Palladium-based system for the polymerization of acrylates. Scope and mechanism

Article abstract of DOI:10.1021/om020435q

The neutral palladium complex [Pd(C₆F₅)Br(NCMe)₂] (1) was found to effect the polymerization of acrylates upon addition of 1 equiv of a monodentate phosphine or pyridine or an excess of halide. Methyl methacrylate was not polymerized, and furthermore, its addition stopped the progress of independently initiated methyl acrylate polymerization in the phosphine-based system. Addition of ethene also inhibited the polymerization of methyl acrylate. However, over 10 mol % incorporation of 1-hexene in the polymer was achieved when the latter was added together with methyl acrylate. The polymerizaion mechanism is discussed.

Full text of DOI:10.1021/om020435q

Organometallics 2002, 21, 4249-4256
4249
P a lla d iu m -Ba sed System for th e P olym er iza tion of
Acr yla tes. Scop e a n d Mech a n ism
Christine Elia, Sharon Elyashiv-Barad, and Ayusman Sen*
Department of Chemistry, The Pennsylvania State University,
University Park, Pennsylvania 16802
Raquel Lo´pez-Ferna´ndez, Ana C. Albe´niz, and Pablo Espinet*
Departamento de Quı´mica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid,
Prado de la Magdalena s/ n, E-47005 Valladolid, Spain
Received J une 3, 2002
The neutral palladium complex [Pd(C6F5)Br(NCMe)2] (1) was found to effect the poly-
merization of acrylates upon addition of 1 equiv of a monodentate phosphine or pyridine or
an excess of halide. Methyl methacrylate was not polymerized, and furthermore, its addition
stopped the progress of independently initiated methyl acrylate polymerization in the
phosphine-based system. Addition of ethene also inhibited the polymerization of methyl
acrylate. However, over 10 mol % incorporation of 1-hexene in the polymer was achieved
when the latter was added together with methyl acrylate. The polymerizaion mechanism is
discussed.
In tr od u ction
reported the catalytic copolymerization of ethene and
acrylates with cationic palladium compounds where an
insertion mechanism is operative.7 However, a maxi-
mum incorporation of ∼15% methyl acrylate in the
copolymer was achieved. Grubbs has also reported a
somewhat related system based on neutral nickel
compounds that is able to polymerize functionalized
alkenes.8 However, this system is ineffective for acry-
lates.
We have presented a preliminary report on the
activity of palladium-pentafluorophenyl-based systems
for the polymerization of acrylates and have been
studying their mechanism and scope.9 Very recently,
Novak has also reported on acrylate polymerization by
palladium compounds where a radical mechanism was
invoked.10 Prompted in part by the Novak report, we
Since the advent of Ziegler-Natta catalysts nearly
40 years ago, innumerable transition-metal complexes
have been developed for the successful polymerization
of simple alkenes through a mechanism involving suc-
cessive insertions of the monomer into a preformed
metal-carbon bond.1,2 However, very few of the cur-
rently known transition-metal-based insertion poly-
merization systems tolerate the presence of hetero-
atoms in the monomer, and none have been able to
homopolymerize alkenes bearing oxygen functionalities
directly adjacent to the CdC bond, such as acrylates.3
As a result, the nearly 2 000 000 000 lb of methacrylic
and acrylic ester based polymers manufactured each
year are exclusively produced by radical and anionic
routes.4 The group-transfer polymerization of acrylates
and polymerization through an enolate mechanism by
early-transition-metal and lanthanide complexes are
also known.4b,5
(5) (a) Webster, O. W.; Sogah, D. Y. In Comprehensive Polymer
Science; Allen, G.; Bevington, J . C., Eds.; Pergamon: New York, 1989;
Vol. 4, p 163. (b) Yasuda, H. In Catalysis in Precision Polymerization;
Kobayashi, S., Ed.; Wiley: New York, 1997; p 189. (c) Chen, E. Y.-X.;
Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1998,
120, 6287-6305. (d) Nguyen, H.; J arvis, A. P.; Lesley, M. J . G.; Kelly,
W. M.; Reddy, S. S.; Taylor, N. J .; Collins, S. Macromolecules 2000,
33, 1508-1510. (e) Collins, S.; Ward, D. G. J . Am. Chem. Soc. 1992,
114, 5460-5462. (f) Deng, H.; Shiono, T.; Soga, K. Macromolecules
1995, 28, 3067-3073. (g) Boffa, L. S.; Novak, B. M. Macromolecules
1994, 27, 6993-6995. (h) Cameron, P. A.; Gibson, V. C.; Graham, A.
J . Macromolecules 2000, 33, 4329-4335. (i) Frauenrath, H.; Keul, H.;
Hocker, H. Macromolecules 2001, 34, 14-19.
(6) Komiya, S.; Yamamoto, A.; Ikeda, S. Bull. Chem. Soc. J pn. 1975,
48, 101-107.
(7) (a) Mecking, S.; J ohnson, L. K.; Wang, L.; Brookhart, M. J . Am.
Chem. Soc. 1998, 120, 888-899. (b) J ohnson, L. K.; Mecking, S.;
Brookhart, M. J . Am. Chem. Soc. 1996, 118, 267-268. (c) Drent, E.;
van Dijk, R.; van Ginkel, R. van Oort, B.; Pugh, R. I. Chem. Commun.
2002, 744-745.
In 1975, Yamamoto reported the polymerization of
methyl acrylate by a ruthenium compound.6 On the
basis of monomer reactivity ratios observed in copoly-
merization reactions, an insertion mechanism was
inferred. More recently, Brookhart and Drent have
* To whom correspondence should be addressed. E-mail: A.S.,
asen@chem.psu.edu; P.E., espinet@qi.uva.es.
(1) (a) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170.
(b) Coates, G. W. Chem. Rev. 2000, 100, 1223-1252. (c) Hlatky, G. G.
Chem. Rev. 2000, 100, 1347-1376. (d) Chen, E. Y.-X.; Marks, T. J .
Chem. Rev. 2000, 100, 1391-1434.
(2) (a) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 429-447. (b) Ittel, S. D.; J ohnson, L. K.; Brookhart,
M. Chem. Rev. 2000, 100, 1169-1203.
(3) Recent review on the polymerization of polar monomers: Boffa,
L. S.; Novak, B. M. Chem. Rev. 2000, 100, 1479-1493.
(4) (a) Odian, G. Principles of Polymerization; Wiley: New York,
1991; p 630. (b) Handbook of Polymer Synthesis; Kricheldorf, H. R.,
Ed.; Marcel Dekker: New York, 1992; Chapter 4.
(8) Younkin, T. R.; Connor, E. F.; Henderson, J . I.; Friedrich, S. K.;
Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460-462.
(9) Sen, A.; Elia, C.; Albe´niz, A. C.; Espinet, P. Abstracts of Papers,
220th National Meeting of the American Chemical Society; Washing-
ton, DC, 2000; American Chemical Society: Washington, DC, 2000.
(10) Tian, G.; Boone, H. W.; Novak, B. M. Macromolecules 2001,
34, 7656-7663.
10.1021/om020435q CCC: $22.00 © 2002 American Chemical Society
Publication on Web 08/31/2002

4250 Organometallics, Vol. 21, No. 20, 2002
Elia et al.
Ch a r t 1. Rela tive Ra tes of â-Hyd r ogen
Sch em e 1
Abstr a ction in th e In ser tion P r od u ct of Meth yl
Meth a cr yla te
Ta ble 1. P olym er iza tion of Meth yl Acr yla te (MA)
w ith [P d (C6F 5)Br (NCMe)2] (1) + Liga n d a
amt
amt
of 1
of MA poly(MA)
solvent (mmol) (% yield)
Mw/
Mn
b
b
(mmol) ligand
Mw
0.012
41.0
51.5
24.7
23.9
11.1
12.4
14.3
trace
63.0
74.3
91.4
78
0.012
0.014
0.012
PPh3
PPh3
PPh3
8.4 × 105 2.4
1.5 × 106 3.9
1.2 × 106 1.9
3.4 × 105 6.8
7.5 × 105 1.5
7.4 × 105 1.7
CHCl3
PhCl
Me2CO
CHCl3
0.005c PPh3
d
0.025
0.021
PMe3
pyridine CHCl3
41.1
64.2
a
Conditions: 1 equiv of ligand per palladium; solvent, 4 mL;
ambient temperature. b Determined by SEC relative to polystyrene
standards. c Conditions: solvent, 2 mL. As a 1 M solution in
d
THF.
CH2C6F5 (4) and MeCH(CO2Me)CH2C6F5 (5). Note that
the â-hydrogen abstraction occurs from the methyl
group following 2,1-insertion of the alkene. A competi-
tion experiment involving the simultaneous addition of
both methyl acrylate and methyl methacrylate to 1
revealed that the reaction of 1 with methyl acrylate
proceeded ca. 1.25 times faster than the corresponding
reaction with methyl methacrylate. Since the coordina-
tion of the bulkier methyl methacrylate is likely to be
disfavored compared to methyl acrylate, this result
suggests that â-hydrogen abstraction from the CH3
group in the inserted methyl methacrylate is much faster
than that from the CH2C6F5 group in either inserted
acrylate or methacrylate. Indeed, in the reaction with
methyl methacrylate, the possible competing â-hydrogen
abstraction product MeC(CO2Me)dCHC6F5 was not
observed by NMR, which suggests that the rates of
â-hydrogen elimination from the two possible sites differ
by at least 2 orders of magnitude, in favor of CH3 (Chart
1).
B. P olym er iza tion in th e P r esen ce of P h osp h in e
or P yr id in e. When 1 equiv of a monodentate phosphine
or pyridine was added to 1 along with excess methyl
acrylate, there was no precipitation of metallic pal-
ladium and the formation of poly(methyl acrylate)
occurred. In the absence of added ligand even reactions
attempted in neat methyl acrylate led to catalyst
decomposition. Table 1 shows selected polymerization
results for methyl acrylate. It must be noted that, as
the polymerizations proceeded, the reaction mixtures
became extremely viscous and stirring was completely
halted. Consequently, mass transfer problems may be
one factor that limits monomer conversion (and in-
creases polydispersity). The best yields were observed
with 1 equiv of PPh3 or pyridine. The use of PMe3 and
AsPh3 resulted in decreased yields, as well as some
catalyst decomposition. The very bulky phosphines
PtBu3 and PtBu2(biphenyl) led to decomposition to
metallic palladium and no polymer formation. The
addition of 2 equiv of ligand per palladium resulted in
the formation of a stable inactive complex, trans-
describe herein details of our palladium-based systems
which polymerize acrylates and present some rather
intriguing features.
Resu lts a n d Discu ssion
A. Sin gle-In ser tion Rea ction s. The addition of 1
equiv of methyl acrylate to the known palladium(II)
compound [Pd(C6F5)Br(NCMe)2] (1)11 resulted in the
formation of two products, CH(CO2Me)dCHC6F5 (2) and
CH2(CO2Me)CH2C6F5 (3), in an approximately 2:1 ratio.
Additionally, the precipitation of metallic palladium was
observed. The overall mechanism is outlined in Scheme
1. In solution 1 exists in equilibrium with the dimer
[Pd2(µ-Br)2(C6F5)2(NCMe)2] plus free MeCN.11 The al-
kene 2 is then formed by insertion of the monomer into
the Pd-C6F5 bond followed by subsequent â-hydrogen
abstraction, whereas the saturated product, 3, is formed
via an eventual hydride transfer and subsequent reduc-
tive elimination. The formation of a similar mixture of
saturated and unsaturated products by reaction of 1
with styrene has been previously studied in detail.12 The
3:2 ratio increases with the percentage of H atom
transfer via a binuclear compound. The formation of 3
is reduced or eliminated in the presence of additional
ligands that preclude the formation of the bridged
species.
The reaction of methyl methacrylate with 1 proceeded
in an analogous manner, forming a 1:1 mixture of the
unsaturated and saturated products CH2dC(CO2Me)-
(11) Albe´niz, A. C.; Espinet, P.; Foces-Foces, C.; Cano, F. H.
Organometallics 1990, 9, 1079-1085. 19F NMR (282 MHz, CDCl3, 293
K): δ -164.5 (m, 2Fmeta), -160.5 (t, 1Fpara), -121.0 (m, 2Fortho).
(12) Albe´niz, A. C.; Espinet, P.; Lin, Y.-S. Organometallics 1997,
16, 4030-4032.

Pd-Based System for the Polymerization of Acrylates
Organometallics, Vol. 21, No. 20, 2002 4251
[Pd(C6F5)Br(PPh3)2].13 As shown in Table 1, the poly-
merization of methyl acrylate can proceed in a variety
of solvents and without solvent.
Ta ble 2. Effect of Ad d ed 1-Hexen e on th e
P olym er iza tion of Meth yl Acr yla tea
amt
amt of
1-hexene
(mmol)
amt of
MA
(mmol)
amt of
polymer incorporated
of 1-hexene
Preliminary investigations into the active species
resulted in some interesting observations. In a stoichio-
metric experiment the addition of 1 equiv of PPh3 to 1
was followed by 19F and 31P NMR spectroscopy. Upon
initial addition of PPh3 to 1 in CDCl3 several species
were observed in solution, including 1, the inactive
bis(phosphine) complex trans-[Pd(C6F5)Br(PPh3)2], and
the dimer [Pd2(µ-Br)2(C6F5)2(PPh3)2] (6) (eq 1). Over time
the only Pd(C6F5) species in solution was the dimer 6.
