
bond forming reaction utilizing organo-tin
intermediates (72% from commercial starting materials
20) ). The copper complexes of the bipyridines 1 and 3
could be obtained by reaction with an appropriate copper(II)
salt yielding blue crystalline materials (2: 93%, 4:
84%, isolated as PF 6 salts). The wireframe models of the
octahedral complexes 2 and 4 are shown in Fig. 1. These
metallo-supramolecular systems were used for the polymerization
of styrene (Scheme 1). The monomer and aluminium
isopropoxide were added to solutions of the complexes
in acetonitrile (1:700 complex to monomer ratio).
After addition of an alkyl halide (e.g. (1-bromoethyl)benzene,
1:1 complex to initiator ratio) the reaction mixtures
were heated to 758C. Samples were taken at different
times and quenched with 1,4-benzoquinone. The role of
the aluminium isopropoxide is yet not clear. We assume
that it acts as Lewis acid lowering the dissociation energy
Scheme 1: