Article
Organometallics, Vol. 29, No. 14, 2010 3127
given R-X system, since ΔHATRP = [BDE(R-X) - BDE-
(Mtnþ1-X)],12 while ΔSATRP should be small. Vinyl acetate is
a monomer that leads to rather strong C-X bonds for X =
Cl or Br.22 In order to activate dormant chains with strong
C-X bonds, metal complexes capable of yielding stronger
Mtnþ1-X bonds are necessary. Under these circumstances,
stronger Mtnþ1-C bonds should also be formed, likely
leading to irreversible OMRP trapping, unless the Mtnþ1-C
bond strength is attenuated by steric interactions, and this is
indeed the case for the CpCrII(nacnac) system, as demon-
strated in our previous contributions.14,15
General Procedures for the Radical Polymerization of Vinyl
Acetate. a. ATRP Procedure: CrII þ Methyl 2-Chloropropionate.
All polymerizations were conducted following the same experi-
mental protocol. As a representative example, the Schlenk tube
was charged with complex 1 (36 mg, 0.086 mmol, 1 equiv) and
methyl 2-chloropropionate (5 μL, 0.043 mmol, 0.5 equiv),
followed by the addition of degassed vinyl acetate (4 mL,
43 mmol, 500 equiv). The Schlenk tube was degassed by three
freeze-vacuum-thaw cycles and then immersed in an oil bath
preheated at 50 ꢀC. At the desired time, the Schlenk flask was
rapidly cooled to room temperature by immersion into iced water
before sample withdrawal. The monomer conversion was deter-
mined gravimetrically after removal of the unconverted monomer
under reduced pressure, and the resulting residue was used for
SEC characterization.
Experimental Section
b. Reverse ATRP Procedure: CrIIICl þ V-70. All polymeriza-
tions were conducted following the same experimental protocol.
A typical experiment is described here as a representative
example with complex 4, VAc, and 0.5 equiv of V-70 (Cr:V-
70:VAc = 1:0.5:500). All operations were carried out under a
protective argon atmosphere. Complex 4 (39.3 mg, 0.086 mmol,
1 equiv) and V-70 (13.2 mg, 0.043 mmol, 0.5 equiv) were
introduced in a Schlenk tube, followed by the addition of
degassed vinyl acetate (4 mL, 43 mmol, 500 equiv). The Schlenk
tube was degassed by three freeze-vacuum-thaw cycles and
then immersed in an oil bath preheated at 50 ꢀC. At the desired
time, the Schlenk flask was rapidly cooled to room temperature
by immersion into iced water before sample withdrawal. The
monomer conversion was determined gravimetrically after re-
moval of the unconverted monomer under reduced pressure,
and the resulting residue was used for SEC characterization.
Materials. All procedures, unless otherwise stated, were car-
ried out under dry, oxygen-free argon or nitrogen, using stan-
dard Schlenk and glovebox techniques. Solvents were dried by
using the method of Grubbs23 or distilled under argon from
appropriate drying agents and degassed by three freeze-
vacuum-thaw cycles prior to use.24 Celite (Aldrich) was dried
overnight at 110 ꢀC before being evacuated and then stored
under argon or nitrogen. Vinyl acetate (VOAc, 99%, Alfa
Aesar) was passed through a neutral alumina column to remove
the stabilizer, dried over calcium hydride, distilled at 90 ꢀC,
degassed by three freeze-vacuum-thaw cycles, and stored
under argon or nitrogen at -20 ꢀC. 2,20-Azobis(4-methoxy-
2,4-dimethylvaleronitrile) (V-70, 96%, Wako) was used as
received. Acetophenone, methyl 2-chloropropionate, chloro-
methyl pivalate, and neopentyl chloride were purchased from
Aldrich, distilled under reduced pressure, and/or degassed by
three freeze-vacuum-thaw cycles before being used. Methyl
propionate, phenacyl chloride, and KN(SiMe3)2 were purchased
c. From a CrIII-Enolate Complex. Compounds CpCrIII
-
(nacnacXyl,Xyl)I (151 mg, 0.275 mmol) and K[OC(OCH3)d
CHCH3] (42.3 mg, 0.335 mmol, 1.22 equiv) were placed in a
Schlenk flask followed by the addition of toluene (15 mL). The
mixture was stirred overnight at room temperature, at which
point the solvent was evaporated in vacuo. The residue was
extracted with hexanes (10 mL) and filtered through Celite (UV/
vis: λmax = 397 nm, 635 nm). The green solution was cooled to
-35 ꢀC to yield 25.1 mg of compound CpCrIII(nacnacXyl,Xyl)-
[OC(OCH3)dCHCH3] (18%). This batch was then dissolved in
VAc (2 mL). Stirring at room temperature for 54 h and then
warming to 50 ꢀC for 21.5 h did not result in any polymerization
process.
from Aldrich and used as received. Complexes CpCrII(nacnacAr,Ar
)
(Ar = Dipp, 1; Xyl, 2),14,16 CpCrIII(nacnacAr,Ar)Cl (Ar = Dipp, 3;
25
Xyl, 4),15,16 CpCrIII(nacnacXyl,Xyl)I,17 and CH3CH(Cl)OOCCH3
were prepared according to literature procedures. K[OC(Ph)d
CH2] and K[OC(OCH3)dCHCH3] were prepared according to
literature procedures outlining the preparation of similar po-
tassium enolate compounds.26
Characterizations. A Varian Cary 100 Bio UV-visible spectro-
photometer was used to conduct measurements using a specially
constructed cell for air-sensitive samples: a Kontes Hi-Vac valve
with PTFE plug was attached by a professional glassblower to a
Hellma 10 mm path length quartz absorption cell with a quartz-
to-glass graded seal. Size exclusion chromatography (SEC) of
poly(vinyl acetate) was carried out in filtered THF (flow rate:
1 mL/min) at 35 ꢀC on a 300 ꢀ 7.5 mm PL gel 5 μm mixed-D
column (Polymer Laboratories), equipped with multiangle light
scattering (Minidawn Tristar, Wyatt Technology Corporation)
and refractive index (RI2000, Sopares) detectors, with a Waters
Synthesis of Compound CpCr(nacnacXyl,Xyl)[OC(Ph)dCH2],
5. A toluene (1 mL) suspension of K[OC(Ph)dCH2] (16.9 mg,
0.107 mmol, 1.11 equiv) was reacted with CpCrIII(nacnacXyl,Xyl)I
(52.9 mg, 0.0963 mmol) dissolved in toluene (4 mL) in a Schlenk
flask overnight at room temperature. The solvent was evapo-
rated in vacuo, and the residue was extracted with hexanes
(2 mL), filtered through Celite, and cooled to -35 ꢀC for 3 days
to yield crystals of 5 (5.6 mg, 11%) suitable for X-ray crystallo-
graphic analysis. Due to the presence of cocrystallized iodide
starting material, satisfactory elemental analysis for 5 was not
obtained. UV/vis (hexanes; λmax, nm (ε, M-1 cm-1)): 407 (7600),
607 (500).
X-ray Characterization of Compound 5. A single crystal of
compound 5 was mounted on a glass fiber and centered on the
optical path of a Bruker X8 APEX II diffractometer with
graphite-monochromated Mo KR radiation. The data were
collected at a temperature of -100.0 ( 0.1 ꢀC in a series of φ
and ω scans in 0.5ꢀ oscillations. Data were collected and
integrated using the Bruker SAINT software package27 and
were corrected for absorption effects using the multiscan tech-
nique (TWINABS)28 and for Lorentz and polarization effects.
˚
column pack (300 ꢀ 7.5 mm, Ultrastyragel 104, 103, 100 A),
equipped with multiangle light scattering (miniDawn Tristar,
Wyatt Technology Corp.) and refractive index (Waters 410)
detectors or at 30 ꢀC on a Polymer Laboratories PL-GPC
50 plus (two PLgel mixC columns in series) with a PL-AS RT
autosampler and PL-RI detector. The isolated polymer samples
were dissolved in THF, and the polymer solutions were filtered
(pore size = 0.45 μm) before chromatographic analysis. The
columns were calibrated against linear polystyrene standards
(Polymer Laboratories).
(22) Gillies, M. B.; Matyjaszewski, K.; Norrby, P.-O.; Pintauer, T.;
Poli, R.; Richard, P. Macromolecules 2003, 36, 8551–8559.
(23) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
(24) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth Heinemann, 1996.
(25) Zakrzewski, J. Chromatographia 2004, 59, 775–777.
(26) Culkin, D. A.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 5816–
5817.
(27) SAINT, version 7.03A; Bruker Analytical X-ray System: Madison,
WI, 1997-2003.
(28) TWINABS, Bruker Nonius scaling and absorption for twinned
crystals, V2008/2; Bruker AXS Inc.: Madison, WI, 2008.