Stereospecific polymerization of chiral oxazolidinone-functionalized alkenes

Article abstract of DOI:10.1021/ma101310n

Acryloyl and vinyl monomers functionalized with a chiral oxazolidinone auxiliary have been successfully polymerized in a stereospecific fashion to highly isotactic, optically active polymers, through either the previously established isospecific coordinat

Full text of DOI:10.1021/ma101310n

7504 Macromolecules 2010, 43, 7504–7514
DOI: 10.1021/ma101310n
Stereospecific Polymerization of Chiral Oxazolidinone-Functionalized
Alkenes
Garret M. Miyake, Daniel A. DiRocco, Qin Liu, Kevin M. Oberg, Ercan Bayram,
Richard G. Finke, Tomislav Rovis, and Eugene Y.-X. Chen*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872
Received June 14, 2010; Revised Manuscript Received August 9, 2010
ABSTRACT: Acryloyl and vinyl monomers functionalized with a chiral oxazolidinone auxiliary have
been successfully polymerized in a stereospecific fashion to highly isotactic, optically active polymers,
through either the previously established isospecific coordination polymerization (for acryloyl monomers) or
a novel isospecific cationic polymerization (for vinyl monomers). Specifically, conjugated chiral acryloylox-
azolidinones, N-acryloyl-(R or S)-4-phenyl-2-oxazolidinone [(R or S)-AOZ], are readily polymerized by
chiral ansa-zirconocenium coordination catalysts, (R,R-, S,S-, or R,R/S,S)-[C2H4(η5-Ind)2]Zrþ(THF)[OC-
(OiPr)dCMe2][MeB(C6F5)3]- (1), in an isospecific manner through a catalyst-site-controlled mechanism,
producing the corresponding optically active chiral polymers, (R or S)-PAOZ. Owing to the nature of
stereocontrol dictated by the chiral catalyst site, even the coordination polymerization of the parent AOZ,
without the chiral side group, also affords PAOZ with nearly quantitative isotacticity. A series of experiments
have shown that the chiral polymers (R or S)-PAOZ exhibit no chiral amplifications, despite having
stereoregularly placed stereogenic centers in the main chain, and the optical activity of the polymers arises
solely from their chiral auxiliary, a consequence of adopting a random-coil secondary structure and thus
having a cryptochiral chain. In sharp contrast, the chiral isotactic polymers derived from nonconjugated
chiral vinyl oxazolidinones, N-vinyl-(R)-4-phenyl-2-oxazolidinone [(R)-VOZ] and its p-hexyloxyphenyl
derivative (R)-HVOZ (designed to solve the solubility issue of the resulting polymer), exhibit substantial
chiral amplifications by virtue of adopting a solution-stable, one-handed helical conformation. The synthesis
of such helical vinyl polymers has been accomplished by the development of a novel isospecific cationic poly-
merization using Lewis and Brønsted acids, such as [Ph3C][B(C6F5)4], BF3 Et2O, and [H(Et2O)2][B(C6F5)4],
3
through a chiral auxiliary-controlled mechanism. Noteworthy is the combination of the near-quantitative
isotactic placement of the stereogenic centers of the polymer main chain with the chiral side groups located
near those stereocenters that renders one-handed helicity of (R)-PVOZ and (R)-PHVOZ. Significantly, this
novel cationic polymerization process, operating at ambient temperature, effectively assembles two elements
of polymer local chirality—side-chain chirality and main-chain chirality—into global chirality in the form of
excess one-handed helicity. Furthermore, the resulting chiral helical vinyl polymers exhibit considerably
higher thermal decomposition temperatures and polymer crystallinity, in comparison to the random-coil
chiral acryloyl polymers, having a similarly high degree of main-chain stereoregularity.
Introduction
enriched stereoregular PVP is synthesized, such a vinyl polymer
with configurational main-chain chirality without chiral side
Optically active synthetic polymers are of considerable current
interest.1 Such polymers are not only fundamentally intriguing
(due to their rich and complex architectures derived from
macromolecular chirality that diverges from that of small mole-
cule chirality) but are also technologically important (due to their
unique chiral arrays that give rise to a number of potential, and in
some cases commercially implemented, applications such as
chiral separation).1 We are particularly interested inthe utilization
of chiral N,O-functionalized polar vinyl polymers2 as potential
chiral polymeric ligands/stabilizers for transition metal nanocluster
catalysts3 en route to asymmetric catalysis.4 This interest stems
from the observation that N,O-functionalized polar vinyl poly-
mers, such as poly(vinylpyrrolidone) (PVP), are among the most
common, effective stabilizers for transition-metal nanoclusters5
and the reasoning that chiral polymers have the suitable length
scale and binding rigidity as well as “high chirality”6 that can
match the extended surface of the nanoclusters. However, optically
active PVP is not accessible; even if enantiomerically pure or
groups cannot be optically active since the entire polymer chain
(by the infinite chain model) contains a mirror plane (for isotactic
polymers) or a glide mirror plane and translational mirror planes
perpendicular to the chain axis (for syndiotactic polymers).1j,l On
the other hand, a polymer assuming a one-handed helical con-
formation is inherently chiral.1 Many polymers are known to
form a helical structure in the solid state; however, they typically
adopt optically inactive, on-average random-coil conformations
in solution due to fast solution dynamics of the polymer chain
with low helix-inversion barriers. Our MM2 modeling indicated
that isotactic PVP would adopt a random-coil conformation,
suggesting that enantiomeric chiral PVP with appreciable mole-
cular weight (MW), if synthesized, would be optically inactive. In
short, there exists a need for the synthesis of optically active,
chiral PVP variants.
A solution to this fundamental problem is to install a chiral
auxiliary into conjugated or nonconjugated vinyl monomers
which, upon polymerization, will lead to optically active poly-
mers. If such polymerization proceeds in a stereospecific manner,
*Corresponding author. E-mail: eugene.chen@colostate.edu.
pubs.acs.org/Macromolecules
Published on Web 09/07/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 18, 2010 7505
Chart 1. Structures of Chiral 2-Oxazolidinone-Funcationzalized Conjugated Acrylamide and Nonconjugated Vinyl Monomers Employed in This Study
Chart 2. Primary Structure of (R)-PVOZ (A) as Well as
MM2-Calculated 41 Helical Structure of [(R)-VOZ]30, Shown from
Top (B), Side (C), and Ribbon (D) Viewsa
has been achieved without LA additives, producing isotactic
polymers with a m/r dyad ratio reaching 92:8.12 Interestingly,
the nonconjugated, unsubstituted VOZ undergoes rapid decom-
position (devinylation) to 2-oxazolidione and acetaldehyde in
acidic aqueous solution with pH< 4.0 so that the radical
polymerization in the presence of poly(methacrylic acid) was
successful only at pH g 4.0.13 This monomer, upon free-radical
polymerization by AIBN, was reported to form water-soluble
polymers with MW ranging from 450 to 100000 (by microiso-
piestic measurements)14 or a water-insoluble polymer which
decomposes at ∼300 °C without melting. An attempt to poly-
merize this monomer by a Ziegler-Natta coordination catalyst
(TiCl3/HONH3þCl-) led to no polymer formation, but instead
the acid-catalyzed devinylation product.15 Acid-catalyzed devi-
nylation of N-vinyl heterocyclic monomers has also been noted
elsewhere.16 VOZ monomers having alkyl or phenyl substituents
at 5-positions can be polymerized by AIBN in dioxane or in
bulk.17 The polymers derived from radical polymerization of
VOZ monomers are essentially atactic.18
As can be seen from the above overview, AOZ and VOZ
monomers have previously been successfully polymerized only by
radical polymerization methods, while the polymerization of (4R
or 4S)-VOZ monomers of our current interest has not been
reported. Furthermore, chiral auxiliary-controlled radical poly-
merization has led to formation of an isotactic copolymer of (4S)-
AOZ with isobutylene, but the isotactic homopolymer of (4Ror 4S)-
AOZ of interest herein was previously unknown. Lastly, no
stereoregular polymers derived from VOZ monomers have been
reported.
