BN Polystyrenes: Emerging Optical Materials and Versatile Intermediates

Article abstract of DOI:10.1021/acs.joc.8b02921

BN polystyrenes are an emerging class of polyolefins functionalized with aromatic side chains in which at least one CC bond is replaced with a BN bond. This class of structures exhibits unusual photophysical properties relative to organic polymers. BN pol

Full text of DOI:10.1021/acs.joc.8b02921

JOCSynopsis
pubs.acs.org/joc
Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
BN Polystyrenes: Emerging Optical Materials and Versatile
Intermediates
Heidi L. van de Wouw and Rebekka S. Klausen*
Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
ABSTRACT: BN polystyrenes are an emerging class of polyolefins
functionalized with aromatic side chains in which at least one CC
bond is replaced with a BN bond. This class of structures exhibits
unusual photophysical properties relative to organic polymers. BN
polystyrenes serve as intermediates in the preparation of functional
polymers, including stereoregular polar polyolefins. The consequen-
ces of BN for CC bond substitution on reactivity and properties are
highlighted.
tyrene is a versatile monomer that is polymerized under free
and controlled radical, thermal, anionic, cationic, and
chains (Chart 1). As a systematic nomenclature for BN aromatic
rings and polymeric derivatives is still evolving,12,13 materials in
Chart 1 are named to emphasize similarity to polystyrene or
other hydrocarbon. BN aromatic heterocycles incorporated as
polyolefin side chains include borazine (Sneddon, 1991),27 1,2-
S
transition-metal-catalyzed conditions.1 Polystyrene’s properties
are exquisitely controlled by synthesis (Figure 1a). Free-radical
polymerization yields amorphous polystyrene (PS) for pack-
aging applications.2 Coordination−insertion polymerization
provides crystalline high-melting stereoregular PS.3 Controlled
radical polymerization (CRP) yields well-defined block
copolymers which self-assemble into nanoscale domains.4
PS derivatives with functionalized side chains are important
materials in their own right. Cross-linking and Friedel−Crafts
sulfonylation provide polystyrenesulfonate (PSS), the major
component of the ion-exchange resin Dowex.5 Polar functional
groups improve adhesion to polar surfaces.6 Borylated polymers
are an active research area.7,8 Tricoordinate boron-function-
alized materials have found wide application in sensing,9
nonlinear optics,10 and n-type charge transport.11 Isosteric and
isoelectronic BN for CC bond substitution in aromatic
compounds is an attractive approach to developing stable
organoboranes.12,13 BN for CC substitution in polymer
backbones, either as poly(aminoborane)s14−19 or as conjugated
polymers (Figure 1b),20−22 is well-precedented.
̈
azaborine and 4-(1,2-azaborine)benzene (Liu and Ja
kle,
̈
nnichsen and Staubitz,
2016),28 N-methyl-1,2-azaborine (So
2017),29 and BN naphthalene (Klausen, 2017).30−33 BN
aromatic rings, as determined by nucleus independent chemical
shift (NICS) calculations, are less aromatic than benzene
(Figure 1c).23,24
SYNTHESIS OF BN AROMATIC VINYL MONOMERS
■
Dewar reported the first syntheses of BN aromatic structures
including BN phenanthrene34 and BN naphthalene35 beginning
in the 1950s. Azaborine, the benzene analogue with the general
formula C4H6BN, has three isomers, which remained elusive for
decades after Dewar’s initial work.12,13 In recent years, modern
methods enabled simplified synthetic access to monocyclic
azaborines.36 Despite advances, there remains a need for
methods suitable for gram-scale and larger syntheses.37
Vinylborazine (VB). Sneddon reported the synthesis and
polymerization of B-vinylborazine (VB) in 1991.27 Rhodium-
catalyzed coupling of borazine (B3N3H6) and acetylene
provided VB.38 Alkylated variants (e.g., B-vinylpentamethylbor-
azine) were synthesized by substitution reactions of B-
haloborazines with Grignard reagents.39
Vinyl monomers with BN aromatic side chains are less
explored. Interest in these structures stems from several
motivations. i. Reactivity. Styrene’s synthetic versatility arises
from the utility of the benzylic reactive intermediates implicated
in styrene polymerization. How does a BN aromatic ring
influence the reactivity of a benzylic radical, anion, or other
reactive intermediate relevant to polymerization? ii. Photo-
physics. BN aromatics have unusual photophysical properties
compared to benzene, including bathochromically shifted
absorption and emission. iii. Postfunctionalization. Organo-
boranes are workhorse intermediates in chemical synthesis.25,26
The C−B bond of B-vinyl polymers introduces opportunities for
postpolymerization chemical modification, addressing chal-
lenges in the synthesis of functional polymers containing both
polar and nonpolar functional groups.