Mw/
Mn
c
c
(g)
(mol %)b
Mw
0.6
2.5
22.7
21.0
16.8
10.0
1.48
1.00
0.42
0.16
trace
0.3
3.8
13.6 × 105 5.3
7.7 × 105
2.4 × 105
1.8 × 105
2.5
3.8
2.0
6.7
13.3d
9.4
a
Conditions: [Pd(C6F5)Br(NCMe)2], 0.009 mmol; 1 equiv of
PPh3; PhCl, 4 mL; total monomer (methyl acrylate + 1-hexene), 2
b
g; ambient temperature. Determined by integration of 1H NMR.
d
c Determined by SEC relative to polystyrene standards. Condi-
PPh3
tions: [Pd(C6F5)Br(NCMe)2], 0.011 mmol.
[Pd(C6F5)Br(NCMe)2] -CH CN8
3
â-hydrogen abstraction. The identity of C6F5CHdCH2
was confirmed by its formation when ethene was added
to a mixture of 1 and 1 equiv of PPh3 and by comparison
with a commercial sample. Because of sterics, 1-hexene
would be expected to insert more slowly than ethene
into the Pd-C6F5 bond. Accordingly, the polymerization
of methyl acrylate was carried out in the presence of
varying amounts of 1-hexene. As shown in Table 2, there
was a decrease in the yield and molecular weight of the
polymer obtained with increasing amounts of added
1-hexene. 1H and 13C NMR spectra of the polymers
revealed up to 10% incorporation of 1-hexene. No
polymer was obtained if a large excess of 1-hexene was
employed.
The experiments described above involving methyl
acrylate and 1-hexene suggest that rapid chain termi-
nation (or transfer) occurs via â-hydrogen abstraction
upon the addition of these monomers to the growing
polymer chain. Support for the importance of â-hydro-
gen abstraction comes from the following set of experi-
ments. A dichloromethane solution containing 1 (0.025
mmol), PPh3, and methyl acrylate (molar ratio 1:1:2000)
was separated into three equal samples. After 25 min
the samples were treated as follows. Sample 1 was
quenched by adding it to a large excess of methanol,
resulting in the precipitation of 0.15 g of polymer.
Methyl methacrylate (635 molar equiv) was added to
sample 2 and the reaction was allowed to continue for
17 h; at the end of this time, quenching with methanol
gave 0.29 g of copolymer (methyl acrylate:methyl meth-
acrylate ratio in polymer 1:3). For sample 3, a volume
of dichloromethane equal to the volume of methyl
methacrylate used in sample 2 was added to avoid a
concentration change; quenching after 17 h yielded 1.27
g of polymer. The above experiment indicates that the
addition of methyl methacrylate produces only a small
amount of the copolymer and inhibits further polymer-
ization of methyl acrylate even after it has been initi-
ated. This reactivity pattern is inconsistent with a
classical radical polymerization pathway but consistent
with a termination step initiated by fast â-H abstraction
from an alkyl chain which is linked to Pd by the last
inserted methyl methacrylate moiety, followed by de-
composition of the resulting hydride complex.
[Pd(C6F5)Br(PPh3)2] + [Pd2(µ-Br)2(C6F5)2(PPh3)2]
(1)
Very interestingly, the isolated dimer did not catalyze
the polymerization of acrylates, and no insertion of
acrylate into the Pd-C6F5 bond was observed. However,
as stated above, the formation of polymer did take place
when methyl acrylate and 1 equiv of PPh3 were simul-
taneously added to 1. At the same time, the 19F NMR
spectrum of the reaction mixture revealed that insertion
of the monomer into the Pd-C6F5 bond had occurred,
as evidenced by the typical shift of the ortho fluorines
of the C6F5 group (a C6F5 group attached to Pd shows a
chemical shift of approximately -118 ppm for the ortho
fluorines,11,13 compared to approximately -140 ppm for
the ortho fluorines for a C6F5 fragment bonded to
carbon;11,12 thus, the large shift of the ortho fluorine
resonances in the 19F NMR spectra is a simple indicator
of monomer insertion into the Pd-C6F5 bond). 19F and
31P NMR monitoring of a solution in which polymeri-
zation was occurring revealed the presence of the
inactive dimer 6, whose concentration increased with
time at the early stages of the reaction. This means that
part of the catalyst was being lost by transformation
into the inactive dimer. Also present were 1, the inactive
bis(phosphine) complex trans-[Pd(C6F5)Br(PPh3)2], and
other unidentified species. It is clear from these experi-
ments that the active species is formed through the
reaction of 1 with 1 equiv of PPh3 in the presence of
methyl acrylate, but it is not the dimer 6. This strongly
suggests that the polymerization must begin with the
species [Pd(C6F5)Br(PPh3)(L)] (L ) MeCN, methyl acry-
late) before it converts to the inactive dimer 6. Obvi-
ously, the concentration of the active species must be
significantly lower than the concentration of 1 initially
added to the reaction mixture.
Apart from methyl acrylate, other acrylates that were
either homopolymerized or copolymerized with methyl
acrylate were ethyl and n-butyl acrylate. Interestingly,
methyl methacrylate was not homopolymerized (see
below).
The addition of ethene to the methyl acrylate poly-
merization system completely inhibited the formation
of poly(methyl acrylate). The formation of C6F5CHdCH2
was observed instead, suggesting the preferential inser-
tion of ethene into the Pd-C6F5 bond followed by
C. P olym er iza t ion in t h e P r esen ce of E xcess
Ha lid e. The addition of an excess of halide to 1 also
resulted in a system capable of polymerizing acrylates.
Some of the results are summarized in Table 3. The
efficacy of added halide was found to decrease in the
order Br- > Cl- > I-. On the other hand, the addition
(13) Uso´n, R.; Fornie´s, J .; Nalda, J . A.; Lozano, M. J .; Espinet, P.
Albe´niz, A. C. Inorg. Chim. Acta 1989, 156, 251-256. 19F NMR (282
MHz, CDCl3, 293 K): δ -162.9 (m, 2Fmeta), -162.8 (t, 1Fpara), -117.7(m,
2Fortho). 31P NMR (121.4 MHz, CDCl3, 293 K): δ 24.6.

4252 Organometallics, Vol. 21, No. 20, 2002
Elia et al.