We hypothesized that the synthetic challenges identified above
could be met by employing isospecific, enantiomeric (R,R or S,S)-
ansa-metallocenium coordination catalysts that were recently
developed for the asymmetric coordination polymerization of
functionalized vinyl monomers such as prochiral acrylmides
leading to optically active, helical vinyl polymers.19 Our reasoning
is threefold: First, we have already shown that catalyst 1 poly-
merizes prochiral conjugated acrylamides with bulky substitu-
ents, such as N,N-diarylacrylamides, to highly isotactic, chiral
polymers adopting a solution-stable, one-handed-helical confor-
mation, where the handedness of the helix is dictated by the
chirality of the catalyst (Chart 3).19 In contrast, if the monomer is
not sufficiently sterically bulky [i.e., methyl methacrylate (MMA)],
the resulting polymer does not form a solution-stable helix, but
insteada random coilconformation. Such low-MW enantiomeric
oligomers can exhibit some optical activity arising from the
absence of mirror planes due to nonequivalent chain-end groups,
but as the MW increases, the effects of the chain-end groups on
the chiroptical properties of the polymers diminish so that the
optical activity decreases to null as the polymer becomes
cryptochiral.20,21 Second, we reasoned that the zirconocene ester
enolate cation 1 can serve as both initiator (the enolate ligand as
nucleophile) and LA catalyst as chelator for the two carbonyls in
the AOZ monomer,22 thus rendering both high activity and high
stereochemical control in AOZ polymerization (vide supra).
Studies in the polymerization of chiral AOZ monomers by the
enantiomeric catalysts 1 will determine if they can produce
isolable, right- and left-handed, solution-stable helical poly-
mers bearing the same chiral auxiliary, trapped in a kinetically
a Carbon, nitrogen, and oxygen are shown in gray, blue, and red,
respectively; hydrogen atoms are omitted for clarity.
then the resulting stereoregular polymers could attain addition-
ally helical chirality due to control of the polymer secondary
structure. Pino and Lorenzi first demonstrated that isotactic vinyl
polymers bearing chiral side groups, such as poly(S)-3-methyl-
1-pentene, can exist in solution with excess one-handed helicity and
that the optical rotation of such polymers increased with increas-
ing isotacticity.7 With this concept and the goal of this work in
mind, three reasons led us to enantiomeric (R or S)-4-phenyl-
2-oxazolidinone-functionalized conjugated and nonconjugated vinyl
monomers N-acryloyl-(R or S)-4-phenyl-2-oxazolidinone [(R or
S)-AOZ] and N-vinyl-(R)-4-phenyl-2-oxazolidinone [(R)-VOZ]
(Chart 1). First, the chiral oxazolidinone group has been used as a
chiral auxiliary in organic synthesis for over 30 years.8 Second,
the resulting N,O-functionalized chiral polymers, (R or S)-PAOZ
and (R)-PVOZ, structurally resemble that of PVP, in addition to
being optically active. Third, introduction of the phenyl group at
the 4-position of the oxazolidinone ring could sterically induce a
solution-stable helical conformation of the isotactic polymer,
thereby effectively assembling two elements of local chirality—
side-chain chirality (stereocenters at 4-positions of the side chain)
and main-chain chirality (stereocenters generated at 2-vinyl carbon
positions during stereoselective polymerization)—into global
chirality (formation of excess one-handed helicity). Indeed,
MM2 modeling of the isotactic [(R)-VOZ]30 predicts a chiral 41
helical structure (Chart 2).
Free-radical polymerization has been previously employed to
polymerize conjugated vinyl monomers bearing chiral auxiliary
groups stereoselectively.9 However, due to unfavorable dipole
interactions between the oxazolidinone and acryloyl carbonyls,
conjugated AOZ with a chiral auxiliary at the 4-position favors a
rotamer which shields the auxiliary away from the reactive center
(cf. left rotamer of (R)-AOZ, Chart 1), providing little stereo-
chemical control in additions to the CdC bond.10 Using a
suitable Lewis acid (LA), such as Sc(OTf)3, should lock the
auxiliary in the preferred conformation for control of stereo-
chemistry (cf. right rotamer of (R)-AOZ, Chart 1) through
bidentate chelation of the LA to both carbonyls of the monomer;
however, such complexation renders the radical and monomer
too electron-deficient to react efficiently for homopolymeriza-
tions. Nevertheless, (4S)-AOZ can be radically copolymerized
with electron-rich isobutylene in the presence of a LA, yielding
isotactic alternating copolymer with a m/r dyad ratio of >95:5.10
In the case of chiral oxazolidine acrylamides, stereocontrolled
(through chiral auxiliary control11) free-radical polymerization

7506 Macromolecules, Vol. 43, No. 18, 2010
Miyake et al.
Chart 3. Asymmetric Polymerization by Enantiomeric Catalysts 1 for the Synthesis of Right- and Left-Handed Helical
Poly[N-phenyl-N-(4-tolyl)acrylamide)s and Their Block Copolymers with MMAa
a Shown on the bottom are the CD spectra of homopolymers and block copolymers by (S,S)-1 (red), rac-1 (green), and (R,R)-1 (blue).19,20
stable state—a case of catalyst site control. Comparative studies
using the racemic catalyst and other catalysts with different
stereochemical control will also reveal whether the chiral auxiliary
will dictate the handedness of the helix, either through an initial
formation of a preferred single-handed helix or through a thermo-
dynamic mutarotation to the preferred helical conformation—
the case of chiral auxiliary control. Third, we reasoned that
being a class of electron-rich monomers, chiral VOZ could be
cationically polymerized by metallocenium or other related cations
and isospecificity rendered by the chiral auxiliary, if devinylation
is overcome by appropriate strategies such as spontaneous
polymer precipitation. As indicated in Chart 2, highly isotactic
(R)-PVOZ will most likely adopt a solution-stable helical struc-
ture, therebyaccomplishing our goal of synthesizing those needed
optically active polymers. Herein we report our findings in testing
each of these three hypotheses.
Acetaldehyde diethyl acetal, aniline, nBuLi (1.6 M in hexanes),
butylated hydroxytoluene (BHT-H, 2,6-di-tert-butyl-4-methyl-
phenol), camphorsulfonic acid, 1,2-dibromobenzene, 1,2-dibro-
moethane, diisopropylamine, indene, lithium dimethylamide,
(2S,4S)-pentanediol (99% ee, [R]20 þ39.8, c = 10, CHCl3),
D
(2R,4R)-pentanediol (97% ee, [R]21 -40.4, c = 10, CHCl3),
D
sodium azide, tetrachlorozirconium, 1,1,3,3-tetramethylguanidine,
triethylamine, and triflic acid as well as (CF3SO2)2O, PhBCl2,
MeMgI (3.0 M in diethyl ether), BF3 Et2O, iBu3Al (neat), AIBN,
3
and CF3COOH were purchased from Aldrich. MMAO (2.2 wt %
Al in heptane) was purchased from Akzo Nobel. Acryloyl chloride
and 2,6-dimethylpyridine were purchased from Alfa Aesar. Tri-
methylaluminum (neat) was purchased from Strem Chemical Co.,
whereas (S)- and (R)-4-phenyl-2-oxazolidinone, isopropyl isobu-
tyrate, and N-methylaniline were purchased from TCI America.
Indene, 1,2-dibromoethane, N,N-dimethylaniline, and acryloyl
chloride were degassed using three freeze-pump-thaw cycles.
Diisopropylamine, triethylamine, (CF3SO2)2O, and PhBCl2
were vacuum-distilled. 2,6-Dimethylpyridine, isopropyl isobu-
tyrate, and aniline were degassed and dried over CaH2 over-
night, followed by vacuum distillation. BHT-H was recrystal-
lized from hexanes prior to use. 1,4-Dioxane (Fisher Scientific)
was degassed, dried over sodium/potassium alloy, and vacuum-
distilled. All other commercial reagents were used as received.