BN Styrene (BNSt) and Substituted Styrenes. Ashe
reported the first synthesis of BNSt in 2009 via ring expansion of
1,2-azaborolide 1.40 The six-step synthesis proceeded in 6%
overall yield (Scheme 1a). The authors reported that the BNSt
anion serves as a π-ligand in metal complexes but did not explore
vinyl polymerization.
Building on Ashe’s 2000 report of ring-closing metathesis
(RCM) routes to 1,2-azaborines,36 in 2013 Liu described a
This Synopsis focuses on “BN polystyrenes”, defined as
polymers arising from vinyl monomers with BN aromatic side
Received: November 15, 2018
Published: January 8, 2019
© XXXX American Chemical Society
A
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Figure 1. (a) Styrene polymerization and applications. (b) Prior work in BN for CC isosterism in polymeric materials. (c) BN aromaticity. NICS(0)πzz
23,24
́
(ppm) values excerpted from Baranac−Stojanovic et al.
azaborinyl)styrene (4-ABSt), respectively.28 Sonnichsen and
Staubitz reported a modification of the RCM/aromatization
strategy to prepare BNMeSt, the N-methyl analogue of BNSt
(four steps, 15% overall yield).29
a
̈
Chart 1. Polyolefins with BN Aromatic Side Chains
BN 2-Vinylnaphthalene (BN2VN). The key step in
Dewar’s 1959 synthesis of BN naphthalene was the
condensation of 2-vinylaniline with trichloroborane, followed
by reduction.35 More recently, Molander reported the synthesis
of dozens of substituted BN naphthalenes via the one-pot
condensation of vinylanilines with dichloroorganoboranes
generated in situ from potassium organotrifluoroborate salts.42
Based on this precedent, we targeted BN 2-vinylnaphthalene
(BN2VN) as a more synthetically accessible monomer
compared to monocyclic structures. We developed a two step,
multigram scale synthesis of BN2VN (55% overall yield, Scheme
2).30,31 2-Aminophenethyl alcohol 5 was converted to 2-
vinylaniline 6 in 63% yield by vacuum distillation in the
presence of potassium hydroxide.43 Condensation with
potassium vinyltrifluoroborate provided BN2VN in 88% yield.
RADICAL POLYMERIZATION
■
Free-radical polymerization of all of the monomers discussed
above has been reported. Table 1 summarizes representative
results. In addition, there is one report each of controlled radical
PVB = poly(vinyl borazine);27 PBNS = BN polystyrene;28 P(4-ABS)
= poly(4-azaborinylstyrene);28 PBNMeS = poly(1-methyl-2-vinyl-1,2-
azaborine);29 PBN2VN = poly(BN 2-vinylnaphthalene);30−32
sPBN2VN = syndiotactic PBN2VN.33 RAFT = reversible addition−
fragmentation chain transfer; CIP = coordination−insertion polymer-
ization.
a
̈
and coordination−insertion polymerization, from Liu and Jakle
and us, respectively. The potential for future investigation is
significant, including both further development of existing
methods, as well as unexplored avenues like anionic polymer-
ization.
