Acyclic guanidines as organic catalysts for living polymerization of lactide

Article abstract of DOI:10.1021/ma901776x

We describe a facile route to structurally diverse guanidinium organic catalysts by reaction of carbodiimides with secondary amines. The efficacy of these catalysts for the living ring-opening polymerization (ROP) of lactide was demonstrated including predictable molecular weights and end-group fidelity. Theoretical studies indicate that the acyclic guanidines are less basic than 1, 5, 7-triazabicyclo[4.4.0]dec-5-ene (TBD), but as for the more active TBD, they catalyze ring opening by activation of the alcohol and by stabilizing the resultant tetrahedral intermediates through hydrogen bonding. These results demonstrate that weak secondary interactions are an important concept for controlled polymerization.

Full text of DOI:10.1021/ma901776x

1660 Macromolecules 2010, 43, 1660–1664
DOI: 10.1021/ma901776x
Acyclic Guanidines as Organic Catalysts for Living Polymerization
of Lactide
Lei Zhang,†,§ Russell C. Pratt,† Fredrik Nederberg,†,‡ Hans W. Horn,† Julia E. Rice,†
Robert M. Waymouth,‡ Charles G. Wade,† and James L. Hedrick*,†
†IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120 and ‡Department of Chemistry,
Stanford University, Stanford, California 94305. §Current address: Milliken Research Corporation,
920 Milliken Road, Spartanburg, SC, 29302.
Received August 7, 2009; Revised Manuscript Received December 8, 2009
ABSTRACT: We describe a facile route to structurally diverse guanidinium organic catalysts by reaction of
carbodiimides with secondary amines. The efficacy of these catalysts for the living ring-opening polymer-
ization (ROP) of lactide was demonstrated including predictable molecular weights and end-group fidelity.
Theoretical studies indicate that the acyclic guanidines are less basic than 1,5,7-triazabicyclo[4.4.0]dec-5-ene
(TBD), but as for the more active TBD, they catalyze ring opening by activation of the alcohol and by
stabilizing the resultant tetrahedral intermediates through hydrogen bonding. These results demonstrate that
weak secondary interactions are an important concept for controlled polymerization.
Ring-opening polymerization (ROP) of cyclic esters, such as
lactide and lactones, is a powerful method for generating poly-
esters with controlled molecular weight and end-group function-
ality; several reviews of the elegant organometallic catalytic
routes to such materials have appeared.1 In addition to poly-
merization techniques that give predictable molecular weights,
narrow molecular weight distributions, and end-group fidelity,
many advanced applications require the removal of undesired
contaminants such as heavy metal ions from catalysts. Toward
this goal, new methods based on organocatalytic ring-opening
polymerization have been developed.2-9 Examples of succes-
sful organocatalysts for the ROP of cyclic esters, carbonates,
and siloxanes include 4-dimethylaminopyridine,3 phosphines,4
N-heterocyclic carbenes (NHC),5 acids,6 bifunctional amino-
thioureas,7 phosphazene bases,8 amidines, and guanidines.9,10
The bicyclic guanidine moiety is a substructure common to
many biologically active natural products and the catalytic role
in natural products has received considerable attention.11
Guanidines and chiral guanidines have been used in several
asymmetric transformations,12 but the synthesis of the catalysts,
particularly the chiral bicyclic guanidine’s is very demanding.13,14
Along these lines, 1,5,7- triazabicyclo[4.4.0]dec-5-ene (TBDHþ,
pKa = 26 in acetonitrile)15 drew our attention as a ROP catalyst
for cyclic esters since it is commercially available and easily
handled with reported use as a transesterification catalyst.16 For
example, in a ROP of lactide in the presence of TBD in a 1000:1:10
monomer/catalyst/initiator monomer conversion was complete in
2 min, rivaling the NHC catalysts in turnover frequency.17 As TBD
is a strong base, our initial hypothesis was that it deprotonates the
alcohol and propagates by an anionic mechanism. However,
solvent effects on the polymerization with TBD were inconsistent
with a purely anionic mechanism. For the TBD-catalyzed ROP of
lactide, the reaction was fastest in CH2Cl2 (>90% conversion in
2 min), slightly slower in THF (60% conversion in 2 min), and
much slower in DMF (0% conversion in 2 min), the opposite of
what would be expected for a conventional anionic polymerization.
These observations and the fact that the much faster rate of
TBD relative to other organic catalyts such as 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBUHþ, pKa = 24) was not entirely consis-
tent with their relative basicities suggested to us the intriguing
possibility that TBD is not functioning merely as a base, but as a
novel bifunctional acylation catalyst; acylation of TBD generates
an activated acyl intermediate with an adjacent imine that func-
tions as a general base to activate the alcohol for nucleophilic
attack (Scheme 1A). Although mechanistic studies implied that
TBD can function both as a base and a nucleophilic acylating
agent, subsequent theoretical studies indicated that a nucleophilic
mechanism had a considerably higher barrier than an alternative
hydrogen-bond-mediated mechanism for transesterification reac-
tions (Scheme 1B).18 The bifunctional hydrogen-bonding capabi-
lities of bicyclic guanidine catalysts were also reported by Corey19
in the enantioselective Strecker reaction of R-aminonitriles and
R-amino acids. Moreover, TBD was shown to react with malonate
esters via nucleophilic attack of both disubstituted nitrogens at
the carbonyl groups to form betaine-like structures.20 In this com-
munication, we describe a facile route to structurally diverse acyclic
guanidines together with their viability as catalysts for the ROP of
lactide. The activity and mechanism of these new catalysts are
compared to the cyclic guanidines previously reported.9,17
The synthesis of the acyclic guanidines was accomplished by
reaction of carbodiimides with secondary amines according to
literature procedures (Scheme 2).21 Three secondary amines were
surveyed, pyrrolidine, TBD, and diphenyl[(2S)-2-pyrrolidine]-
methanol. Pyrrolidine is readily available and is expected to
generate a sterically unencumbered guanidine, 1, whereas
diphenyl[(2S)-2-pyrrolidine]methanol generates a bulky chiral
guanidine, 3, that is likely to have important ramifications in
stereoselective polymerizations. The addition of TBD to dicyclo-
hexylcarbodiimide (DCC) generates a biguanidine 2 that is con-
siderably more basic than TBD alone and has been shown to be a
potent catalyst for the transesterification of vegetable oils to
generate biofuels.10 DCC was reacted neat at elevated temp-
erature with either pyrrolidine, TBD, or diphenyl[(2S)-2-pyrroli-
dine]methanol. Once the DCC melted, a homogeneous solution
was formed, and the reaction was allowed to proceed overnight to
*To whom correspondence should be addressed. E-mail: hedrick@
almaden.ibm.com.
pubs.acs.org/Macromolecules
Published on Web 01/05/2010
r 2010 American Chemical Society