Borate salts [Ph3C][B(C6F5)4] and [HN(Me2)Ph][B(C6F5)4] as
well as borane B(C6F5)3 were obtained as a research gift from
Boulder Scientific Co.; the borane was further purified by
Experimental Section
Materials, Reagents, and Methods. All syntheses and manip-
ulations of air- and moisture-sensitive materials were carried out
in flame-dried Schlenk-type glassware on a dual-manifold
Schlenk line, a high-vacuum line, or in an argon or nitrogen-
filled glovebox. HPLC-grade, nonstabilized organic solvents
were sparged extensively with nitrogen during filling of the
solvent reservoir and then dried by passage through activated
alumina (for THF, Et2O, and CH2Cl2) followed by passage
through Q-5-supported copper catalyst (for toluene and
hexanes) stainless steel columns. HPLC-grade DMF was de-
gassed and dried over CaH2 overnight, followed by vacuum
transfer. Toluene-d8 and benzene-d6 were degassed, dried over
sodium/potassium alloy, and filtered before use, whereas
CDCl3, CD2Cl2, and DMSO-d6 were degassed and dried over
recrystallization from hexanes at -35 °C. The (C6F5)3B THF
3
adduct was prepared by addition of THF to a toluene solution of
the borane followed by removal of the volatiles and drying in
vacuo. Literature procedures were employed for the preparation
of the following compounds and metallocene complexes: (S)-
AOZ,23 (R)-AOZ,23 AOZ,24 (R)-VOZ,25 [H(Et2O)2]þ[B(C6F5)4]-,26
LiOC(OiPr)dCMe2,27 (EBI)H2 [EBI = C2H4(η5-Ind)2],28 rac-
(EBI)Zr(NMe2)2,29 rac-(EBI)ZrMe2,29 rac-(EBI)ZrMe(OTf),30
rac-(EBI)ZrMe[OC(OiPr)dCMe2],30 rac-(EBI)Zrþ(THF)[OC-
(OiPr)dCMe2][MeB(C6F5)3]- (rac-1),30 (S,S)-(EBI)ZrCl2,31 (R,R)-
(EBI)ZrCl2,31 (S,S)-(EBI)ZrMe[OC(OiPr)dCMe2],19,20 (R,R)-
(EBI)ZrMe[OC(OiPr)dCMe2],19,20 (S,S)-(1),19,20 and (R,R)-(1).19,20
˚
activated Davison 4 A molecular sieves. NMR spectra were
recorded on a Varian Inova 300, 400, or 500 MHz (for polymer
tacticity analysis) spectrometer. Chemical shifts for 1H and 13
C
spectra were referenced to internal solvent resonances and are
reported as parts per million relative to tetramethylsilane,
whereas 19F NMR spectra were referenced to external CFCl3.

Article
(R)-Methyl 2-((tert-Butoxycarbonyl)amino)-2-(4-(hexyloxy)-
Macromolecules, Vol. 43, No. 18, 2010 7507
interfaced to a dual-manifold Schlenk line with an external
temperature bath for runs at other temperatures. In a typical
procedure for polymerizations of conjugated acryloyl oxazoli-
phenyl)acetate. To a flame-dried flask with a magnetic stir bar
was added (R)-methyl 2-((tert-butoxycarbonyl)amino)-2-(4-hy-
droxyphenyl)acetate (19.4 g, 68.8 mmol, 1.0 equiv), anhydrous
K2CO3 (23.8 g, 172 mmol, 2.5 equiv), and anhydrous DMF
(250 mL). The mixture solution was cooled to 0 °C, after which
1-iodohexane (25.4 mL, 172 mmol, 2.5 equiv) was added and the
reaction was allowed to stir overnight at room temperature.
Diethyl ether (1000 mL) was added, and the mixture washed
with water (2 ꢀ 500 mL), saturated KHSO4 (500 mL), and brine
(500 mL). The solution was dried with MgSO4 and concentrated
to give a yellow oil which was purified by silica gel chromato-
dinones (AOZ), predetermined amounts of B(C6F5)3 THF and
3
the appropriate metallocene ester enolate precatalyst in a 1:1
molar ratio were premixed in 5 mL of CH2Cl2 and stirred for 10
min to cleanly generate the corresponding cationic ester enolate
catalyst.19,20,32 The amount of catalyst employed was determined
by the [monomer] to [catalyst] ratio specified in the polymeri-
zation tables. Monomer (0.737 mmol) was quickly added as a
solid to the vigorously stirring solution, and the polymerization
was allowed to proceed for 3 h with continuous stirring. For poly-
merization of vinyloxazolidines (VOZ), monomer (1.06 mmol)
was dissolved in the solvent described in the polymerization
tables, before addition of initiator as a solid or solution via
syringe, and the polymerization was allowed to stir for the time
specified in the polymerization tables. After the measured time
interval, a 0.2 mL aliquot was taken from the reaction mixture
via syringe and quickly quenched into a 4 mL vial containing
0.6 mL of undried “wet”CDCl3 stabilizedby250ppm of BHT-H;
graphy (9:1 hexanes:EtOAc) to yield the desired product as a
21
clear oil (14.1 g, 56%). Rf = 0.18 (9:1 hexanes:EtOAc). [R]D
=
-45.5 (c = 0.013 g/mL, MeOH). 1H NMR (400 MHz, CDCl3):
δ 7.22 (d, J = 8.5 Hz, 2H), 6.82 (m, 2H), 5.44 (bd, J = 6.4 Hz,
1H), 5.21 (bd, J = 7.2 Hz, 2H), 3.90 (t, J = 6.6 Hz, 2H), 3.67
(s, 3H), 1.73 (m, 2H), 1.39 (s, 9H), 1.29 (m, 5H) 0.87 (m, 3H). 13C
NMR (100 MHz, CDCl3): δ 172.1, 159.4, 155.0, 128.9, 128.5,
115.0, 80.2, 68.2, 57.2, 52.8, 31.7, 29.4, 28.5, 25.9, 22.8, 14.2. IR
(NaCl, neat): 3440, 3380, 2955, 2933, 2872, 1747, 1717, 1612,
1511, 1247, 1169 cm-1. HRMS (ESIþ) calcd for C20H31NNaO5:
365.2202; found: 365.2217.
1
the quenched aliquots were analyzed by H NMR to obtain
monomer conversion data. The polymerization was immedi-
ately quenched after the removal of the aliquot by the addition
of 5 mL 5% HCl-acidified methanol. The quenched mixture was
precipitated into 50 mL of methanol, stirred for 1 h, filtered or
centrifuged, washed with methanol, and dried in a vacuum oven
at 50 °C overnight to a constant weight.
For polymerizations carried out at other temperatures, the
catalyst (or monomer solution) was loaded in a 25 mL Schlenk
flask equipped with a stir bar and a septum cap inside the
glovebox. The charged Schlenk flask was taken out of the
glovebox, interfaced to a dual-manifold Schlenk line, and immersed
in a pre-equilibrated bath at desired temperature. The poly-
merization was started by adding rapidly the monomer (or
catalyst solution) via gastight syringe under positive N2 pressure.
The remaining procedures were the same as those ambient-
temperature polymerization runs. Polymerizations using Cs-
ligated metallocenium catalysts for the synthesis of syndiotactic
polymers followed the literature procedure.33
Polymer Characterizations. Gel permeation chromatography
(GPC) analyses of the polymers were carried out at 40 °C and a
flow rate of 1.0 mL/min, with DMF as the eluent, on a Waters
University 1500 GPC instrument equipped with four 5 μm PL
gel columns (Polymer Laboratories) and calibrated using 10
PMMA standards. Chromatograms were processed with Waters
Empower software (version 2002); number-average molecular
weight (Mn) and molecular weight distribution (MWD = Mw/Mn)
of polymers were given relative to PMMA standards. Glass
transition temperatures (Tg) of the polymers were measured by
differential scanning calorimetry (DSC) on a DSC 2920 (TA
Instruments). Polymer samples were first heated to 150 °C at
20 °C/min, equilibrated at this temperature for 4 min, then
cooled to 30 °C at 20 °C/min, held at this temperature for 4 min,
and reheated to 230 °C (for AOZ polymers) or 390 °C (for VOZ
polymers) at 10 °C/min. All Tg values were obtained from the
second scan, after removing the thermal history from the first
heating cycle. Maximum rate decomposition temperatures
(Tmax) and decomposition onset temperatures (Tonset) of the
polymers were measured by thermal gravimetric analysis (TGA)
on a TGA 2950 thermogravimetric analyzer (TA Instruments).