RCM/aromatization sequence (Scheme 1b) to 1,2-azaborines41
of the general type 4 which serve as common intermediates in
the preparation of functionalized monocyclic BN aromatics. In
Free-Radical Homopolymerization. Sneddon reported
VB polymerization initiated by azoisobutyronitrile (AIBN),
which provided two fractions: a major fraction consisting of
soluble, moderate molecular weight (Mn = 10.7 kDa) material
̈
2016, Liu and Jakle described derivatization of 4 with vinyl and
p-styryl organometallic reagents to provide BNSt and 4-(1,2-
B
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Scheme 1. (a) Ashe: Ring Expansion and Deprotection of 1,2-Azaborolide 1 Provides BNSt.40 (b) Liu: Ring-Closing Metathesis
a
(RCM) and Aromatization Provides 4, a Versatile Intermediate in the Synthesis of BN Aromatic Vinyl Monomers29,41
,
a
LDA = lithium diisopropylamide; TMS = trimethylsilyl; TBAF = tetrabutylammonium fluoride; TBS = tert-butyldimethylsilyl.
a
Scheme 2. Synthesis of BN2VN (55% Overall Yield)30,
bonds are strong, with bond dissociation enthalpies ranging
between 105−112 kcal mol−1 (CAM-B3LY/6-311G(d,p)).32
To date, only BN2VN polymerization has been carried out on
multigram. BNSt polymerization was reported on a 10 mg scale
and BNMeSt polymerization on a ca. 150 mg scale.
All BN polystyrenes showed optical properties distinct from
hydrocarbon analogues (Table 2); for example, PBN2VN is pale
yellow, whereas P2VN is white. In general, the longest
wavelength absorption (λmax) bathochromically shifted and
intensified upon BN substitution. The magnitude of the
bathochromic shift varied from 16 nm (PS vs PBNS) to 43
nm (P2VN vs PBN2VN). The emission spectra of BN
polystyrenes also differed from hydrocarbon polymers, with
larger Stokes shifts seen in BN polystyrenes than in hydrocarbon
polymers. Similar trends are observed in molecular systems,
including the monomers themselves.48
Controlled Radical Polymerization. Controlled radical
polymerization (CRP) introduces a dynamic equilibrium
between propagating and dormant polymer chains (Scheme
3a).51 The low concentration of propagating polymer chain ends
suppresses chain recombination and other termination events
that contribute to a broad distribution of chain lengths
a
CPME = cyclopentyl methyl ether.
assigned to a polyolefin with borazine side chains and a smaller
fraction (<15−20%) of high molecular weight material arising
from cross-linking of the borazine side chains.27,44 Alkylated
vinylborazines did not polymerize under free-radical con-
ditions.39,45
Free-radical polymerization of vinyl monomers with 1,2-
azaborine-derived substituents proceeds under similar con-
ditions as reported for VB (AIBN, 70−90 °C). No side-chain
cross-linking was observed. BN2VN, BNMeSt, and 4-ABSt
polymerization provided high molecular weight polymers
(Table 1), but PBNS was only isolated in low molecular weight
(Mn = 1.6 kDa). The origin of this divergent reactivity is not yet
clear. Free-radical polymerization of vinyl boronic esters is
known,46,47 suggesting that α-boryl radicals are generally
suitable for polymerization, an observation further supported
by the high reactivity of BN2VN and BNMeSt. A potential
explanation is chain transfer to monomer, the phenomenon in
which the polymer radical abstracts a weakly bonded atom from
the monomer. The formation of a molecular radical at the
expense of a polymeric radical results in short chain lengths.