Note
Macromolecules, Vol. 43, No. 3, 2010 1661
Scheme 1. ROP of Cyclic Esters through Dual Activation by TBDa
Figure 1. 1H NMR study. Hydroxy group of benzyl alcohol (bottom
spectrum) shift downfield with the addition of noncyclic guanidine, 1
(middle spectrum), and TBD (top spectrum).
a (A) acyl mechanism and (B) hydrogen bonding mechanism.
Scheme 4. Single Turnover Reaction of Lactide with Alcohol Using
Noncyclic Guanidine Catalyst 1
Scheme 2. Synthesis of Acyclic Guanidines
However, vinyl acetate reacts with benzyl alcohol giving benzyl
acetate over the course of 12 h inthe presenceof1, which issimilar
to that observed for DBU.9 As for TBD, the addition of 1 to
benzyl alcohol in toluene shifts the alcohol-proton from 0.85 to
3.45 ppm; minor shifts for the R-methylene protons of benzyl
alcohol are also observed, indicating a strongly hydrogen bonded
complex.
Scheme 3. Acyl Transfer Reactions with TBD and 1
To determine if the acyclic guanidines could mediate the ring-
opening of lactide, a single turnover experiment was carried out
where an excess of benzyl alcohol was treated with lactide in the
presence of guanidine 1 in toluene-d8 (Scheme 4). An aliquot
removed upon mixing revealed the formation of the benzyl
dilactate; after two days, further transesterification to benzyl
lactate was observed.
Having demonstrated that 1 ring-opens lactide in the presence
of stoichiometric benzyl alcohol, each of the guanidines 1-3 was
surveyed for the catalytic ROP of lactide. Polymerization of
L-lactide in CH2Cl2 (2 M) with 1 mol % of 1 relative to monomer
generate a viscous oil/gel. GC/MS results showed that quantitative
conversion of starting material was accomplished in ∼12 h.
Compounds 1 and 2 were purified by Kugelrohr distillation and
3 was purified by column chromatography.
The catalytic activity and mechanism of synthesized acyclic
guanidines were studied and compared with that of TBD. While
theoretical studies implicate an alcohol activation mechanism,10
in a model reaction it was shown that TBD could be acylated by
reaction with vinyl acetate followed by removal of the low-boiling
acetaldehyde (Scheme 3). Subsequent addition of excess benzyl
alcohol completed the cycle by the formation of methyl benzoate,
releasing the TBD-catalyst.10 In contrast, none of the acyclic
guanidines reacted with vinyl acetate to give acylated products.
(with 1 mol % of pyrenebutanol relative to monomer as the
initiator) generated polylactide in approximately 40 min with
near quantitative conversion of monomer to polymer (∼99%).
The resulting polymer had a number average molecular weight
Mn = 17 600 with a PDI (PDI = Mw/Mn) of 1.06. Concurrent
monitoring of both the refractive index and UV absorbance by
gel permeation chromatography (GPC) (Figure 2A) together
1
with H NMR spectroscopy (Figure 2B) shows that the UV-
active pyrenebutanol initiator is fully incorporated into the
polymer, demonstrating end-group fidelity and supports the
initiation of the polymerization by pyrenebutanol. Aliquots
taken during the polymerization with benzyl alcohol show a
linear increase in molecular weight with conversion, a trend that