Polymer samples were heated from ambient temperatures to
600 °C at a rate of 20 °C/min. Values for Tmax were obtained
from derivative (wt %/°C) vs temperature (°C), while Tonset
values (initial and end temperatures) were obtained from wt %
vs temperature (°C) plots.
(R)-4-(4-(Hexyloxy)phenyl)oxazolidin-2-one. To a solution of
LiAlH4 (1.61 g, 42.5 mmol, 1.1 equiv) in THF (200 mL) was added
dropwise a solution of (R)-methyl 2-((tert-butoxycarbonyl)-
amino)-2-(4-(hexyloxy)phenyl)acetate (14.1 g, 38.7 mmol, 1.0 equiv)
in THF (150 mL). The reaction was stirred at room temperature
until the starting material was consumed by TLC analysis, after
which 10% KOH was added and the reaction mixture filtered
and concentrated to yield an off-white solid. The solid was
dissolved in THF (400 mL) and cooled to 0 °C, after which
thionyl chloride (22.4 mL, 309 mmol, 8.0 equiv) was added
dropwise, and the solution was stirred for an additional 3 h at
0 °C, then warmed to room temperature, and stirred overnight.
The reaction was concentrated to give a viscous oil that was crys-
tallized with hexanes and filtered, yielding the desired product as
a white amorphous solid (7.01 g, 68%). Rf = 0.22 (1:1 hexanes:
EtOAc). [R]D21 = -12.5 (c = 0.8 g/dL, MeOH). 1H NMR (400
MHz, CDCl3): δ 7.20 (m, 2H), 6.85 (m, 2H), 5.87 (bs, 1H), 4.86
(m, 1H), 4.64 (m, 1H), 4.12 (m, 1H), 3.91 (m, 2H), 1.74 (m, 2H),
1.42 (m, 2H), 1.29 (m, 4 H), 0.87 (m, 3H). 13C NMR (100 MHz,
CDCl3): δ 159.8, 159.7, 131.3, 127.5, 115.3, 72.9, 68.3, 56.2, 31.7,
29.3, 25.9, 22.8, 14.2. IR (NaCl, neat): 3284, 2932, 2860, 1756,
1613, 1514, 1246, 1032 cm-1. HRMS(ESIþ) calcdforC15H22NO3:
263.1521; found: 263.1526.
(R)-4-(4-(Hexyloxy)phenyl)-3-vinyloxazolidin-2-one(HVOZ).
To a flame-dried flask was added palladium(II) trifluoroacetate
(63 mg, 0.19 mmol, 0.05 equiv), 1,10-phenanthroline (34 mg,
0.19 mmol, 0.05 equiv), and n-butyl vinyl ether (4.9 mL, 37.9
mmol, 10.0 equiv). This mixture was stirred for 5 min, followed
by the addition of (R)-4-(4-(hexyloxy)phenyl)oxazolidin-2-one
(1.0 g, 3.79 mmol, 1.0 equiv). The reaction was heated to 75 °C
for 12 h, filtered through Celite, and concentrated. Purification
of the crude product by silica gel chromatography gave a viscous
oil which was crystallized with pentanes and filtered to yield the
desired product as a white solid (1.07 g, 98%). Rf = 0.15 (9:1
hexanes:EtOAc). [R]D21 = -44.8° (c = 1.70 g/dL, CH2Cl2); mp
(°C): 52-53. 1H NMR (400 MHz, CDCl3): δ 7.14 (d, J = 8.5 Hz,
2H), 6.87 (d, J = 8.4 Hz, 2H), 6.77 (dd, J = 16.0, 9.3 Hz, 1H),
4.95 (dd, J = 9.0, 5.4 Hz, 1H), 4.66 (m, 1H), 4.28 (d, J = 9.3 Hz,
1H), 4.08 (m, 2H), 3.91 (t, J = 6.5 Hz, 2H), 1.74 (m, 2H), 1.42
(m, 2H), 1.30 (m, 4H), 0.87 (m, 3H). 13C NMR (100 MHz,
CDCl3): δ 159.7, 155.9, 129.8, 129.0, 127.3, 115.4, 96.0, 71.0,
68.3, 58.0, 31.7, 29.4, 25.9, 22.8, 14.2. IR (NaCl, neat): 2932,
2871, 1765, 1639, 1613, 1514, 1394, 1246 cm-1. HRMS (ESIþ)
calcd for C17H24NO3: 289.1678; found 289.1679.
Optical rotations were measured on an Autopol III Auto-
matic Polarimeter at 23 °C. The measurements were conducted
on 0.2 g/dL polymer solutions in CHCl3. Circular dichroism
(CD) spectra were obtained from an Aviv model 202 CD
spectrometer. CD analysis was conducted on polymer solutions
General Polymerization Procedures. Polymerizations were
performed in 30 mL glass reactors inside the glovebox for
ambient temperature (∼25 °C) runs or in 25 mL Schlenk flasks

7508 Macromolecules, Vol. 43, No. 18, 2010
Miyake et al.
with concentrations of 0.2 g/dL in CHCl3. Powder X-ray
diffraction (XRD) analyses were performed on powder samples
with a Scintag X2 Advanced Diffraction System using Cu KR
Table 1. Selected Results of Polymerization of (R and S)-AOZ by
Chiral Catalysts 1a
run
no.
monomer
form
catalyst
form
convb 103Mn MWDc [R]23
(%) (g/mol) (Mw/Mn) (deg)
c
d
D
˚
(λ = 1.540 562 A) radiation and a Peltier detector on the diffracted-
beam side. In all cases measurements were performed with a step
size of 0.02° with 1.2 s per step. The tacticity of (R)-PAOZ and
(R)-PHVOZ was analyzed by 13C NMR according to the
procedures established for polyacrylamides34 and for the parent
PVOZ,18 respectively. NMR data of the polymers representing
each of three classes of the polymers described in this study are
listed below.
1
2
3
4
5
6
(R)-AOZ
(R)-AOZ
(R)-AOZ
(S)-AOZ
(S)-AOZ
(S)-AOZ
rac-1
100
100
100
100
100
100
11.1
6.77
7.88
9.98
7.23
8.27
2.41
1.77
1.42
2.07
1.77
1.74
-158
(R,R)-1
(S,S)-1
rac-1
(R,R)-1
(S,S)-1
-160
-154
þ182
þ170
þ185
a Carried out in 5 mL of CH2Cl2 at ambient temperature (∼25 °C);
2 mol % catalyst. b Conversion measured by 1H NMR. c Determined by
GPC relative to PMMA standards. d 0.2 g/dL, CHCl3.
Poly(N-acryloyl-2-oxazolidinone) (PAOZ). 1H NMR (DMSO-
d6, 500 MHz, 100 °C) for PAOZ: δ 4.37 (m, CH2O, 2H), 3.87 (m,
CH2N, 2H), 3.73, 3.68, 3.60 (m, CH, unresolved triads, 1H),
1.86, 1.71, 1.67, 1.55, 1.48 (m, CH2, unresolved diads and tetrads,
2H). 13C NMR (DMSO-d6, 125 MHz, 100 °C): δ 174.4 (CdO,
rr þ mr), 174.3 (CdO, mm), 152.7 (CdO, ring), 61.50 (CH2O),
42.09 (CH2N), 37.24 (CH), 34.75 (CH2).
current system, it could also be attributable to the possibility
that the enantiomeric monomer was preferentially polymer-
ized by one enantiomer of the racemic catalyst. To test this
hypothesis, we investigated the ability of the (R,R)-1 enan-
tiomer to effect the kinetic resolution polymerization of
rac-(R/S)-AOZ, ideally polymerizing one enantiomer pre-
ferentially via a large stereoselectivity factor while resolving
the other enantiomerically pure. The actual experiment
showed that (R,R)-1 was unable to kinetically resolve (R/S)-
AOZ, as several aliquots taken during the course of polym-
erization, when analyzed by chiral HPLC, revealed no
enantiomeric excess of the unreacted monomer, demonstrat-
ing that each enantiomer of the catalyst polymerizes the
enantiomeric monomers with equal efficacy. Furthermore,
the specific rotations of the isolated polymers are strikingly
similar to their respective monomers. More specifically,
[R]23D of (R)-AOZ is -159°, while the isolated polymer has
the same value (within experimental error) of [R]23D = -158°.