However, calculations suggest that azaborine’s C−H and N−H
(dispersity, D̵) in free-radical polymerization. CRP is desirable
for applications requiring control of molecular weight,
dispersity, and end group structure. Notable examples include
atom transfer radical polymerization (ATRP), nitroxide
mediated polymerization (NMP), and reversible addition−
fragmentation chain transfer (RAFT) polymerization. RAFT
polymerization is convenient, as typically a chain-transfer agent
(CTA) is simply added to free-radical polymerizations.52
a
Table 1. Free-Radical Polymerization of BN Aromatic Vinyl Monomers
entry
monomer (solvent)
radical initiator
temp (°C)
time (h)
yield (%)
25
Mn (kDa)
D
̵
ref
1
2
3
4
5
VB (C6H6)
AIBN (1 mol %)
ACHN (1 mol %)
AIBN (1 mol %)
AIBN (1 mol %)
AIBN (2.5 mol %)
80
90
85
70
70
20
72
72
24
20
10.7
1.6
16.5
20.5
38.5
1.68
1.33
1.51
3.87
3.91
27
28
29
31
28
b
BNSt (C6D6)
BNMeSt (neat)
BN2VN (neat)
4-ABSt (THF)
50
51
75
b
>99
a
b
AIBN = 2,2′-azobis(2-methylpropionitrile); ACHN = 1,1′-azobis(cyanocyclohexane). Percent conversion.
C
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Table 2. Selected BN Polystyrene Photophysical Data Compared to Hydrocarbon Polymers
BN aromatic side chains
hydrocarbon aromatic side chains
a
b
a
b
entry
polymer
λmax (nm)
λem (nm)
ref
polymer
λmax (nm)
λem (nm)
ref
1
2
3
4
PBNS
277
299
279
320
344
369
nd
28
28
29
30
PS
261
nd
265
277
278
313
nd
49
50
29
30
P(4-ABS)
PBNMeS
PBN2VN
PVBP
P2MeS
P2VN
398
335, 401
a
b
Longest wavelength absorption. Wavelength emitted after excitation at λmax. PVBP = poly(vinyl biphenyl); P2MeS = poly(2-methylstyrene);
P2VN = poly(2-vinylnaphthalene).
divided by M1−M2 crosspolymerization and r2 describes the
ratio of the rate constants for M2−M2 homopolymerization
divided by M2−M1 cross-polymerization. Alternating copoly-
mers arise when r1 = r2 ≪ 0, indicating a preference for cross-
polymerization. This behavior is seen for structurally dissimilar
monomers, such as the radical copolymerization of electron-rich
styrene with electron-poor maleic anhydride. Statistical
copolymerizations arise when r1 and r2 are both close to 1 and
is typically only observed with structurally similar monomers.
Copolymerization of BN aromatic vinyl monomers with
hydrocarbons is therefore an opportunity to assess the styrene-
like reactivity of the organoborane. Copolymerizations with 2-
methylstyrene (2MeSt), 2-vinylnaphthalene (2VN), and styrene
have been reported. These studies uniformly suggest that BN
aromatic vinyl monomers have comparable, but somewhat
lower, reactivity compared to the analogous hydrocarbons.
BN-Methylstyrene (BNMeSt) and 2-Methylstyrene.
Scheme 3. (a) Controlled Radical Polymerization by
a
̈
Reversible Deactivation. (b) Liu and Jakle: RAFT
b
Polymerization of BNSt and 4-ABSt28,
̈
Sonnichsen and Staubitz contrasted the homopolymerizations
of BNMeSt and 2MeSt and also reported the free-radical
copolymerization of 2MeSt and BNMeSt.29 2MeSt polymer-
ization proceeded to higher conversion and provided higher
molecular weight polymer than observed for free-radical
polymerization of BNMeSt under similar reaction conditions
(P2MeS, Mn = 46.7 kDa; PBNMeS, Mn = 16.5 kDa). A binary
free-radical copolymerization (1:1 molar ratio of monomers)
provided the atactic statistical copolymer P(2MeS-co-BNMeS)
(Mn = 21.8 kDa) which was determined to be enriched in 2MeSt
by NMR spectroscopy. The authors concluded that the relative
rates of 2MeSt and BNMeSt homo- and crosspolymerization are
of comparable magnitude, but 2MeSt is more reactive. Reactivity
ratios were not reported.
a
b
Green sphere represents a generic deactivating agent. DDMAT = 2-
(dodecylthiocarbonothioylthio)-2-methylpropionic acid; DMF =
dimethylformamide; ACHN = 1,1′-azobis(cyanocyclohexane).