Ta ble 3. Hom op olym er iza tion of Meth yl Acr yla te
in th e P r esen ce of Ad d ed Ha lid e Liga n d a
amt of
amt of 1
(mmol)
MA poly(MA)
Mw/
Mn
b
b
halide
solvent (mmol)
(%)
Mw
0.012
41.0
44.4
23.3
23.2
23.3
23.6
23.5
trace
trace
53.0
75.0
31.6
trace
trace
trace
0.0092 NBu4Cl
0.0069 NBu4Cl
0.0075 NBu4Br
0.0073 NBu4I
PhCl
PhCl
PhCl
8.5 × 105 5.3
3.7 × 105 3.4
14.5 × 105 9.7
0.011
NH4BPh4 PhCl
0.0069 NBu4BF4 PhCl
0.0092 NBu4Cl
acetone 22.2
a
Conditions: 10 equiv of ligand/Pd; solvent, 4 mL; 24 h; room
temperature. b Determined by SEC relative to poly(styrene) stand-
ards.
Ta ble 4. Effect of Rea ction Tim e on th e
P olym er iza tion of Meth yl Acr yla te (MA)a
reacn
time (h)
amt of MA
(mmol)
poly(MA)
(%)
Mw/
Mn
b
b
Mw
0.17
1
4
23.3
23.3
23.2
23.2
4.0
11.1
24.4
71.0
10.6 × 105
8.0 × 105
7.0 × 105
4.7 × 105
3.2
4.2
3.9
2.6
F igu r e 1. Plot of (f - 1)/F vs f/F2 for the copolymerization
of methyl acrylate and methyl methacrylate (F ) M1/M2, f
) m1/m2, M ) monomer composition, m ) polymer com-
position, r1 and r2 ) reactivity ratios for monomers 1 and
2, respectively).
12
a
Conditions: Pd(C6F5)Br(NCCH3)2, 0.007 mmol; NBu4Br, 10
equiv/Pd; PhCl, 4 mL; room temperature. Determined by SEC
b
relative to poly(styrene) standards.
1
Ta ble 5. Effect of Mon om er F eed Ra tio on th e
P olym er iza tion of Meth yl Acr yla te (MA) a n d
Meth yl Meth a cr yla te (MMA)a
into the polymer increased, as determined by H NMR
integration. At the same time, there was a drop in the
molecular weight and, especially, the yield of the
polymer, reminiscent of the copolymerization reactions
of 1-hexene.
amt of
MA
amt of
MMA
amt of
polymer
(g)
polymer
composition
Mw/
c
c
(mmol) (mmol)
(MA:MMA)b
Mw
Mn
The polymers formed in the copolymerization of
methyl acrylate and methyl methacrylate were analyzed
by size exclusion chromatography (SEC) and NMR
spectroscopy. SEC showed unimodal distributions, sug-
gesting that the formed polymers are copolymers and
not mixtures of homopolymers. The random nature of
the copolymers formed was verified by 1H and 13C NMR
spectroscopy. The NMR investigation was aided by
synthesizing copolymers using methyl methacrylate-d8.
The data from Table 5 were used to determine the
reactivity ratio of the two monomers by the Fineman-
Ross method14 (see Figure 1) and yielded the following
values: r(MA) ) 0.66, r(MMA) ) 2.53. These are in good
agreement with those reported in the literature for
radical copolymerization of the two monomers.15
The copolymerization of methyl acrylate with 1-hex-
ene in the presence of halide was also studied. These
reactions afforded copolymers with a maximum of 13%
1-hexene incorporated into the polymer, as determined
by integration of the 1H NMR spectra. Like the co-
polymerizations carried out in the presence of 1 equiv
of phosphine, as the amount of added 1-hexene in the
monomer feed increased, the incorporation of 1-hexene
into the polymer increased while the copolymer yield
and molecular weight decreased. SEC analysis showed
unimodal distributions, implying the formation of co-
polymers. NMR spectroscopy showed the random nature
of the copolymers. Figure 2 shows a 13C NMR spectrum
of a methyl acrylate-hexene copolymer with resonances
at 175.1 (-C(O)O), 52.1 (-OCH3), 41.5 (-CH-), and
15.7
11.2
7.9
6.5
10.4
13.0
11.3
0.66
0.48
0.35
1:0.8
1:1.9
1:3.7
8.4 × 105
7.0 × 105
4.7 × 105
8.6
7.8
4.8
12.3d
a
Conditions: Pd(C6F5)Br(NCCH3)2, 0.0092 mmol; NBu4I, 10
equiv/Pd; PhCl, 4 mL; total monomer (MA and MMA), 2.0 g; 25 h,
room temperature. Determined by integration of 1H NMR. c De-
b
d
termined by SEC relative to poly(styrene) standards. Conditions:
Pd(C6F5)Br(NCCH3)2, 0.014 mmol; PPh3, 1 equiv/Pd; 17 h.
of noncoordinating anions such as ammonium tetra-
phenylborate (NH4BPh4) and tetrabutylammonium tetra-
fluoroborate (NBu4BF4) resulted in catalyst decomposi-
tion and no polymer formation. This suggests that (a) a
coordinating anion is required and (b) the ammonium
cation has no effect on the system. By using varying
amounts of NBu4Br, it was determined that the optimal
polymerization yield was obtained when 10 equiv of Br-
was added. Table 4 shows selected data for the poly-
merization of methyl acrylate in the presence of 10 equiv
of NBu4Br per palladium for reaction times varying from
10 min to 12 h. While higher yields of poly(methyl
acrylate) were achieved as the reaction time was
increased, the polymer molecular weight remained
relatively unchanged.
While the homopolymerization of methyl methacry-
late in the presence of halide afforded very low yields
of polymer (<10%), the copolymerization with methyl
acrylate was more successful. The composition of the
methyl acrylate-methyl methacrylate copolymers was
controlled by varying the monomer feed ratio, as pre-
sented in Table 5. As the ratio of methyl acrylate to
methyl methacrylate in the feed decreased (Table 5,
entries 1-3), the incorporation of methyl methacrylate
(14) Fineman, M.; Ross, S. D. J . Polym. Sci. 1950, 5, 256.
(15) Greenley, R. Z. In Polymer Handbook; Brandrup, J ., Immergut,
E. H., Grulke, E. A., Eds.; Wiley: New York, 1999; p II/181.

Pd-Based System for the Polymerization of Acrylates
Organometallics, Vol. 21, No. 20, 2002 4253
F igu r e 2. 13C NMR spectrum of a copolymer of methyl
acrylate and 1-hexene in CDCl3.