Likewise, the specific rotation of (S)-AOZ is þ159°, and it
is þ182° for its derived polymer. Significantly, the minimal
to no change in both magnitude and sign of specific
rotations of these polymers, in comparison to their respec-
tive monomers, is drastically different than the chiral helical
polymer examples discussed above, where helix formation
resulted in polymers with largely different optical rotations
(and sometime in signs as well) than their corresponding
monomers.
Poly[N-acryloyl-(R)-4-phenyl-2-oxazolidinone] [(R)-PAOZ].
1H NMR (DMSO-d6, 500 MHz, 100 °C): δ 7.28 (m, Ar, 5H),
5.38 (m, CH, 1H), 4.64-4.14 (m, CH2O, 2H), 3.84, 3.76, 3.67 (m,
CH, unresolved triads, 1H), 1.88, 1.74, 1.46 (m, CH2, 2H). 13
C
NMR (DMSO-d6, 125 MHz, 100 °C): δ 174.3 (CdO, rr þ mr),
173.9 (CdO, mm), 153.0, (CdO, ring), 139.7, 129.1, 128.3, 125.9
(Ar), 70.18 (CH2O), 57.98 (CHN), 39.04 (CH), 36.01 (CH2).
Poly[N-vinyl-(R)-4-(4-(hexyloxy)phenyl)-2-oxazolidinone]
[(R)-PHVOZ]. 1H NMR (500 MHz, CDCl3, 50 °C): δ 7.62 (bs,
Ar, 2H), 6.99 (bs, Ar, 2H), 4.53 (m, 1H), 4.25 (m, 1H), 4.03 (m,
2H), 3.77 (m, 1H), 1.82 (bs, 2H), 1.49 (bs, 2H), 1.35 (bs, 5H), 0.89
(bs, 4H), 0.77 (bs, 1 H). 13C NMR (CDCl3, 125 MHz, 50 °C): δ
159.6 (C-O), 158.0 (CdO, mmmm), 132.4, 129.6, 115.2 (Ar),
70.26 (CH2O), 68.24 (CH2O), 56.23 (CHN), 48.54 (CH), 35.64
(CH2), 31.60 (CH2), 29.36 (CH2), 25.84 (CH2), 22.56 (CH2),
13.90 (CH3).
Results and Discussion
Stereospecific Coordination Polymerization of Chiral Acry-
loyl Oxazolidinones. As a control and test to examine the
compatibility of the oxazolidinone functionality attached to
the acryloyl monomer with the cationic metallocenium co-
ordination catalyst, we first polymerized the parent unsub-
stituted, prochiral acryloyl-2-oxazolidinone (AOZ) with rac-1
at ambient temperature. In accordance with the stereospecific
coordination polymerization of N,N-diarylacrylamides19,20
and N,N-dialkylacrylamides34a-c by such metallocene cata-
lysts, the polymerization of AOZ by rac-1 is rapid and
produces highly stereoregular, but optically inactive, PAOZ
with a near-quantitative isotacticity (mm%) of ∼99% as
determined by 13C NMR (see Experimental Section). Since
AOZ contains no chiral auxiliary, the observed stereochem-
istry must be attributed to the catalyst-site-controlled poly-
merization rendered by the C2-ligated chiral catalyst.2 Next,
we polymerized both (R)- and (S)-AOZ monomers using rac-
1 (2 mol %) for 3 h at ambient temperature, achieving
quantitative monomer conversions and affording the corre-
sponding isotactic, optically active polymers, (R)-PAOZ and
(S)-PAOZ, with MW’s = 11.1 and 9.98 kg/mol, respectively
(runs 1 and 4, Table 1). The observed MW’s are close to the
calculated MW of 10.9 kg/mol, and therefore the polymer-
ization shows its control over the resulting polymer MW.
The polymers exhibit unimodal MWD’s, but they are rela-
tively broad (>2.0), as compared to the typically narrow
MWD’s (<1.2) observed for poly(alkyl methacrylate)s and
poly(alkyl acrylamide)s produced by rac-1.30,32,34b The for-
mation of polymers with relatively broad, unimodal MWD’s
are normally attributed to the slower rate of chain initiation
than the rate of chain propagation for polymerization sys-
tems with single-site catalysts.2 However, in the case of the
Efforts in explaining the above results obtained in the
polymerization of (R and S)-AOZ monomers have led to the
formulation of the following three hypotheses (possible
scenarios): (1) the polymers produced by rac-1 form an equal
mixture of right- and left-handed helical structures, where
the helicity is determined by the chirality of the catalyst, and
therefore the optical activity arising from the secondary
structures cancel each other out; (2) one-handed chiral helical
polymers are produced, but the helicity is dictated by the
chiral side groups of the isotactic polymers; or (3) the poly-
mers produced adopt random-coil conformations, where the
stereoregular main chain becomes cryptochiral, and therefore
the optical activity is controlled by the chiral auxiliary. Several
lines of key evidence detailed below unequivocally disprove
hypotheses 1 and 2 and thus show that the third scenario is
strongly suggested for the present chiral AOZ polymers.
First, we employed enantiomeric catalysts 1 for the poly-
merization of (R and S)-AOZ since the results will reveal if
excess one-handed helicity could be formed or not. (R)-AOZ
was polymerized efficiently by (R,R)- and (S,S)-1, affording
the corresponding polymers with MW’s = 6.77 and 7.88 kg/mol
and MWD’s = 1.77 and 1.42, respectively (runs 2 and 3, Table 1).
Intriguingly, the specific rotations of all the polymers derived
from (R)-AOZ are rather similar (i.e., [R]23 varied from a
narrow range from -154° to -160°), regardless of the form of
the catalyst utilized (runs 1-3). Furthermore, all the polymers
D

Article
Macromolecules, Vol. 43, No. 18, 2010 7509
Figure 1. CD spectra of (R)-AOZ polymers produced by catalysts (S,S)-
1 (blue), (R,R)-1 (red), and rac-1 (green).
Figure 2. CD spectra of (S)-AOZ polymers produced by catalysts
(S,S)-1 (blue), (R,R)-1 (red), and rac-1 (green).
show nearly identical Cotton effects, as revealed by their CD
spectra (Figure 1), which is in sharp contrast to the chiral
helical polymers with one-handed helicity being dictated
by the chirality of the catalyst (vide supra). Since the poly-
mers derived from (R)-AOZ are optically indistinguishable,
these results clearly ruled out hypothesis 1, which assumes
that each enantiomeric catalyst produces AOZ polymer
kinetically trapped in the right- or left-handed, solution-
stable helix.
Second, the study of the chiroptical properties of the
polymers derived from (S)-AOZ using three different
forms of catalyst 1 provides additional evidence disproving
hypothesis 1. As in the case of the (R)-AOZ monomer, (S)-
AOZ was quantitatively polymerized by the enanantiomeric
catalysts to isotactic, optically active polymers with MW’s =
7.23 and 8.27 kg/mol and MWD’s = 1.77 and 1.74 by (R,R)-
and (S,S)-1, respectively (runs 5 and 6, Table 1). Identical to
the observations with the polymers composed of (R)-AOZ,
the (S)-AOZ-based polymers exhibited rather similar opti-
cal rotations, regardless of catalyst form employed (i.e.,
[R]23D = þ182°, þ170°, and þ185° for polymers produced by
rac-, (R,R)-, and (S,S)-1, respectively) and again nearly
identical CD spectra (Figure 2). Hence, these results strongly
back the above conclusion (point 1), based on the findings
in the polymerization of (R)-AOZ, that the enantiomeric
catalysts do not convert the enantiomeric AOZ to a kineti-
cally trapped right- or left-handed solution-stable helical
polymer.
Third, we ascertained the possibility of the chiral side
groups of the polymer dictating helicity (hypothesis 2) due
to the observed large change in CD spectra for the isolated
polymers, as compared to their respective monomers (Figure 3).