Different CTA structures are optimal for styrenes, acrylates, and
The physical properties of the copolymer are intermediate
between the parent homopolymers. P2MeS is an indefinitely
stable white solid, while PBNMeS decomposes in air from a
white to a brown solid within 72 h. P(2MeS-co-BNMeS)
discolors more slowly than PBNMeS. A single copolymer glass
transition temperature (Tg), the temperature at which the
material transforms from a glassy to rubbery state, was observed
at 114 °C. This single Tg, intermediate between the
homopolymers (P2MeS, Tg = 132 °C; PBNMeS, Tg = 85 °C),
supported microstructural assignment to a statistical copolymer.
BN2VN and 2-Vinylnaphthalene. In 2017, we described
the free-radical copolymerization of BN2VN and 2VN.30 Gel
permeation chromatography (GPC) revealed a unimodal
distribution of copolymer molecular weights. A bimodal
molecular weight distribution would suggest formation of two
homopolymers instead of a single copolymer. A suite of NMR
spectroscopies (1H, 11B, 13C) aided copolymer characterization.
A UV−vis spectroscopic assay based on the different absorption
intensities at 320 nm of the BN naphthalene and naphthalene
side chains quantified BN2VN incorporation. Based on this
assay, a close agreement between monomer feed ratio and
other monomers.
̈
Liu and Jakle reported RAFT polymerization of both BNSt
and 4-ABSt using the chain-transfer agent 2-(dodecylthiocarbo-
nothioylthio)-2-methylpropionic acid (DDMAT) (Scheme 3b),
a CTA optimized for styrene polymerization. Only P(4-ABS)
demonstrated one of the signatures of CRP, chain extension
upon introduction of additional monomer. Chain extension
indicates end-group fidelity, that the chain cap can still be
reversibly cleaved to allow further polymerization.
The outlook for BN aromatic vinyl monomer CRP is very
promising. Future efforts will undoubtedly continue to explore
methods in addition to RAFT, as well as the preparation of block
copolymers containing BN aromatic side chains.
COPOLYMERIZATION
■
Copolymerization of two or more monomers is an essential
strategy for tuning the physical properties of a polymer. The
binary copolymerization behavior of two monomers M1 and M2
is described by their reactivity ratios r1 and r2, where r1 is the
ratio of the rate constants for M1−M1 homopolymerization
D
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copolymer composition was determined. P(BN2VN-co-2VN)
was prepared with between 9.2 and 74 wt % BN2VN.
Copolymer molecular weights (Mn) ranged between 6.0 and
8.4 kDa, with the highest molecular weight copolymers obtained
from copolymers enriched in 2VN. Our results again suggested
that the vinyl borane has comparable, but lower, reactivity than
the hydrocarbon analog.
BN2VN and Styrene. In early 2018, we reported free-radical
copolymerization of styrene and BN2VN. An optical assay based
on the selective absorption at 320 nm of the BN naphthalene
side chain confirmed BN2VN incorporation into the copolymer
at levels commensurate with the feed ratio.31 Reactivity ratios for
BN2VN and styrene were determined from low conversion free-
radical copolymerizations using both traditional linearization
methods and modern nonlinear least-squares statistical analysis
(Table 3 and Figure 2).32 The reactivity ratios (r1(BN2VN) =
POSTPOLYMERIZATION MODIFICATION
■
Organoboranes are versatile intermediates in synthetic chem-
istry, serving as precursors to alcohols, amines, and other
functional groups.25,26 This versatility highlights the potential
for boron-functionalized polymers to serve as a platform to
functional materials, but the challenge of introducing boron into
the polymer limits the scope of relevant polymer architectures.