35.0 ppm (-CH2-) resulting from consecutive methyl
acrylate units. In addition, peaks at 175.9, 37.5-32.4,
27.9, 23.0, and 14.2 ppm were observed for the methyl
acrylate-hexene sequence. The absence of a resonance
at 43.4 ppm implies that consecutive hexene units such
as HHH, HHA, and HAH, where H ) 1-hexene and A
) methyl acrylate, are not present.
D. Mech a n istic Stu d ies. The three possible mech-
anisms for the palladium-mediated polymerization of
acrylates are anionic, radical, and insertion. The first
can be discounted on the basis of the fact that the
addition of the anionic polymerization terminating agent
methanol (139 equiv per palladium) had no effect on
either the yield or the molecular weight of the poly-
(methyl acrylate) formed.
Distinguishing between the other two mechanisms is,
however, more difficult and the mechanistic results that
we have obtained display facets that can fit either
mechanism. For example, the observation under stoi-
chiometric conditions of products derived from the
insertion of the monomer into the Pd-C6F5 bond and
the decrease in polymer molecular weight upon the
addition of 1-hexene are consistent with the initiation
and termination steps, respectively, of an insertion
mechanism. Other observations not fitting into a clas-
sical radical mechanism are as follows. Neither styrene
nor methyl methacrylate, which readily undergo free
radical or metal based ATRP polymerization,16 were
efficiently polymerized by either the phosphine- or
halide-based systems. Moreover, in the phosphine-based
system, the polymerization of methyl acrylate was
quickly halted when methyl methacrylate was added to
the reaction mixture (see section B), as discussed before.
This is in sharp contrast with typical radical-initiated
polymerizations (e.g., by AIBN), which readily convert
a mixture of methyl acrylate and methyl methacrylate
to the corresponding copolymer. Indeed, the copolymer
in the latter case is richer in methyl methacrylate
because of its higher reactivity compared to methyl
acrylate.15
F igu r e 3. ESR spectrum, at 70 °C, of the poly(methyl
acrylate) radical species detected during the AIBN-initiated
polymerization of methyl acrylate.
Control experiments involving the polymerization of
methyl acrylate in the presence of 1-hexene using AIBN
as a free radical initiator reveal the incorporation of
1-hexene in poly(methyl acrylate) at levels comparable
to those observed with our systems.
The tacticity of the poly(methyl acrylate) formed by
the phosphine and halide-based systems was ascer-
tained from analysis of the methine-coupled and
-decoupled 1H NMR spectra, which provided information
concerning the dyad (m, r), triad (mm, mr, rr), and
tetrad (mmm, mmr + rmm, rmr) sequences (m ) meso,
r ) racemic) in the polymer chain.18 The observed values
matched those calculated using Bernoullian statistics,
suggesting an atactic polymer.19 Furthermore, the
values were also in agreement with those for a polymer
made using AIBN as the initiator. Finally, the copoly-
mer of methyl acrylate and methyl methacrylate pro-
duced by the halide-based system is richer in methyl
methacrylate, in good agreement with the data reported
in the literature for radical polymerization of the two
monomers.15
Finally, under standard reaction conditions, the ad-
dition of 5 equiv (per palladium) of 2,6-tert-butyl-4-
methylphenol, a free radical inhibitor, did not slow the
polymerization of methyl acrylate, but it has been
suggested that 2,6-tert-butyl-4-methylphenol is not an
efficient inhibitor for free radical polymerization of
acrylates.20 More potent inhibitors are TEMPO and
galvinoxyl. The addition of a few equivalents of these
inhibitors to either the phosphine- or halide-based
system effectively halted the polymerization.
The inhibition of polymerization by TEMPO or gal-
vinoxyl has been widely used as a diagnostic test for
radical polymerizations. Unfortunately, there is always
the possibility that, because of their high reactivity,
these radical scavengers can react and deactivate metal-
based catalysts that effect polymerization through an
insertion mechanism. To examine this possibility, we
In two parallel polymerizations of methyl acrylate,
one initiated by AIBN and the other catalyzed by 1 +
PPh3, the radical •CH(CO2Me)CH2R17 could be detected
by ESR in the first (Figure 3) but not in the second
experiment.
On the other hand, observations that may be more
consistent with a radical mechanism are as follows.
(18) (a) Matsuzaki, K.; Uryu, T.; Ishida, A.; Ohki, T.; Takeuchi, M.
J . Polym. Sci.: A-1 1967, 5, 2167-2177. (b) Suzuki, T.; Santee, E. R.;
Harwood, H. J .; Vogl, O.; Tanaka, T. J . Polym. Sci., Polym. Lett. Ed.
1974, 12, 635-640. (c) Kawamura, T.; Toshima, N.; Matsuzaki, K.
Macromol. Chem. Phys. 1995, 196, 3415-3424.
(16) Reviews: (a) Matyjaszewski, K.; Xia, J . Chem. Rev. 2001, 101,
2921-2990. (b) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev.
2001, 101, 3689-3746.
(17) (a) Sugiyama, Y. Chem. Lett. 1996, 951-952. (b) Sugiyama, Y.
Bull. Chem. Soc. J pn. 1997, 70, 1827-1831.
(19) Odian, G. Principles of Polymerization; Wiley: New York, 1991;
p 675.
(20) Matyjaszewski, K. Macromolecules 1998, 31, 4710-4717.

4254 Organometallics, Vol. 21, No. 20, 2002
Elia et al.
Sch em e 2
have investigated the reaction with galvinoxyl. The
addition of 1 equiv of galvinoxyl to 1 in CDCl3 at room
temperature did not result in any discernible reaction.
When the reaction was repeated in the presence of 1
equiv of methyl acrylate, the rapid formation of
CH(CO2Me)dCHC6F5 (2) was observed (86% yield in 30
min). Curiously, unlike the reaction of 1 with methyl
acrylate in the absence of galvinoxyl, the simultaneous
formation of CH2(CO2Me)CH2C6F5 (3) did not occur (see
Scheme 1). The formation of 2 was much slower (10%
in 18 h) in the reaction 1 + 1 equiv of methyl acrylate
+ 1 equiv of galvinoxyl + 5 equiv of NBu4Br.
Several conclusions can be drawn from these observa-
tions. First, it is clear that Pd-C6F5 bond homolysis
•
does not occur and C6F5 is not trapped by galvinoxyl.
