Ring-opening polymerization of heterocycles with palladium insertion catalysts: Observation of a multifunctional polymerization initiator

Article abstract of DOI:10.1021/om990265q

The palladium insertion polymerization catalysts (bipy)Pd(CH₃)L⁺OTf^- have been found to mediate a second class of polymerization reaction: the ring-opening polymerization of heterocycles. Unlike traditional cationic ring-opening processes, this polymerization is found to proceed via an unusual series of metal-based reactions, including novel steps which allow the coupling of insertion monomers (CO and norbornene) with ring-opened polymers.

Full text of DOI:10.1021/om990265q

Organometallics 1999, 18, 3953-3955
3953
Rin g-Op en in g P olym er iza tion of Heter ocycles w ith
P a lla d iu m In ser tion Ca ta lysts: Obser va tion of a
Mu ltifu n ction a l P olym er iza tion In itia tor
Ngiap Kie Lim, Karin J . Yaccato, Rania D. Dghaym, and Bruce A. Arndtsen*
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal,
Quebec H3A 2K6, Canada
Received April 14, 1999
Summary: The palladium insertion polymerization cata-
lysts (bipy)Pd(CH3)L+OTf- have been found to mediate
a second class of polymerization reaction: the ring-
opening polymerization of heterocycles. Unlike tradi-
tional cationic ring-opening processes, this polymeriza-
tion is found to proceed via an unusual series of metal-
based reactions, including novel steps which allow the
coupling of insertion monomers (CO and norbornene)
with ring-opened polymers.
and end groups, and allows the novel combination of
cationic ring-opening polymerization with sequential
olefin/CO insertion to generate new polymers.
Cationic palladium-methyl complexes with various
dative ligands, (bipy)Pd(CH3)L+OTf- (bipy ) 2,2′-bipyri-
dyl; OTf ) OSO2CF3; 1-4), can be prepared by reaction
of (bipy)Pd(CH3)Cl with AgOTf in the presence of L (eq
1).7 The addition of 1 atm of CO to CH2Cl2 solutions of
The cationic ring-opening polymerization of hetero-
cycles has become of significant importance in polymer
synthesis, due in part to the diverse variety of func-
tionalized materials that can be prepared by this
method, including natural and/or biodegradable poly-
mers.1 In contrast to the well-known transition-metal-
catalyzed synthesis of polyolefins, where the catalyst
can affect various polymerization properties (e.g., se-
quential monomer selectivity, polymer molecular weight
and tacticity),2 the synthesis of ring-opened heterocycles
is often mediated by compounds whose role is to initiate
the polymerization process. This provides less op-
portunity to “tune” polyheterocycle properties through
catalyst choice and has stimulated research to develop
initiators which function beyond the simple activation
of polymerization, such as living polymerization initia-
tors,1 chiral initiators,3 enzyme initiators,4 and ring-
opening insertion systems.5,6 We report here that the
organo-palladium complexes (bipy)Pd(R)L+OTf-, which
are known insertion polymerization catalysts,2 can
mediate a second type of polymerization reaction: the
ring-opening polymerization of heterocycles. In contrast
to typical initiators, mechanistic studies suggest this
polymerization proceeds via an unusual series of metal-
based reactions. The manipulation of these steps pro-
vides significant control over polymer molecular weight
1-4 leads to the formation of acyl complexes (bipy)Pd-
(COCH3)L+OTf- (5-8). When this reaction of 1-4 (ca.
20 mg, 0.05 mmol) with 1 atm of CO is performed at 70
°C in THF, the reaction solution slowly darkens and
becomes viscous. Filtration and removal of solvent yields
1
0.33-0.98 g of a glassy solid (Table 1).8 The H and 13C
NMR of this material shows it to be the ring-opened
polymer of tetrahydrofuran. As shown in eq 1, com-
plexes 1-4 under 1 atm of CO are effective for the
cationic ring opening of a number of heterocycles, such
as dioxolane and lactones.
Metal complexes that can mediate two distinct types
of polymerizations, i.e., insertion and heterocyclic ring-
opening polymerization, have been observed in rare
instances.6,9 These systems typically involve highly
electropositive metal centers, which mediate ring-open-
ing polymerization as Lewis acid initiators, by binding
strongly to the heteroatom to begin the cationic ring-
opening process. In contrast, palladium complexes such
as 1-4 are considered to interact only weakly with
heteroatom substituents (e.g. ethers and esters), a
feature that has made them of much interest in the
insertion polymerization of functionalized olefins.2 It is
(1) For reviews, see: (a) Cationic Polymerization; Matyjaszewski,
K., Ed.; Marcel Dekker: New York, 1996. (b) Ring-Opening Polymer-
ization; Brunelle, D. J ., Ed.; Hanser: Munich, 1993. (c) Yagci, Y.;
Mishra, M. K. In Macromolecular Design: Concept and Practice;
Mishra, M. K., Ed.; Polymer Frontiers: New York, 1994; p 391.
