Communications
1.77; run 9). The polymerization carried out in dichloro-
methane performed similarly to that done in toluene (run 11
versus 10).
Next, we investigated the effectiveness of these acid/base
pairs for the polymerization of renewable monomers MBL
and MMBL, which are cyclic analogues of MMA. a-Methyl-
ene-g-butyrolactone (MBL), or tulipalin A, which is a natural
substance found in tulips, and the MBL ring is an integral
building block of many natural products.[14] The g-methyl
derivative, MMBL, is readily prepared by a two-step process
from the cellulosic biomass-derived levulinic acid.[15] Despite
being a heterogeneous process (because of the insolubility of
the resulting polymer), the polymerization of MBL by
(tBu)3P/Al(C6F5)3 in dichloromethane achieved greater than
90% polymer yield in 1 hour (run 12). The PMBL produced
has a medium Mn value of 4.48 ꢀ 104 Da and a relatively broad
MWD of 2.18. Likewise, the tBuNHC/Al(C6F5)3 pair is also
quite effective for MBL polymerization, but the MW of the
resulting PMBL is approximately four times higher (run 13
versus 12). Again, the use of Ph3P as the base brought about
the formation of bimodal polymer products (ca. 32% high
MW fraction and ca. 68% low MW fraction; run 14). Thanks
to the good solubility of PMMBL in dichloromethane, the
polymerization of MMBL by these acid/base pairs is homo-
geneous and highly effective. Specifically, the polymerization
by (tBu)3P/Al(C6F5)3 achieved quantitative monomer con-
version in ten minutes, thus giving a high MW polymer,
essentially an atactic polymer (Mn = 1.92 ꢀ 105 Da, MWD =
2.28, mr= 47.0%; run 15). Both NHC bases are highly active
for MMBL polymerization, with a high TOF value of 4.8 ꢀ
104 hÀ1 (runs 16 and 17), but the Mn of 1.39 ꢀ 105 Da of the
PMMBL produced by tBuNHC is about twice that produced by
MesNHC, and the polymer also exhibits a much more narrow
MWD (1.15 for run 16, see Figure 1 for GPC trace, versus
1.42 for run 17). The bimodal behavior of Ph3P is once again
manifested in the MMBL polymerization (ca. 43% high MW
fraction and ca. 57% low MW fraction; run 18, see Figure 1
for GPC trace)
To understand the above-described intriguing polymeri-
zation behavior observed for the base/alane pairs, which is a
function of not only the form of the alane and the structure of
the base, but also of the addition sequence of acid, base, and
monomer, we examined a series of relevant reactions between
the alane (in different forms) and five different bases
employed in this study.
First, (tBu)3P and C7H8·Al(C6F5)3 react rapidly at room
temperature in C7D8 to form at least five different products
according to NMR spectroscopy. One product can be
identified as the anticipated adduct (tBu)3P/Al(C6F5)3 (1),
but only as a minor species in the product mixture. This rapid
consumption (decomposition) of the acid and the base upon
direct mixing as well as the formation of multiple undeter-
mined products (presumably a result of several types of
Figure 1. Gel permeation chromatography (GPC) traces of PMMBL
produced by tBuNHC (left trace, run 16) and by (tBu)3P (right trace,
run 18).
90%), but upon warming many other products began to
emerge. To avoid the complications brought about by toluene
being coordinated to the alane, we also examined the reaction
of the unsolvated Al(C6F5)3 with (tBu)3P in C6D6 at room
temperature, thus revealing the formation of the adduct
(77%), plus two other minor species.[11] This observation
indicates that several products derived from the reaction
using C7H8·Al(C6F5)3 must be related to toluene being
coordinated to the alane. The broad signal appearing at d =
48.7 ppm in the 31P NMR spectrum (and relevant signals in
1
the H NMR spectrum) for the adduct suggests an equilibri-
um established between the free acid/base FLP and the acid/
base adduct.[16] Most significantly, the reaction of (tBu)3P with
MMA·Al(C6F5)3 at room temperature in C6D6 (or CD2Cl2)
cleanly generates zwitterionic phosphonium enolaluminate
(tBu)3PCH2C(Me) = C(OMe)OAl(C6F5)3 (2) as two isomers
(Z/E) in a 7:3 ratio.[11,17] This addition pattern is similar to the
1,4-addition of FLPs to 1,3-dienes[18] and the 1,2-addition of
FLPs to terminal alkynes.[19] Zwitterion 2 can be readily
characterized by NMR spectroscopy (here using the major
isomer as an illustration) for the phosphonium cation
(tBu)3P+[18] [d = 49.0 ppm (s) in the 31P NMR spectrum; d =
1.52 ppm (d, tBu) in the 1H NMR spectrum)], for the
À[20]
enolaluminate anion OAl(C6F5)3
[d = À123.4 (m, 6F, o-
F), À157.8 (t, 3F, p-F), À164.4 (m, 6F, m-F) in the 19F NMR
spectrum], and for the remaining ester enolate moiety
CH2C(Me) = C(OMe)O [d = 3.50 (s, 3H, OMe), 3.19 (d,
2H, PCH2), 1.63 ppm (s, 3H, CMe) in the 1H NMR
=
spectrum], Figure 2. Phosphonium enolaluminate 2 is stable
in solution at room temperature for up to 2 hours. Addition of
an excess of MMA to this solution, with or without an
additional equivalent of Al(C6F5)3, led to rapid polymeri-
zation, therefore confirming zwitterion 2 is the active species
responsible for the rapid polymerization observed in the
above polymerization procedure. Interestingly, although
(tBu)3P reacts with MMA·B(C6F5)3 to form analogous
À
reactions occurring concurrently, such as C H activation,
nucleophilic attack of the aryl ring, etc.) explain why this acid/
base pair, when premixed before addition of MMA, is inactive
for the polymerization (see above). When the reaction was
started at À758C, adduct 1 (a sharp signal at d = 42.6 ppm in
the 31P NMR spectrum) was formed as the major product (>
=
zwitterionic phosphonium enolborate (tBu)3PCH2C(Me) C-
(OMe)OB(C6F5)3 (3),[11] it is inactive towards polymerization
of MMA, with or without an additional equivalent of the
borane. This observation is reminiscent of our previous
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 10158 –10162
Zhang, Yuetao