There are two possible ways that 2 (but not 3) can be
formed in the reaction 1 + 1 equiv of methyl acrylate +
1 equiv of galvinoxyl. One way is that C6F5CH2CH-
(CO2Me)• is generated by homolysis following the inser-
tion of acrylate into the Pd-C6F5 bond of 1 and that 2
is formed from this radical through hydrogen ab-
straction by galvinoxyl. The second possibility is that
â-hydrogen abstraction from Pd-CH(CO2Me)CH2C6F5
results in the formation of 2 and Pd-H (see Scheme 1)
and the galvinoxyl simply abstracts a hydrogen atom
from the metal hydride, resulting in catalyst decomposi-
tion (the formation of metallic palladium is observed in
the reaction). The first pathway is easily discounted for
the following reason. If the C6F5CH2CH(CO2Me)• radical
is formed in the reaction of 1 with methyl acrylate, then
polyacrylate formation should occur even in the absence
of added ligand (phosphine or halide). The second
possibility, the reaction of galvinoxyl with a putative
Pd hydride formed by â-hydrogen abstraction from Pd-
CH(CO2Me)CH2C6F5, finds full support in independent
studies on other systems involving isolated hydrides, or
in catalyzed olefin isomerization reactions, accepted to
occur by an insertion/â-hydrogen abstraction mecha-
nism.21
All the evidence, taken together, appears to favor a
radical pathway for the polymerizations. Nevertheless,
it is clear that a classical free radical mechanism is not
being followed; rather, it occurs in conjunction with
insertion and â-hydrogen elimination steps. This is
evident from several observations, including (a) the
pronounced effect of added ligands, (b) the failure of
monomers such as styrene and methyl methacrylate to
undergo polymerization, and (c) the ability of methyl
methacrylate to suppress polymerization when added
together with methyl acrylate and to inhibit further
polymerization of methyl acrylate when added after the
polymerization has been initiated in the phosphine-
based system.
into the Pd-H bond to occur. The bond homolysis step
is reversible and at any given time the concentration of
radicals is low. This explains our inability to detect the
radical by ESR while a signal was observed in the
classical radical polymerization initiated by AIBN.
Similar pathways for radical generation has been
observed in stable free radical polymerizations (SFRP)22
and in cobalt-mediated radical polymerizations.23
The actual polymerization occurs by successive addi-
tion of acrylate monomer to the alkyl radical (Scheme
2, step E). The growing radical chain is in equilibrium
with the corresponding palladium-bound polymeric
alkyl (steps B and F ). Chain termination by â-hydrogen
abstraction can occur from the latter (step C). As our
experiments suggest, the latter step will be significantly
faster if the last added monomer is methacrylate or
1-hexene rather than acrylate. Indeed, if methyl meth-
acrylate is present from the outset, no polymerization
occurs because of facile â-hydrogen abstraction. If it is
added after acrylate polymerization has started, rapid
termination of the growing chains occurs. Because of
the relatively high molecular weight of the polymers
obtained even at low conversions, it has not been
possible to identify the end groups.
The deactivation of the catalyst is associated with
â-hydrogen abstraction (step C) and irreversible decom-
position of the resulting hydride (step D). Since ir-
reversible decomposition competes with insertion and
reentry into the polymerization system, the actual
behavior depends on the last alkene added to the
growing chain and on the ancillary ligands on pal-
ladium. Methyl methacrylate and 1-hexene facilitate the
formation of hydride by â-hydrogen elimination com-
pared to methyl acrylate, and the decomposition rate
of the catalyst increases.
A proposed mechanism that reconciles our observa-
tions is shown in Scheme 2. The first step involves the
insertion of acrylate into the Pd-C6F5 bond (step A).
In the absence of added ligand, this is quickly followed
by â-hydrogen abstraction (step C) and the decomposi-
tion of the resultant palladium hydride (step D). In the
presence of coordinating ligands, steps C and D are
retarded, thereby allowing the competing palladium-
carbon bond homolysis and/or the insertion of acrylate
(22) Le Grognec, E.; Claverie, J .; Poli, R. J . Am. Chem. Soc. 2001,
123, 9513-9524.
(21) Albe´niz, A. C.; Espinet, P.; Lo´pez-Ferna´ndez, R.; Sen, A. J . Am.
Chem. Soc., in press.
(23) Review: Gridnev, A. A.; Ittel, S. D. Chem. Rev. 2001, 101,
3611-3660.

Pd-Based System for the Polymerization of Acrylates
Organometallics, Vol. 21, No. 20, 2002 4255
prior to use. 1-Hexene and styrene were purchased from
Aldrich and deoxygenated before use. 2,2′-Azobis(isobutyro-
nitrile) (AIBN), galvinoxyl, pyridine, TEMPO, tetrabutyl-
ammonium bromide (NBu4Br), tetrabutylammonium chloride
(NBu4Cl), tetrabutylammonium iodide (NBu4I), tetrabutyl-
ammonium tetrafluoroborate (NBu4BF4), ammonium tetra-
phenylborate (NH4BPh4), trimethyl phosphine (PMe3), and
triphenylphosphine (PPh3) were purchased from Aldrich.
trans-Pd(C6F5)Br(CH3CN)2 (1) was prepared according to the
literature procedure.11 Ethene was purchased from MG In-
dustries. All the reactions described were carried out under
an inert atmosphere. Polymer tacticity was determined by
integration of the methine-coupled and -decoupled region in
Phosphines and excess halide ions act as ancillary
ligands in Scheme 2. Their effects are complex, as they
can influence differently the rates of individual steps
in the polymerization process. The presence of either
class of ligands will be expected to retard the â-hydrogen
elimination step (in the case of excess halide through
the formation of anionic alkyl complexes, e.g. [Pd2(µ-
X)2(CH(CO2Me)CH2Pol)2X2]2-).24 However, the initial
alkene insertion step will also be retarded. This is
evident from the observation that the formation of 2 in
the reaction 1 + 1 equiv of methyl acrylate + 1 equiv of
galvinoxyl + 5 equiv of NBu4Br is slower than in the
absence of excess bromide. The phosphine provides an
additional deactivation mechanism (path G) not ob-
served with halide ions.
The deactivating effect of galvinoxyl or TEMPO is
2-fold. First, they react with the growing chains in the
radical polymerization process. Additionally, they can
react with the Pd hydride formed by â-hydrogen ab-
straction,21 preventing its reentrance to initiate new
chain growth. The first is presumably the faster of the
two processes. In any case, it is clear that the suppres-
sion of polymerization by the addition of highly reactive
radical traps cannot be relied upon as an infallible
diagnostic test for radical polymerization in metal-based
polymerization systems, since radical traps can also
interrupt hydride insertion based reactions.
1
the H NMR spectrum. The extent of monomer incorporation
in the copolymers was determined by integration of the
methoxy signals (methyl acrylate/methyl methacrylate copoly-
mers) or methoxy/methyl signals (methyl acrylate/1-hexene
copolymers) in the 1H NMR spectra.