(2) (a) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (b)
Drent, E.; Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663. (c) Abu-
Surrah, A. S.; Rieger, B. Angew. Chem., Int. Ed. Engl. 1996, 35, 2475.
(3) For examples, see: (a) Nakano, T.; Okamoto, Y.; Hatada, K. J .
Am. Chem. Soc. 1992, 114, 1318. (b) Novak, B. M.; Goodwin, A. A.;
Patten, T. E.; Deming, T. J . Polym. Prepr. 1996, 37, 446.
(4) Bisht, K. S.; Deng, F.; Gross, R. A.; Kaplan, D. L.; Swift, G. J .
Am. Chem. Soc. 1998, 120, 1363.
(7) (a) Dghaym, R. D.; Yaccato, K. J .; Arndtsen, B. A. Organome-
tallics 1998, 17, 4. (b) van Asselt, R.; Gielens, E. E. C. G.; Rulke, R.
E.; Vrieze, K.; Elsevier, C. J . J . Am. Chem. Soc. 1994, 116, 977.
(8) Isolated polymers were washed with organic solvents and/or
precipitated from methanol until the only 1H and 13C NMR peaks noted
were those of the polymer.
(5) Aida, T.; Inoue, S. J . Am. Chem. Soc. 1985, 107, 1358.
(6) Yasuda, H.; Tamamoto, H.; Yamashita, M.; Yokota, K.; Naka-
mura, A.; Miyake, S.; Kai, Y.; Kanehisa, N. Macromolecules 1993, 26,
7134.
(9) For examples of cationic and insertion polymerization catalysts,
see: Hayakawa, M.; Mitani, M.; Yamada, T.; Mukaiyama, T. Macromol.
Rapid Commun. 1996, 17, 865. Quyoum, R.; Wang, Q.; Tudoret, M.
J .; Baird, M. C. J . Am. Chem. Soc. 1994, 116, 6435.
10.1021/om990265q CCC: $15.00 © 1999 American Chemical Society
Publication on Web 09/10/1999

3954 Organometallics, Vol. 18, No. 20, 1999
Communications
weights, ranging from Mw ) 8.8 × 104 for 4 to Mw ) 1.9
Ta ble 1. P olym er iza tion w ith Com p lexes 1-4 a n d
12a
1
× 104 with 2 (Table 1). In addition, the H NMR of the
b
b
poly-THF generated with 1 shows the incorporation of
the PhNdC(H)Tol ligand into the polymer as an immo-
nium group (1H NMR: δ 2.38 (s, 3H), 4.45 (t, 2H), 7.3-
7.8 (m, 9H), 9.34 (s, 1H)).11 When >0.06 M PhNd
C(H)Tol is added to the polymerization, the incorporation
of this immonium moiety into the polymer is quantita-
tive.12 Similarly, a -CH2(CH3)NdC(H)Tol+ unit is
formed in the poly-THF generated with 2.13 No evidence
substrate
initiator [L], mmol yield, g Mn
Mw
THF
THF
THF
THF
1
2
3
4
4
4
4
4
4
4
4
12
0.44
0.06
0.55
0.92
0.13
0.07
1.42
8.02
4.33
1.06
3.01
3.12
2.4
1.3
3.3
3.5
1.1
8.2
4.9
1.9
7.2
7.8
2.1
1.5
THF
0.25c
0.50c
THF
THFd
13.8 19.6
dioxolane
butyrolactone
ꢀ-capralactone
THF
1.4
0.5
2.8
19.1 26.7
15.5 22.1
3.3
0.75
4.1
t
for BuNdC(H)Tol or NCCH3 incorporation is noted with
3 and 4, respectively, and instead small amounts of bipy
(<20%) are found in the larger Mw polymers. COSY and
TOCSY NMR experiments demonstrate these immo-
nium units are end groups on the alkyl terminus of the
poly-THF. The incorporation of imine into the poly-THF
as an immonium group and the lower polymer Mw
observed with the more basic imines on initiators 1 and
2 suggest that this ligand (L) is involved in termination
of THF cationic polymerization by nucleophilic attack
on the growing polymer. To test this hypothesis, in-
creasing amounts of free Tol(H)CdNPh were added to
THF polymerization with 2. This results in similarly
decreasing polymer Mw (Table 1), consistent with the
role of free imine in polymer termination.