Chung et al. functionalized alkenyl side chains in high-vinyl
polybutadiene via hydroboration−oxidation,53 while Hartwig
and Hillmyer demonstrated catalytic C−H activation of
poly(ethylethylene) (PEE)54 to yield partially hydroxylated
polymers (Scheme 4). Hillmyer et al. also reported C−H
functionalization of the commodity polymers linear low-density
polyethylene (LLDPE) and polypropylene.55,56
Scheme 4. Borane Strategy for Polyalcohol Synthesis
Table 3. Reactivity Ratios for the Copolymerization of BN
Aromatic Vinyl Monomers and Styrene
M1
VB
BN2VN
r1
M2
r2
ref
0.078
0.423
styrene
styrene
4.02
2.30
27
31
Challenges in the vinyl polymerization of trialkylboranes
include deactivation of the double bond and the oxidative and
hydrolytic instability of trialkylboranes. Chung reported
Ziegler−Natta polymerization of the mono-hydroboration
products derived from long-chain dienes, which were converted
to polyalcohols by hydroperoxide oxidation.57 A four-carbon or
longer spacer was needed between the double bond and boron
to avoid suppressing reactivity. Vinylboronic ester polymer-
ization has been reported, but polymer sensitivity in air to
hydrolysis and cross-linking limited characterization.46,47
Nonetheless, the significant commercial interest in the
incorporation of a controlled amount of a polar functional
group into a nonpolar polymer continues to motivate research
into strategies for polymer functionalization. Polar functional
groups play an essential role in modifying a plastic’s strength,
toughness, melting point, and solvent compatibility.58,59
Examples of commercial polar−nonpolar copolymers include
acrylonitrile-butadiene-styrene (ABS) and ethylene-vinyl ac-
etate (EVA).2,60 Challenges in the direct copolymerization of
polar and nonpolar monomers are many but include the
incompatibility of a Lewis basic functional group with the
polymerization catalyst and significant mismatch in reactivity
ratios.61−63
Figure 2. Klausen: styrene−BN2VN reactivity ratios. (a) Mayo−Lewis
plot for the statistical copolymerization of BN2VN and styrene with
AIBN.32 Dashed line represents a random copolymerization (r1 = r2 =
1); solid curve is a fit to the experimental data and indicates a statistical
copolymerization where r1 = 0.423 and r2 = 2.30. FBN2VN = fractional
composition of BN2VN in the copolymer; f BN2VN = fractional
composition of BN2VN in the monomer feed. (b) Reactivity ratios
of statistical copolymerization of BN2VN and styrene with AIBN by
Fineman−Ross (FR), Kelen−Tudos (KT), and nonlinear least-squares
̈
̈
(NLLS) analysis. Dashed ellipsoid represents the 95% confidence
interval of NLLS analysis.32
0.423 and r2(St) = 2.30) indicated a statistical copolymerization.
Styrene is more reactive than BN2VN and at low conversion
styrene-enriched copolymers were obtained. These insights
enabled control over copolymer properties, such as the Tg.
Sneddon reported the radical copolymerization of VB and
styrene in 1991.27 The reactivity ratios (r1(VB) = 0.078 and
r2(St) = 4.02) indicated a greater reactivity mismatch between
styrene and VB than between styrene and BN2VN. We attribute
this reactivity difference to the increased aromaticity of BN
naphthalene compared to borazine.23,24,32
BN2VN polymers show exciting potential as a solution to the
challenge of preparing hydroxyl-functionalized polyolefins via
postpolymerization modification. In the following section, we
summarize oxidative postfunctionalization of PBN2VN-stat-PS
yielding statistical styrene-vinyl alcohol copolymers.
Styrene (St) and vinyl acetate (VAc) are a comonomer pair
with fundamentally incompatible reactivity: St is far more
reactive than VAc, and no VAc is incorporated in a binary
copolymerization (r1(St) = 55 and r2(VAc) = 0.01).64,65 The
reactivity mismatch between VAc and conjugated monomers
limits VAc’s utility as a precursor to hydroxy-functionalized
copolymers, even though poly(vinyl acetate) is a commodity
E
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polymer and precursor to poly(vinyl alcohol) (PVA) via side-
chain saponification.66
The efficient statistical copolymerization of styrene and
BN2VN suggested a solution to this challenge via oxidative
conversion of the C−B bond into a C−O bond. We showed that
alkaline hydrogen peroxide converted PBN2VN-co-PS to PVA-
co-PS (Scheme 5a). Copolymers with variable BN2VN content
organoborane side chain was observed. Overoxidation of the BN
naphthalene side chain resulted in formation of indole and boric
acid as byproducts. A methanol azeotrope removed residual
boric acid as trimethyl borate. Infrared spectroscopy provided
evidence of the characteristic hydroxy group stretching
frequency (Scheme 5b).