R ea ct ion s of 1 w it h Met h yl Acr yla t e a n d Met h yl
Meth a cr yla te. To a solution of 1 (0.0200 g, 0.046 mmol) in
CDCl3 (0.6 mL) was added methyl acrylate (0.004 mL, 0.046
mmol). The mixture was let stand for 2 h, and decomposition
to metallic palladium was observed. It was checked by 19F and
1H NMR spectra, and a mixture of 2 and 3 (2:3 ) 2:1) was
found.
2: 19F NMR (282 MHz, CDCl3, 293 K) δ -160.8 (m, 2Fmeta),
-150.4 (t, 1Fpara), -138.8 (m, 2Fortho); 1H NMR (300 MHz,
CDCl3, 293 K) δ 7.67 (d, J (H,H) ) 16.0 Hz, 1H; CH(C6F5)),
6.77 (d, J (H,H) ) 16.0 Hz, 1H; CH(CO2CH3)); MS (EI) m/z
(relative intensity) 252 (M+, 46), 221 (100), 193 (65), 173 (10),
143 (36), 123 (16), 117 (8), 59 (2).
3: 19F NMR (282 MHz, CDCl3, 293 K) δ -161.8 (m, 2Fmeta),
-156.1 (t, 1Fpara), -142.9 (m, 2Fortho); 1H NMR (300 MHz,
CDCl3, 293 K) δ 3.05 (t, J (H,H) ) 7 Hz, 2H; CH2(C6F5)), 2.64
(t, J ) 7 Hz, 2H; CH2(CO2CH3)); MS (EI) m/z (relative
intensity) 254 (M+, 44), 223 (17), 195 (71), 194 (100), 181 (77),
145 (16), 143 (11), 59 (12).
The reaction of 1 with methyl methacrylate was carried out
in the same way, and a mixture of compounds 4 and 5 was
obtained (4:5 ) 1:1).
Con clu sion
We have discovered new palladium-based systems for
the homopolymerization of acrylates and their copoly-
merization with simple 1-alkenes. The copolymers in-
variably contain much more acrylate than simple al-
kene. In this respect, the activity of our neutral and
anionic palladium complexes is a mirror image of that
provided by cationic complexes, which efficiently poly-
merize nonfunctionalized alkenes but not acrylates.7
The polymerization occurs by a free radical mechanism
that is tied to a â-H elimination chain termination/
transfer step. Further mechanistic studies on this and
related systems are in progress.
4: 19F NMR (282 MHz, CDCl3, 293 K) δ -162.8 (m, 2Fmeta),
-156.7 (t, 1Fpara), -142.9 (m, 2Fortho); 1H NMR (300 MHz,
CDCl3, 293 K) δ 6.3 (s, 1H; H1)*, 5.45 (s, 1H; H2), 3.8 (s, 3H;
OCH3), 3.7 (s, 2H; CH2C6F5); MS (EI) m/z (relative intensity)
266 (M+, 95), 235 (47), 207 (48), 206 (48), 205 (20), 203 (27),
187 (100), 181 (81), 59 (10). In the 1H NMR spectrum, H1 is
trans to CH2C6F5 and H2 is trans to CO2CH3.
5: 19F NMR (282 MHz, CDCl3, 293 K) δ -162.9 (m, 2Fmeta),
-157 (t, 1Fpara), -143.2 (m, 2Fortho); 1H NMR (300 MHz, CDCl3,
293 K) δ 3.68 (s, 3H; OCH3), 3.1 (m, 1H; CH), 2.8 (m, 2H;
CH2C6F5), 1.2 (d, J (H,H) ) 8.9 Hz, 3H; CH3); MS(EI) m/z
(relative intensity) 268 (M+, 22), 253 (19), 237 (6), 221 (11),
209 (11), 208 (26), 193 (11), 181 (100), 59 (7).
Exp er im en ta l Section
Gen er a l Con sid er a tion s. 1H, 13C, 19F, and 31P NMR
spectra were recorded on Bruker AC-300, PX-300, RX-400, and
AMX2-500 instruments. Chemical shifts are reported in δ
(ppm) downfield from Me4Si (1H), CFCl3 (19F), or H3PO4 (31P).
The spectra were recorded at 293 K. ESR spectra were
recorded on a Bruker ER83CS/ER 041 X6 instrument. Size
exclusion chromatography (SEC) was carried out on a Waters
SEC system using a three-column bed (Styragel 7.8 × 300 mm
columns: 100-10 000, 500-30 000, and 5000-6 000 000 D)
and Waters 410 differential refractometer. SEC samples were
run in CHCl3 at room temperature and calibrated to polysty-
rene standards. Solvents were dried over CaH2 and distilled
and deoxygenated before use. Methyl acrylate, ethyl acrylate,
butyl acrylate, methyl methacrylate, and methyl methacrylate-
d8 were purchased from Aldrich, distilled, and deoxygenated
To a solution of 1 (0.0250 g, 0.057 mmol) in CDCl3 (0.6 mL)
was added methyl acrylate (0.0051 mL, 0.057 mmol) and
galvinoxyl (0.0240 g, 0.057 mmol). The mixture was monitored
by 19F NMR, and after 30 min, compound 2 accounted for 86%
of the C6F5-containing compounds (100% after 2 h).
Syn th esis of [P d 2(µ-Br )2(C6F 5)2(P P h 3)2] (6).25 A solution
of PPh3 (0.105 g, 0.400 mmol) in CH2Cl2 (6 mL) was added
dropwise to a stirred solution of 1 (0.175 mg, 0.400 mmol) in
CH2Cl2 (20 mL). The mixture was stirred for 50 min and the
solvent evaporated to dryness. Et2O (5 mL) was added to the
residue, and a yellow solid was obtained which was filtered,
washed with Et2O, and air-dried. Yield: 0.186 mg (75%). Anal.
Calcd for C48H30Br2F10P2Pd2: C, 46.82; H, 2.45. Found: C,
45.98; H, 2.70. 19F NMR (282 MHz, CDCl3, 293 K): δ -163.1
(m, 2Fmeta), -160.8 (t, 2Fpara), -118.6 (m, 4Fortho). 31P NMR
(121.4 MHz, CDCl3, 293 K): δ 33.6 (t, J (P,F) ) 10.2 Hz).
P olym er iza tion s in th e P r esen ce of P h osp h in e or
P yr id in e. P olym er iza tion of Acr yla tes. To a solution of
(24) For anionic complexes formed by similar systems see: (a) Uso´n,
R.; Fornie´s, J .; Nalda, J . A.; Lozano, M. J .; Espinet, P.; Albe´niz, A. C.
Inorg. Chim. Acta 1989, 156, 251-256. (b) Albe´niz, A. C.; Espinet, P.;
Mart´ın-Ruiz, B.; Milstein, D. J . Am. Chem. Soc. 2001, 123, 11504-
11505.