0.05e
THF
a
Conditions: 10 g of substrate, 0.05 mmol of catalyst, and 1
atm of CO heated to 70 °C for 24 h. ×104. c Added PhNdC(H)Tol.
b
100 g of substrate. e Added norbornene.
d
Sch em e 1
1
The poly-THF generated with 1-4 also has a H NMR
resonance at δ 2.01 (s) which integrates to one methyl
group per polymer chain.12 This resonance becomes a
doublet (2J C-H ) 6.9 Hz) when 13CO is employed in the
polymerization. Both IR (νCO 1739.9 cm-1) and 13C NMR
(δ 171.2) analysis show that a carbonyl is also incorpo-
rated on the polymer. These spectral features correlate
with a [CH3CO] moiety on the poly-THF in the form of
an ester end group.14 The presence of this unit as the
other polymer end group was determined by 13C NMR
decoupling experiments, which shows three-bond cou-
pling between the 13C-labeled carbonyl carbon and the
-OCH2- hydrogens on the poly-THF (3J C-H ) 2.7 Hz).
Considering the incorporation of 13CO into this end
group, the ester undoubtedly arises from coupling of the
poly-THF with the palladium-acyl ligand.
While the mechanistic details of this reaction have
not been fully elucidated, the incorporation of two
palladium ligands from 1-4 (Pd-imine and Pd-meth-
yl), and external CO, into the poly-THF suggests ring-
opening polymerization occurs via a series of steps.
Plausible mechanisms to account for our observations
are outlined in Scheme 1. Palladium-acyl complexes
have been previously postulated by Tanaka to react with
THF via free RCO+ to form 4-chlorobutyl alkyl acetate,
suggesting MeCO+ (path A) as a possible intermediate.15
Nevertheless, the formation of intermediate 9 implies
THF coordination to Pd is also important to the polym-
erization. Indeed, it was found that the chloride-
substituted (bipy)Pd(COMe)Cl, which does not coordi-
nate THF to Pd, is also inactive for THF polymerization
also interesting to note that when 1-4 are heated to
70 °C in THF for several days in the absence of CO
ligand, only traces (<20 mg) of polymer are obtained,
providing an unusual carbon monoxide “on” switch for
the polymerization. These data all suggest an unusual
role of the palladium complexes in initiating ring
opening and prompted our closer examination of the
polymerization process.
Monitoring the reaction of THF with (bipy)Pd(CH3)-
NCCH3+OTf- (4; 7 mg, 0.02 mmol in 1 mL of d8-THF
and 1 atm of 13CO) by 1H and 13C NMR reveals the
immediate quantitative formation of free acetonitrile
and a new palladium-acyl fragment, (bipy)Pd(13-
COCH3)+ (9), at room temperature.10 The 13C NMR of
this complex shows a single labeled 13CO resonance (δ
218.3, Pd13COCH3), implying that the THF solvent,
rather than a second 13CO, occupies the fourth coordi-
nation site (Scheme 1). Heating of 9 to 70 °C reveals no
further reaction intermediates and instead leads to the
slow disappearance of 9 and gelling of the solvent due
to formation of poly-THF. This rapid insertion of CO
into the palladium-methyl bond prior to ring opening
suggests the palladium-acyl complex 9 is the active
initiator for polymerization. Further evidence for the
involvement of the 9 in the polymerization is found in
the structure of the poly-THF (vide infra).
(11) For comparison, 1H NMR of Tol(H)CdN(Ph)CH3+OTf-: δ 2.38
(s, 3H), 4.16 (s, 3H), 7.2-7.8 (m, 9H), 9.30 (s, 1H).
Closer examination of the isolated poly-THF reveals
these palladium complexes allow more control over the
ring opening than simply providing the correct initiation
conditions. Variation of the dative ligand (L) on 1-4
leads to significantly different poly-THF molecular
(12) End-group quantitation was determined by comparison of Mn
calculated from 1H NMR to Mn from GPC data. For complete polymer
characterization, see the Supporting Information.
(13) 1H NMR of Tol(H)CdN(CH3)CH2 end group: δ 2.49 (s, 3H),
+
3.76 (s, 3H), 4.20 (t, 2H), 7.2-7.8 (m, 4H), 9.15 (s, 1H).
(14) Ethyl acetate: νCO 1742 cm-1; δ 170.3 (CO).