BN2VN enabled the first synthesis of statistical styrene-vinyl
alcohol (SVA) copolymers. SVA copolymers showed tunable
variation in physical properties with hydroxy content. Solubility
in polar, protic solvents like methanol increased with increasing
hydroxy concentration. Differential scanning calorimetry (DSC)
also showed systematic variation in the Tg. While PBN2VN-co-
PS had a Tg intermediate between PS and PBN2VN, after
oxidation, the Tg was intermediate between PS and PVA.
In the following section, we highlight a different application of
postfunctionalization of BN aromatic side chains.
Scheme 5. Klausen: Postfunctionalization of BN2VN
Copolymers. (a) Typical Reaction Conditions for Oxidation
of PBN2VN-co-PS to PVA-co-PS. (b) Cropped IR Spectra
Highlighting the Disappearance of BN2VN ν(NH) Feature
a
and Appearance of PVA ν(OH) Features after Oxidation
COORDINATION−INSERTION POLYMERIZATION
■
Transition-metal complexes catalyze coordination−insertion
polymerization of ethylene, propylene, and other vinyl
monomers, yielding high molecular weight commercial
polymers. Rational ligand design in homogeneous catalysts has
yielded highly stereoregular polymers.67
Polymer physical properties depend on the stereochemical
relationships between adjacent repeat units in a polymer chain
(Scheme 6a,b). There are three types of tactic, or stereoregular,
polymers: atactic, syndiotactic, and isotactic. If the stereo-
chemical configuration of two adjacent repeating units is the
same, it is a meso (m) diad, while in a racemo (r) diad the
configurations are different in the two repeating units. Two
adjacent racemo diads make a rr triad, two adjacent meso diads
make a mm diad, and adjacent meso and racemo diads make a mr
triad. A perfectly syndiotactic macromolecular chain consists of
exclusively rr triads, a perfectly isotactic chain consists
a
IR spectra reprinted with permission from ref 31. Copyright 2018
Wiley.
(13−58 wt %) and molecular weights (Mn = 12.8−39.6 kDa)
were investigated. In all cases, complete consumption of the
Scheme 6. (a) Stereoregular Polymer Nomenclature. (b) Variation in Physical Properties with Tacticity. (c) Klausen: Synthesis of
a
Syndiotactic PBN2VN by Coordination−insertion Polymerization
a
1
Oxidation yields syndiotactic poly(vinyl alcohol) (sPVA). Inset shows cropped H NMR spectra (400 MHz, DMSO-d6) of sPVA and aPVA
highlighting the hydroxy triads. Spectrum reprinted with permission from ref 33.
F
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ORCID
exclusively of mm triads, and an atactic chain consists of a
statistical distribution of rr, mm, and mr triads.
Heidi L. van de Wouw: 0000-0002-2115-6325
Rebekka S. Klausen: 0000-0003-4724-4195
The synthesis of highly stereoregular polar polyolefins is
limited by the challenge of identifying catalysts compatible with
polar functional groups.59,61 Open coordination sites on the
metal interact with carbonyl, amine, and other functional
groups, resulting in catalyst and/or polymer decomposition.
Nonetheless, stereoregular polar polyolefins have interesting
properties and applications. PVA hydrogels have potential
therapeutic applications, and high molecular weight PVA fibrils
exhibit high tensile strength.68−70 Tacticity has a profound effect
on both applications as relative stereochemistry influences the
extent of intra- and intermolecular hydrogen bonding between
hydroxy groups.71,72 Synthetic routes to sPVA include cationic
polymerization of vinyl ethers with bulky protecting groups73−75
or radical polymerization of vinyl pivalate,76,77 followed by
protecting group cleavage. Syndiotacticties are typically modest,
and cationic polymerizations yield low molecular weight
materials. The limited substrate scope of cationic polymerization
and the poor reactivity of vinyl ester derived radicals also impose
a limitation on the ability to tune PVA’s properties through
copolymerization with nonpolar monomers.