(25) Uso´n, R.; Royo, P.; Fornie´s, J .; Mart´ınez, F. J . Organomet.
Chem. 1975, 90, 367-374.

4256 Organometallics, Vol. 21, No. 20, 2002
Elia et al.
PPh3 (0.0032 g, 0.012 mmol) in PhCl (2 mL) was added a
solution of methyl acrylate (2.0554 g, 23.875 mmol) in PhCl
(2 mL). This mixture was added to 1 (0.0052 g, 0.012 mmol),
and the reaction proceeded at room temperature for 20 h,
although stirring was halted after the initial 2.5 h. The
polymer was precipitated from MeOH, the methanol was
decanted, and the polymer was dried under vacuum to yield
1.88 g of poly(methyl acrylate) (91%). 1H NMR (300 MHz,
CDCl3, 293 K): δ 3.68 (s, 6H; OCH3), 2.34 (b, 2H; CH(CO2-
CH3)s, CH(CO2CH3)i)*, 1.95 (b, 1H; CHi), 1.68 (b, 2H; CH2s),
1.51 (b, 1H; CH′i) (i ) isotactic, s - syndiotactic). Polymer
tacticity was determined by integration of the methine coupled
and decoupled region of the 1H NMR spectra.
removed from the drybox. The reaction mixture was stirred
at room temperature. The polymer was precipitated from
MeOH, the MeOH was decanted, and the polymer was dried
under vacuum.
Cop olym er iza tion of Meth yl Acr yla te w ith 1-Hexen e.
The polymerization was carried out by following a procedure
analogous to that employed for the copolymerization of methyl
acrylate and methyl methacrylate.
AIBN-In itia ted Cop olym er iza tion s of Meth yl Acr yla te.
A mixture of methyl acrylate (0.8610 g, 10.001 mmol) and
1-hexene (1.1120 g, 13.213 mmol) in PhCl (2 mL) was added
to a solution of AIBN (0.0045 g, 0.027 mmol) in PhCl (2 mL).
The reaction proceeded at 56 °C for 23 h. The polymer was
precipitated from MeOH, the methanol was decanted, and the
copolymer was dried under vacuum to yield 0.317 g (16% yield,
13.7% hexene incorporation). The amount of hexene incorpo-
The same procedure was used for other monomers, ligands,
and/or solvents (see Table 1).
Poly(ethyl acrylate): 65% yield; 1H NMR (300 MHz, CDCl3,
293 K) δ 4.13 (b, OCH2CH3), 2.41-2.26 (b), 1,95 (b), 1.69(b),
1.28 (t, b, OCH2CH3); Mw (Mw/Mn) ) 18.2 × 105 (2.7).
Poly(butyl acrylate): 75% yield; 1H NMR (300 MHz, CDCl3,
293 K) δ 4.04 (b, OCH2CH2CH2CH3), 2.38-2.18 (b), 2-1.85
(b), 1.68-1.54 (b), 1.38 (b), 0.91 (b); Mw (Mw/Mn) ) 7.3 × 105
(11.5).
1
rated was determined by integration of H NMR signals.
The copolymerization of methyl acrylate and methyl meth-
acrylate initiated by AIBN was carried out in a similar way,
but using a monomer ratio (methyl acrylate:methyl meth-
acrylate) of 1.1:1 and a temperature of 100 °C for 3 h. The
copolymer obtained (68% yield) was methyl methacrylate rich
(methyl acrylate:methyl methacrylate ) 1:1.25).
ESR Exp er im en ts. A mixture of methyl acrylate (0.2390
g, 2.776 mmol) and AIBN (0.0005 g, 0.003 mmol) in CH2Cl2
(0.6 mL) was charged in an ESR tube. The sample was heated
to 70 °C inside the probe, and a radical species was detected.
A mixture of methyl acrylate (0.2390 g, 2.776 mmol) and
PPh3 (0.0007 g, 0.003 mmol) in CH2Cl2 (0.4 mL) was added to
a solution of 1 (0.0012 g, 0.003 mmol) in CH2Cl2 (0.2 mL), and
the mixture was charged in an ESR tube. Spectra were taken
in the temperature range 25-50 °C, after mixing and when
the polymerization was taking place, and a viscous solution
was present (1 and 3 h later). No signal was detected.
Cop olym er iza tion of Meth yl Acr yla te a n d 1-Hexen e.
To a solution of PPh3 (0.0030 g, 0.011 mmol) in PhCl (2 mL)
was added a mixture of methyl acrylate (0.8604 g, 9.994 mmol)
and 1-hexene (1.1104 g, 13.194 mmol) in PhCl (2 mL). This
mixture was added to 1 (0.0050 g, 0.011 mmol), and the
reaction proceeded at room temperature for 21 h. The polymer
was precipitated from MeOH, the methanol was decanted, and
the polymer was dried under vaccum to yield 0.162 g of
copolymer (8.2% yield, 9.4% hexene incorporation). The amount
1
of hexene incorporated was determined by intergration of H
NMR signals.
P olym er iza tion s in th e P r esen ce of Excess Ha lid e.
P olym er iza tion of Acr yla tes. To a solution of the appropri-
ate halide in PhCl (4 mL) was added methyl acrylate. The
mixture was then added to 1, and the flask was capped with
a rubber septum and removed from the drybox. The reaction
mixture was stirred at room temperature for the desired
reaction time. CHCl3 was added to the viscous crude product.
The polymer was precipitated from MeOH, the MeOH was
decanted, and the polymer was dried under vacuum. The same
procedure was followed when other solvents were used.
Cop olym er iza tion of Meth yl Acr yla te w ith Meth yl
Meth a cr yla te (or Meth yl Meth a cr yla te-d 8). To a solution
of halide (10 equiv/Pd) in PhCl (4 mL) was added methyl
acrylate and methyl methacrylate (or methyl methacrylate-
d8) in the appropriate proportion. The mixture was then added
to 1, and the flask was capped with a rubber septum and
Ack n ow led gm en t. We thank the U.S. Department
of Energy, Office of Basic Energy Sciences (Contract No.
DE-FG02-84ER13295), the Direccio´n General de Inves-
tigacio´n (Grant No. BQU2001-2015), and the J unta de
Castilla y Leo´n (Grant Nos. VA80/99 and VA17/00B) for
financial support. The collaboration was greatly facili-
tated by grants from Iberdrola and the Comisio´n Con-
junta Hispano-Norteamericana de Cooperacio´n Cient´ı-
fica y Tecnolo´gica (Project 20009). We thank Prof.
Garc´ıa-Herbosa and the University of Burgos (Spain)
for ESR facilities. A fellowship to R.L.-F. from the MEC
is also gratefully acknowledged.
OM020435Q