(15) Tanaka, M.; Koyanagi, M.; Kobayashi, T. Tetrahedron Lett.
1981, 22, 3875 and references therein.
(10) 1H NMR of 9: δ 2.82 (d, 3H), 7.70 (t, 2H), 8.19 (t, 2H), 8.48 (d,
2H), 8.57 (d, 2H).

Communications
Organometallics, Vol. 18, No. 20, 1999 3955
of 13CO, norbornene, and 13CO into (bipy)Pd(CH3)Cl to
form 12,18 followed by addition of AgOTf in THF and
heating to 70 °C. 13C NMR of the isolated polymer 11
reveals the presence of 13C label in both a ketone (δ
207.7) and ester (δ 174.9) end group, while COSY and
13C decoupling experiments show connectivity between
the ester carbonyl carbon and both the norbornyl to
poly-THF fragments, demonstrating this polymer is
indeed the novel product of coupling of olefin and CO
insertion monomers with ring-opening polymerization.
In conclusion, the palladium insertion polymerization
catalysts (bipy)Pd(R)L+OTf- have been found to initiate
THF ring-opening polymerization with significant ligand
control over polymer molecular weights and end groups.
In addition to mediating two distinct classes of polym-
erization on a single metal site, these palladium com-
plexes provide a single-pot method to couple insertion
monomers onto ring-opened heterocycles. To our knowl-
edge, the latter is not precedented in metal-mediated
polymerizations6,19 and provides potential access to new
classes of block copolymers incorporating insertion and
ring-opened monomers. Experiments directed toward
the generation of these materials with complexes 1-4
are currently underway.
Sch em e 2
under our conditions. The initiation of ring opening from
9 could occur through direct acylation of THF (path B),
or the polymerization of THF on palladium (path C),
followed by reductive elimination.17 The termination of
either of these ring-opening reactions by nucleophilic
attack of L would generate the observed immonium end
groups. Further studies are currently in progress to
distinguish between these mechanistic possibilities.
A particularly exciting aspect of this polymerization
is the coupling of the acyl fragment in 9 with the THF
polymer. Since this acyl moiety is also the ligand into
which olefin/CO insertion polymerization has been
found to occur,2 the coupling step suggests these pal-
ladium complexes could provide a unique system to
combine monomers polymerized by two distinct mech-
anisms, insertion (olefin and carbon monoxide) and
cationic ring opening (THF), into novel copolymers.1,6
Experiments toward this end were performed in a
controlled manner by reaction of 4 (0.05 mmol) with 1
equiv of norbornene (0.05 mmol), AgOTf, and 1 atm of
13CO in THF at 70 °C (Scheme 2). This yields 3.01 g of
Ack n ow led gm en t. We thank the NSERC of Canada
and Fonds FCAR du Quebec for their financial support.
R.D.D. thanks McGill University for the McFee Memo-
rial Fellowship.
Su p p or tin g In for m a tion Ava ila ble: Text and figures
giving preparation details and characterization data on 1-3,
5-7, 9, and polymers. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
a white solid, which H NMR analysis shows to be poly-
THF (11) with the quantitative incorporation of a single
norbornyl (δ 1.17-3.22 (m)) and methyl ketone (δ 2.13
(d, 3H)) fragment. An identical polymer (11) can be
generated by first carrying out the sequential insertion
OM990265Q
(18) Markies, B. A.; Kruis, D.; Rietveld, M. H. P.; Verkerk, K. A.
N.; Boersma, J .; Kooijman, H.; Lakin, M. T.; Spek, A. L.; van Koten,
G. J . Am. Chem. Soc. 1995, 117, 5263.
(19) Organic initiators for simultaneous free radical and ring-
opening polymerization have been recently prepared: (a) Weimer, M.
W.; Scherman, O. A.; Sogah, D. Y. Macromolecules 1998, 31, 8425. (b)
Mecerreyes, D.; Moineau, G.; Dobois, P.; J erome, E.; Hedrick, J . L.;
Hawker, C. J .; Malmstrom, K. E.; Trollsas, M. Angew. Chem., Int. Ed.
1998, 37, 1274.
(16) Effenberger, F.; Sohn, E.; Epple, G. Chem. Ber. 1983, 116, 1195.
(17) The role of CO insertion in path C could be to effect the Lewis
acidity of palladium. Such ancillary ligand effects have been previously
observed for THF ring opening: Borkowsky, S. L.; J ordan, R. F.; Hinch,
G. D. Organometallics 1991, 10, 1268.