Notes
The authors declare the following competing financial
interest(s): A patent application has been submitted by Johns
Hopkins University based on an aspect of this research with
H.L.V.D.W. and R.S.K. as co-inventors.
Biographies
We demonstrated that syndioselective BN2VN coordina-
tion−insertion polymerization followed by stereoretentive
oxidation provided sPVA by an orthogonal mechanism with
the potential to address these limitations.33 Homogeneous
monocyclopentadienyl complexes78,79 activated by Lewis acids
are known to catalyze syndioselective styrene polymerization
and are also effective BN2VN polymerization catalysts.
BN2VN’s aromaticity, and the dative interaction between
neighboring elements, reduced the Lewis acidity and basicity
of boron and nitrogen, resulting in compatibility with the
oxophilic Ti catalyst. Additionally, BN2VN’s aromaticity
suggested its ability to intercept the mechanism of styrene
syndioselective polymerization.80 Indeed, we found that
Cp*TiMe3 and B(C6F5)3 provided syndiotactic PBN2VN
Heidi L. van de Wouw earned her B.S. degrees in chemistry and in
environmental studies and planning with an emphasis in water quality
under advisement from Prof. Carmen F. Works and Prof. Stephen A.
Norwick (Sonoma State University). She is currently a graduate student
at Johns Hopkins University with Prof. Rebekka S. Klausen, where her
research focuses on the synthesis and study of stable aromatic
organoborane materials.
1
(sPBN2VN) in high yield (Scheme 6c). H and 13C NMR
studies, as well as thermal properties, suggested a highly
stereoregular structure. Sodium hydroperoxide oxidation of
1
sPBN2VN yielded sPVA, as confirmed by H NMR and IR
spectroscopy.
OUTLOOK
■
The studies highlighted herein showcase the potential for BN
polystyrenes to impact fundamental and applied polymer
science. BN aromatic vinyl monomers exhibit styrene-like
reactivity and versatility in several contexts, including facile
radical copolymerization with aromatic hydrocarbons and the
ability to intercept reaction mechanisms dependent on
aromaticity. The unusual photophysical properties of BN
polystyrenes not only point to sensing applications but also
facilitate quantitative copolymer characterization. New direc-
tions for BN polystyrenes as intermediates in the preparation of
functional copolymers and stereoregular polymers were
emphasized. Future work will continue to expand the potential
of these materials, including the preparation of other BN
aromatic isomers (e.g., 1,3- or 1,4-azaborines) and the further
development of modern polymerization chemistries.
Prof. Rebekka S. Klausen earned her Ph.D. with Prof. Eric N. Jacobsen
(Harvard University) in 2011, after which she pursued postdoctoral
research with Prof. Colin Nuckolls (Columbia University) from 2011 to
2013. She established her independent research group at Johns
Hopkins University in 2013, where her work focuses on the
development of innovative synthetic strategies to atomically precise
materials.
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation under Grant No. CHE-1752791. We thank
all current and former Klausen group members who have
contributed to the development of BN aromatic vinyl
monomers: Dr. Shehani N. Mendis, Yuyang Ji, Jae Young Lee,
Tiffany Zhou, Elorm C. Awuyah, and Jodie I. Baris. H.L.V.D.W.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: klausen@jhu.edu.
G
DOI: 10.1021/acs.joc.8b02921
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(21) Zhang, W.; Li, G.; Xu, L.; Zhuo, Y.; Wan, W.; Yan, N.; He, G.
9,10-Azaboraphenanthrene-Containing Small Molecules and Conju-
gated Polymers: Synthesis and Their Application in Chemodosimeters
for the Ratiometric Detection of Fluoride Ions. Chem. Sci. 2018, 9 (19),
4444−4450.
thanks the JHU Department of Chemistry for the Ernest M.
Marks Award and the Harry and Cleio Greer Fellowship.
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