Again, this possibility is inconsistent with the observation
that there was no chiral amplification and exhibited only
minimal differences in specific rotations of the polymers than
their respective monomers, as shown by the examples de-
scribed above. A hypothetical helix-helix stereomutation
was also not observed for these polymers, as the specific
rotation of the solution did not change immediately after
dissolution or after 4 days.
Figure 3. CD spectra of chiral AOZ monomers: (S)-AOZ (blue) and
(R)-AOZ (red).
dictates the formation of a single-handed helical polymer
during the course of polymerization, then we can reveal
this phenomenon through copolymerizations, specifically,
through investigating chiral amplifications manifestable by
the majority rules35 or “sergeants and soldiers” effects.36 In
both effects, a small chiral bias results in a chiral amplifica-
tion in the entire polymer chain, leading to highly optically
active polymers. To this end, we copolymerized nonequivalent
amounts of (R)- and (S)-AOZ by rac-1 (since no kinetic
resolution polymerization proceeded, we can employ the
racemic catalyst directly) (runs 7-9, Table 2). The copolymers
with 10, 20, and 40% ee of (R)-AOZ exhibited very similar
MW and MWD, but no chiral amplification was observed
in their CD spectra. Furthermore, in examining specific rota-
tions of these polymers, there was just an additive effect in
optical rotation, not a chiral amplification. Specifically, the poly-
Fourth, the results of our copolymerization studies pro-
vide additional evidence for disproof of hypothesis 2. We
reasoned that if the chiral auxiliary of the AOZ monomers
mers with 10, 20, and 40% ee of (R)-AOZ had [R]23
=
D
-15.9°, -33.8°, and -70.7° (from run 7 to 9), increasingly

7510 Macromolecules, Vol. 43, No. 18, 2010
Miyake et al.
Table 2. Selected Results of Copolymerization of (R)- with (S)-AOZ
by rac-1a
run (R)-AOZ (S)-AOZ
(mol %)
conv
(%)
103Mn
(g/mol) (Mw/Mn)
MWD
[R]23
(deg)
D
no.
(mol %)
7
8
9
55
60
70
45
40
30
100
100
100
8.34
8.79
8.89
1.37
1.39
1.42
-15.9
-33.8
-70.7
a See footnotes in Table 1 for explanations.
Table 3. Selected Results of Copolymerization of Achiral AOZ with
Chiral (S)-AOZ by rac-1a
run
no.
(S)-AOZ
(%)
AOZ
(%)
conv
(%)
103Mn
(g/mol)
MWD
(Mw/Mn)
[R]23
(deg)
D
10
11
12
13
0
4
10
20
100
96
90
80
100
100
100
100
6.03
7.40
7.87
8.67
1.69
1.41
1.43
1.43
0.0
þ18.3
þ28.1
þ35.8
a See footnotes in Table 1 for explanations.
linearly with % ee, as was observed in vinyl polymers not
forming excess one-handed helicity.37
To examine the ability of these chiral monomers to
influence helicity through the “sergeants and soldiers” effect,
we copolymerized (S)-AOZ with the structurally similar,
prochiral acryloyl-2-oxazolidinone (AOZ). As a control,
we polymerized AOZ alone with rac-1. The optically inactive
polymer produced by rac-1 exhibited a MW of 6.03 kg/mol
and a MWD of 1.69 (run 10, Table 3). Next, we incorporated
4, 10, and 20 mol % of chiral (S)-AOZ into the monomer
feed, quantitatively producing the corresponding optically
active copolymers (runs 11-13, Table 3). However, the
specific rotation increased only linearly with incorporation
of the chiral monomer, and also no large Cotton effects were
observed in the CD spectra, again indicating the lack of
chiral amplification. Overall, the combination of the results
obtained in the homopolymerization of the enantiomeric
monomers with the chiral amplification studies, through
copolymerization of the mixed chiral-chiral and chiral-
achiral monomer feeds, led to the conclusion that the side
groups of the repeat units are not bulky enough and too far
(three atoms) away from the isotactically placed stereogenic
centers of the polymer backbone to sterically induce helicity
into the polymer. Instead, random-coil secondary structures
form.
Fifth, disproof of hypotheses 1 and 2 by the above four sets
of experiments led to the key, third hypothesis that remains
consistent with, and thus supported by, all the data: the
chiral isotactic AOZ polymers adopt a random-coil confor-
mation, and the optical activity is dictated by the chiral
auxiliary. This scenario was further supported by the obser-
vation that all (R)-AOZ polymers with different main-chain
stereoconfigurations—specifically, the isotactic polymer by
C2-ligated metallocenium catalyst 1, the syndiotactic poly-
mer by the Cs-ligated metallocenium catalysts,33 and the
atactic polymer by the free radical initiator (AIBN)—showed
nearly identical Cotton effects in their CD spectra (Figure 4).
Sixth, modeling of isotactic [(R)-AOZ]30 by MM2 calcula-
tions also led to a random-coil chain secondary structure
(Figure 5), thus providing additional support to hypothesis 3.
This modeling result is in sharp contrast to the calculated
helical structure for isotactic [(R)-VOZ]30 (cf. Chart 2), the
discussion of which immediately follows.
Figure 4. CD spectra of (R)-AOZ-derived polymers with different
main-chain stereoconfigurations: isotactic polymer (blue), syndiotactic
polymer (red), and atactic polymer (green).
Figure 5. Modeled random-coil structure of [(R)-AOZ]30 shown in the
space-filling mode. Carbon, nitrogen, and oxygen are shown in gray,
blue, and red, respectively; hydrogen atoms are omitted for clarity.
polymer. Accordingly, we reasoned that if we brought the
chiral side groups closer to the stereocenters of the polymer
main chain in polymers derived from nonconjugated N-
vinyl-4-(R)-phenyl-2-oxazolidinone [(R)-VOZ)], where the
chiral auxiliary would now be only two atoms away from
the main-chain stereocenters, then the side groups could be
effective in rendering solution-stable helical polymers.
However, two challenges are present in coordination
polymerization of (R)-VOZ. First, it is a nonconjugated vinyl
monomer, so it cannot be polymerized via a coordination
conjugate-addition mechanism by zirconocenium ester en-
olate catalysts such as 1.2 Second, it is a heteroatom (N,O)-
functionalized vinyl monomer, so it cannot be polymerized
(at least directly) via a coordination insertion mechanism by
metallocenium alkyl catalysts such as rac-(EBI)ZrMeþMeB-
- 2
(C6F5)3
. A potential strategy here is to use protected
(coordinated) hetereoatom-functionalized vinyl monomers
with aluminum Lewis acids, which can be readily removed
postpolymerization.38 Initial attempts to polymerize (R)-VOZ
with rac-(EBI)ZrMe2 activated with 500 equiv of MAO—
which we reasoned could not only abstract the methyl group
to form the active zirconcenium catalyst species39 but also
protect the heteroatoms of (R)-VOZ from interfering with
the migratory insertion polymerization process—failed to
form any isolable polymer after 24 h of reaction. Next, we
Stereospecific Cationic Polymerization of Chiral Vinylox-
azolidinones. As the above studies have shown, the chiral
auxiliary of the repeat units in the chiral AOZ polymers are not
bulky enough, and too far (three atoms) away from the polymer
backbone stereocenters, to sterically induce helicity into the

Article
Macromolecules, Vol. 43, No. 18, 2010 7511
Scheme 1. Stepwise Activation of rac-(EBI)ZrMe2 by
[Ph3C][B(C6F5)4]
all common organic solvents and concentrated acids tested,
even at elevated temperatures (up to the boiling points of the
solvents). The polymer yield was held nearly constant of
∼60% for runs at {(R)-VOZ}:{[Ph3C][B(C6F5)4]} = 50 at
25 °C for 2 h in toluene or CH2Cl2, but the polymerization in
THF was almost completely shut down (0.65% yield). The
[Ph3C][B(C6F5)4] initiator can be substituted by other cationic
initiators such as BF3 Et2O. From a viewpoint of devinylation
3
that plagues the polymerization of the unsubstituted VOZ13,15
and other N-vinyl heterocyclic monomers16 using acids, it is
intriguing and actually fortunate that instantaneous precipi-
tation of the resulting polymer prevented such devinylation
to a large extent, enabling us to achieve the first successful
cationic polymerization of VOZ monomers with good poly-
mer yields up to 80% (at 0 °C) and also the first successful
synthesis of highly isotactic PVOZ (vide infra).