1662 Macromolecules, Vol. 43, No. 3, 2010
Zhang et al.
Figure 2. (A) Overlay of signals from refractive index (RI) and UV absorbance GPC detectors, (B) NMR spectrum of synthesized PLLA with R H
peak of pyrenebutanol marked. (PLLA with Mn of 17600 and PDI of 1.06 was prepared using catalyst 1 with L-LA/catalyst 1/pyrenebutanol =
100:1:1).
Figure 4. Comparison of the adduct of 1 and TBD with methanol;
binding energies and hydrogen bond lengths relevant for the catalytic
activation of the alcohol initiating the ROP.
Figure 3. Mn (b) and PDI (O) versus conversion for ROP of
L-LA
catalyzed by noncyclic guanidine catalyst 1 in CH2Cl2. ( -LA/catalyst
L
1/benzyl alcohol = 100:1:1). Molecular weight from GPC using
polystyrene standards.
is typically associated with a living polymerization (Figure 3).
The PDIs remain narrow throughout the polymerization
(<1.15). Higher targeted degrees of polymerization (DPs),
200 and 400, gave molecular weights of 28000 g/mol (PDI =
1.13) and 37000 g/mol (PDI = 1.14), respectively. The differ-
ences of the three acyclic guanidines in activity and selectivity
were revealed by comparing the three catalysts in a one-to-one
ratio of catalyst to benzyl alcohol initiator in CH2Cl2 (2 M) with
targeted polylactide DPs of 100. A quenched aliquot (20 min
polymerization time) showed quantitative conversion of mono-
mer to polymer (Mn = 37 000 g/mol, PDI = 1.49) for catalyst 2,
whereas polymers generated from catalysts 1 and 3 showed
conversions of 50 and 70% (Mn = 18 000 and 33 000 g/mol
and PDIs of 1.13 and 1.04, respectively). The higher activityof2 is
accompanied by the loss in selectivity and control; presumably
this catalyst is more active both for ring-opening and for
transesterification of the resulting polymer, leading to broader
polydispersities. Catalyst 3 shows both high activity and selecti-
vity, but little stereoselectivity for the ROP of rac-lactide (Pi, the
probability of forming a new i dyad, = 0.56) at room temperature
and at 0 °C (Pi = 0.62), which we attribute to the remote position
of the stereogenic center.
Figure 5. Structural comparison of the new guanidine catalyst (1) and
TBD.
Catalyst 1 forms a more weakly bound adduct with methanol
than TBD, as apparent in the binding energy (-7.0 kcal/mol (1)
versus -7.8 kcal/mol (TBD)) as well as the N*----H-OCH3
˚
˚
hydrogen bond distance (1.818 A (1) vs 1.792 A (TBD)),
indicating 1 to be a weaker base than TBD (see Figure 4). These
trends correlate with the observed shifts in 1H resonances of
the hydroxyl H for benzyl alcohol upon binding to the guanidine:
Δδ = 2.26 ppm (1) versus ∼7.0 ppm (TBD).
To gain an understanding of the catalytic activity of this new
class of guanidines we compared the structures of 1 and its adduct
with methanol with the corresponding structures for TBD, using
B3LYP/aug-cc-pVTZ//B3LYP/6-31þG*22 density functional cal-
culations23 in the presence of a continuum dielectric ε=2.379
(toluene) using IEF-cPCM24 as implemented in GAMESS-US.25
We attribute the lower basicity of 1 (relative to TBD) to the
observation that the five-membered pyrrolidine ring in 1 is rotated
out-of-plane (dihedral angle 50°(1) vs 10°(TBD), Figure 5) due to
nonbonding interactions between the pyrrolidine ring and the

Note
Macromolecules, Vol. 43, No. 3, 2010 1663
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