The insolubility of the resulting non-cross-linking (R)-PVOZ
in all common organic solvents suggests a rigid-rod-like chiral
polymer adopting a one-handed helical conformation—as
predicted by modeling (cf. Chart 2)—a result of having a
highly isotactic backbone stereoconfiguration generated
through a novel chiral auxiliary-controlled, isospecific cationic
polymerization mechanism. However, to provide concrete
evidence for such a polymer structure, the insolubility associated
with (R)-PVOZ must be solved to allow characterization of
the polymer. To this end, we synthesized the p-hexyloxy-
substituted derivative, (R)-4-(4-(hexyloxy)phenyl)-3-vinyl-
oxazolidin-2-one [(R)-HVOZ)], according to Scheme 2,
and subsequently investigated its cationic polymerization
behavior.
precomplexed (R)-VOZ with 0-3 equiv of iBu3Al and sub-
jected the complexed monomer to polymerization by
rac-(EBI)ZrMe2 activated with equimolar B(C6F5)3 [which
generates in situ the active species, rac-(EBI)ZrMeþMeB-
(C6F5)3
]
- 39 at ambient temperature or 80 °C for up to 24 h.
However, this also yielded no polymer products. We also
repeated these polymerization procedures, but using [Ph3C]-
[B(C6F5)4] as the activator. For the runs using iBu3Al as the
complexing agent, no polymerization was observed at various
reaction temperature and time. Surprisingly, in the absence
i
of Bu3Al, the polymerization at ambient temperature by
in situ activation of rac-(EBI)ZrMe2 with equimolar [Ph3C]-
[B(C6F5)4] proceeded rapidly, with the polymer immediately
crashing out of solution.
This intriguing, exciting result raised the question of what
is the actual catalyst species responsible for the successful
polymerization of (R)-VOZ? Note that zirconocenium spe-
Gratifyingly, (R)-HVOZ was also polymerized by [Ph3C]-
[B(C6F5)4], and the polymerization remained homogeneous
during the course of polymerization. The resulting polymers
are soluble in common organic solvents, thus enabling character-
izations of the polymers by GPC for MW and MWD,
NMR for tacticity, and optical rotation and CD for optical
activity. However, the isolated polymer yield was very low
(6.0%, run 14, Table 4), even after extended reaction time
(24 h) at ambient temperature. Nonetheless, the polymer had
a relatively narrow MWD of 1.65 and a MW of 7.63 kg/mol.
Considering side reactions at ambient temperature often
observed for cationic polymerization, we lowered the poly-
merization temperature to 0 °C (run 15) and -20 °C (run 16),
but doing so did not improve the polymer yield. The use of
-
cies such as rac-(EBI)ZrMeþMeB(C6F5)3 are inactive for
this polymerization. However, it is known that activation of
the metallocene dimethyl precatalyst with [Ph3C][B(C6F5)4]
proceeds in a stepwise fashion: the first step is the rapid
methide abstraction to form the transient μ-Me dimer, a
result of stabilization of the initially formed metallocene
cation by the other half of the neutral dimethyl species,
instead of the anion [B(C6F5)4]-, due to its extremely weak
coordination nature.40 The slower proceeding step is the
gradual conversion of the stable dimer to the highly unstable,
reactive mononuclear zirconocenium species by the other
half of [Ph3C][B(C6F5)4] (Scheme 1). Thus, the complexity of
this activation process presented four possible species being
responsible for the observed polymerization activity: the
mononuclear zirconocenium cation, the dinuclear cation,
the dimethyl precatalyst, and the activator [Ph3C][B(C6F5)4].
Control experiments subsequently ruled out the neutral
precatalyst rac-(EBI)ZrMe2 and the mononuclear zircono-
cenium cation rac-(EBI)ZrMeþ as the active species for this
polymerization. The independently prepared dinuclear
complex40 from the reaction of 2 equiv of rac-(EBI)ZrMe2
with 1 equiv of [Ph3C][B(C6F5)4] also exhibited no polymer-
ization activity. Lastly, we found that addition of a catalytic
amount of [Ph3C][B(C6F5)4] to a toluene solution of (R)-
VOZ resulted in rapid polymerization with the polymer
crashing out of solution, the same phenomenon as seen in
the polymerization by in situ activation of rac-(EBI)ZrMe2
with equimolar [Ph3C][B(C6F5)4] (vide supra). These results
conclusively pointed to [Ph3C][B(C6F5)4]—intended as the
activator for the metallocene precatalyst—as the actual
active species responsible for the observed polymerization
activity. Unfortunately, all isolated polymers using different
{(R)-VOZ}:{[Ph3C][B(C6F5)4]} ratios(5-50), solvents (toluene
and CH2Cl2), and temperature (0 and 25 °C) are insoluble in
BF3 Et2O as initiator enhanced the polymer yield somewhat
3
(to ∼10%, run 17) at 25 °C, but variations in polymerization
temperature (runs 18 and 19) lowered the yield. Brønsted
acids were also examined, including [H(Et2O)2]þ[B(C6F5)4]-
(run 20) and [HN(Me2)Ph]þ[B(C6F5)4]-, but the polymer
yield was never higher than 10%.
Hypothesizing that devinylation may be the cause for the
low polymer yield seen in the cationic polymerization of (R)-
HVOZ, we investigated the stoichiometric reaction of (R)-
HVOZ with [Ph3C][B(C6F5)4]. Indeed, the isolated product
from the reaction was (R)-4-(4-(hexyloxy)phenyl)oxazolidin-
2-one, the devinylation product. Likewise, the polymerization
reaction of (R)-HVOZ with a catalytic amount of [Ph3C]-
[B(C6F5)4] led to (after quenching the reaction, separating
the MeOH insoluble polymer fraction, and removing the
solvent) isolation of almost exclusively the oxazolidinone.
Although acid-catalyzed decomposition of N-vinyl hetero-
cyclicmonomers viadevinylation,13,15 even inthe solid state,16
is known, it was surprising to see the sharp contrast in the
extent of the monomer decomposition between the hetero-
geneous polymerization of (R)-VOZ and the homogeneous
polymerization of (R)-HVOZ. A further study through
monitoring the reaction of [Ph3C][B(C6F5)4] with 5 equiv

7512 Macromolecules, Vol. 43, No. 18, 2010
Miyake et al.
Scheme 2. Outlined Synthesis of (R)-HVOZ
Table 4. Selected Results of Cationic Polymerization of (R)-HVOZa
run
no.
temp
(°C)
yield
(%)
103Mn
(g/mol) (Mw/Mn)
MWD
initiator
14
15
16
17
18
19
20
[Ph3C][B(C6F5)4]
[Ph3C][B(C6F5)4]
[Ph3C][B(C6F5)4]
25
0
-20
25
0
-20
25
6.0
2.0
5.8
9.5
6.9
6.3
9.5
7.63
7.11
5.20
6.53
6.68
5.26
7.31
1.65
1.69
1.81
1.60
1.79
1.78
1.64
BF3 Et2O
3
BF3 Et2O
3
BF3 Et2O
3
[H(Et2O)2][B(C6F5)4]
a See footnotes in Table 1 for explanations.
Scheme 3. Proposed Chiral Auxiliary-Controlled Isospecific Cationic
Polymerization of (R)-(H)VOZ
Figure 6. Carbonyl as well as main-chain CH and CH2 regions in the
13C NMR (125 MHz, CDCl3, 50 °C) of (R)-PHVOZ produced by
[Ph3C][B(C6F5)4] at ambient temperature.
of (R)-HVOZ revealed a valuable insight: isotactic polymer
was formed initially, but over time the backbone methylene
proton signals corresponding to an isotactic configuration
disappeared, and the proton signal for the oxazolidinone
appeared. Hence, this experiment showed that some poly-
merization initially occurs, followed by decomposition to
give oxazolidinone, thus competing with the direct devinylation
of the monomer. This valuable insight also explains the drastic
difference in polymer yields between the polymerizations of (R)-
VOZ and (R)-HVOZ: the hexyloxy substitution in (R)-HVOZ
should have minimal to none effects on the rate of monomer
devinylation, but instead it is the insolubility of (R)-PVOZ,
upon forming and crashing out of solution, that prevents it
from decomposition. Obviously, this mechanism of devinyla-
tion prevention does not apply to the soluble (R)-PHVOZ.
On the other hand, the solubility of (R)-PHVOZ allowed
us to investigate its tacticity and optical properties, despite
the observed low polymer yield. As anticipated, analysis
of the 13C NMR spectrum of (R)-PHVOZ produced by
[Ph3C][B(C6F5)4] at ambient temperature clearly reveals its
high isotacticity, as evidenced by the single mmmm pentad
peak in the CdO region which is collaborated by the single
methine and methylene backbone carbon peaks (Figure 6).
Outlined in Scheme 3 is the proposed chiral auxiliary-con-
trolled isospecific cationic polymerization for the production
of isotactic, chiral vinyl oxazolidinone-functionalized vinyl
polymers, where the concept of stereocontrol in repeated
vinyl additions is analogous to the stereocontrol observed in
the free radical polymerization of chiral acrylamides.10-12
More importantly, combination of this near-quantitatively
Figure 7. CD spectra of monomer (R)-HVOZ (blue) and polymer (R)-
PHVOZ (red).
isotactic placement of the stereogenic centers of the polymer
main chain with the chiral side groups located near those
stereocenters of the backbone rendered one-handed helicity
for (R)-PVOZ and (R)-PHVOZ. Owing to the insolubility of
(R)-PVOZ, its helical structure was only inferred by model-
ing (cf. Chart 2). Thanks to the solubility of (R)-PHVOZ, the
helical structure is now directly supported by the experi-
mental results, including the greatly changed specific rota-
tion, in both magnitude and sign, in going from the monomer
(R)-HVOZ to the polymer (R)-PHVOZ (-44.8 °C to þ156;
run 14, Table 4) as well as the drastically different CD spectra
between the monomer and the polymer (Figure 7). Each of

Article
Macromolecules, Vol. 43, No. 18, 2010 7513
Figure 8. TGA plot of (R)-PVOZ produced by [Ph3C][B(C6F5)4] in
CH2Cl2.
Figure 10. Overlay XRD plots of (R)-PAOZ (red, run 3, Table 1) and
(R)-PVOZ produced by [Ph3C][B(C6F5)4] in CH2Cl2.
temperatures of Tinitial = 351 °C, Tend = 412 °C, and Tmax
390 °C (Figure 9).
=
The XRD plots show significant differences in the crystal-
linity in the as-quenched polymers, (R)-PVOZ and (R)-
PAOZ (Figure 10). Although both isotactic polymers exhibit
three distinct scattering peaks [d spacing: 1.03, 0.58, and
0.38 nm for (R)-PVOZ], the sharpness and intensity of the
scattering peaks of (R)-PVOZ is significantly greater, indi-
cating a higher degree of crystallinity, as compared to (R)-
PAOZ. Overall, these characterizations demonstrated that
the chiral isotactic vinyl polymer (R)-PVOZ is considerably
more thermally stable and more crystalline than the chiral
isotactic acrylamide polymer (R)-PAOZ, characteristics attri-
butable to the rigid-rod-like, helical structure of the chiral
vinyl polymer.
Conclusions
Chiral oxazolidinone-functionalized alkenes have been suc-
cessfully polymerized at ambient temperature in a stereospecific
fashion, leading to the corresponding highly isotactic, optically
active polymers. Depending on whether the monomer is con-
jugated or not, the polymerization proceeds through one of two
mechanisms. For conjugated chiral acryloyl oxazolidinones,
(R or S)-AOZ, isospecific coordination polymerization is brought
about by chiral catalysts 1 in both racemic and enantiomeric
forms. This polymerization is catalyst-site controlled, producing
highly isotactic, optically active polymers (R or S)-PAOZ. Owing
to the nature of chiral catalyst site control, the coordination
polymerization of the parent AOZ without the chiral side group
also affords PAOZ with nearly quantitative isotacticity. These
oxazolidinone-functionalized, chiral isotactic poly(acrylamide)s
adopt a random-coil structure, thus having a cryptochiral chain
and exhibiting no chiral amplifications; their optical activity
arises solely from the chiral auxiliary. These results are rationa-
lized by the chiral side groups in these chiral poly(acrylamide)s
not being sufficiently bulky and located too far away from the
isotactically placed stereocenters of the main chain to induce
sterically a solution-stable helical conformation.
Figure 9. TGA derivative plots of (R)-PAOZ (red, run 3, Table 1) and
(R)-PVOZ produced by [Ph3C][B(C6F5)4] in CH2Cl2.
these observables represents chiral amplifications character-
istic of helical structure formation.
Physical Properties of Stereoregular (R)-PAOZ and (R)-
PVOZ. Polymer thermal transition, decomposition, and
crystallinity were analyzed by DSC, TGA, and XRD, respec-
tively, and comparisons were made between the rigid-rod-
like helical polymer (R)-PVOZ, produced via isospecific
cationic polymerization by [Ph3C][B(C6F5)4], and the random-
coil polymer (R)-PAOZ, produced via isospecific coordination
polymerization by catalyst 1. In the DSC trace, no Tg was
observed for (R)-PVOZ in the conditions employed, which is
not surprising for such a highly isotactic and crystalline
polymer.34b,c On the other hand, (R)-PAOZ exhibited a high
Tg of 196 °C. In the TGA trace, (R)-PVOZ showed a very
narrow, one-step decomposition window with high decom-
position temperatures of Tinitial = 435 °C, Tend = 481 °C
(Figure 8), and Tmax = 460 °C. In comparison, (R)-PAOZ
showed in its TGA trace a relatively broader, one-step
decomposition window, with much lower decomposition
In order to polymerize nonconjugated chiral vinyl oxazolidi-
nones, we successfully developed a novel ambient-temperature
isospecific cationic polymerization using Lewis and Brønsted acids.

7514 Macromolecules, Vol. 43, No. 18, 2010
Miyake et al.
This polymerization is chiral auxiliary controlled and also pro-
duces highly isotactic, optically active polymers, (R)-PVOZ and
(R)-PHVOZ. However, in sharp contrast to the chiral acrylamide
polymers, these vinyl polymers adopt a solution-stable helical
conformation, thereby manifesting substantial chiral amplifications.
Both modeling of the insoluble (R)-PVOZ and experimental
results obtained from the soluble (R)-PHVOZ polymer have
yielded the same result: a chiral helical structure. Synthetically,
the facile acid-catalyzed devinylation presented a major challenge
to the homogeneous, stereospecific cationic polymerization of
(R)-HVOZ, thus severally limiting its polymer yield. Efforts are
underway to search for more effective strategies to eliminate or
largely suppress such side reactions.
The chiral helical vinyl polymers synthesized herein are of
particular interest for two key reasons. First, they effectively
assemble two elements of polymer local chirality—side-chain
chirality and main-chain chirality—into global chirality in the
form of excess one-handed helicity. Second, these N,O-functio-
nalized chiral vinyl polymers represent chiral variants of structu-
rally similar PVP, the currently most widely employed effective
ligand/stabilizer in transition-metal nanocluster chemistry. Such
globally assembled helical chiral polymers already showed their
superior physical properties such as having considerably higher
thermal decomposition temperatures and polymer crystallinity as
compared to the random coil chiral acryloyl polymers having
similarly high main-chain stereoregularity. Our research in utiliz-
ing both classes of chiral polymers synthesized herein as chiral
ligands/stabilizers for transition-metal nanoclusters and their
subsequent asymmetric catalysis is currently underway, the
results of which will appear elsewhere in due course.
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Acknowledgment. This work was supported by the National
Science Foundation (NSF-0756633). We thank Prof. Alan Kennan
of CSU for access to his CD spectrometer and Boulder Scientific
Co. for the research gifts of B(C6F5)3, [Ph3C]þ[B(C6F5)4]-, and
[HN(Me2)Ph]þ[B(C6F5)4]-.
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