Cobalt-mediated radical polymerization routes to poly(vinyl ester) block copolymers

Article abstract of DOI:10.1002/pola.24445

Cobalt-mediated radical polymerizations (CMRPs) utilizing redox initiation are demonstrated to produce poly(vinyl ester) homopolymers derived from vinyl pivalate (VPv) and vinyl benzoate (VBz), and their block copolymers with vinyl acetate (VAc). Combinin

Full text of DOI:10.1002/pola.24445

Cobalt-Mediated Radical Polymerization Routes to
Poly(vinyl ester) Block Copolymers
DAVID N. BUNCK, GREGORY P. SORENSON, MAHESH K. MAHANTHAPPA
Department of Chemistry, University of Wisconsin–Madison, 1101 University Ave., Madison, Wisconsin 53706
Received 27 July 2010; accepted 11 October 2010
DOI: 10.1002/pola.24445
Published online 18 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Cobalt-mediated radical polymerizations (CMRPs)
utilizing redox initiation are demonstrated to produce poly(vinyl
ester) homopolymers derived from vinyl pivalate (VPv) and vinyl
benzoate (VBz), and their block copolymers with vinyl acetate
interpreted in terms of the decreased nucleophilicity of these
less electron donating propagating polymer chain ends. Based
on these results, we demonstrate that sequential CMRP reac-
tions present a viable route to microphase separated poly(vinyl
ester) block copolymers as shown by small-angle X-ray scatter-
ing analyses. VC 2010 Wiley Periodicals, Inc. J Polym Sci Part A:
(
VAc). Combining anhydrous Co(acac)
2
, lauroyl peroxide, citric
acid trisodium salt, and VPv at 30 C results in controlled poly-
merizations that yield homopolymers with M
¼ 2.5–27 kg/mol
with /M 1.20–1.30. Homopolymerizations of scrupu-
ꢀ
n
Polym Chem 49: 242–249, 2011
M
w
n
¼
lously purified VBz proceed with lower levels of control as evi-
denced by broader polydispersities over a range of molecular
KEYWORDS: biodegradable; block copolymers; cobalt-mediated
radical polymerization (CMRP); degenerate transfer polymeriza-
tion; radical polymerization; self-assembly; vinyl acetate; vinyl ester
weights (M
n
¼ 4–16 kg/mol; M /M
w n
¼ 1.34–1.65), which may be
INTRODUCTION Controlled/living polymerizations furnish
access to macromolecular materials with well-defined molec-
ular architectures, molecular weights, and molecular weight
PVAc is commonly used in settings ranging from biomedical
2
0
21
devices to paper-coating and adhesive applications. The
properties of other vinyl ester homopolymers are, however,
quite limited and have prevented their widespread use. Sur-
1
distributions. Each controlled polymerization technique is
well suited for the (co)polymerization of monomers of a
prisingly, the sequential block copolymerization of vinyl
2
3,4
12,13
given structural type. Anionic and cationic
polymeriza-
esters has only recently been reported
and the proper-
tions are commercially practiced for the synthesis of poly-
mers derived from styrene, dienes, isobutylene, and vinyl
ether monomers. Controlled free radical processes such as
ties of poly(vinyl ester) block copolymers remain unknown.
Poly(vinyl ester) block copolymers are expected to possess a
subset of the desirable properties of poly(acrylates) and
polyolefins, with the added benefit of chemical and biochem-
5
,6
atom transfer radical polymerization,
radical polymerization, and reversible-addition fragmenta-
nitroxide-mediated
7
20
ical degradability.
8
,9
tion chain transfer (RAFT) polymerizations facilitate con-
trolled enchainment of styrenic and acrylic monomers, with
limited applications to conjugated dienes. Vinyl ester mono-
mers have remained conspicuously absent from the list of
monomers amenable to controlled polymerizations until rela-
Degenerate chain transfer polymerizations have recently
emerged as a useful tool for controlled polymerizations of
vinyl esters and other highly reactive, nonconjugated mono-
mers such as N-vinyl pyrrolidone. Iodine transfer polymer-
ization enables the synthesis of poly(vinyl acetate) (PVAc)
with well-defined molecular weights, relatively narrow mo-
1
0–15
tively recently.
The quest for new, environmentally benign materials moti-
vates the search for families of economically produced mono-
mers and polymers with widely tunable properties, process-
lecular weight distributions (M
w
/M
n
¼ 1.30–1.40), and con-
2
2–24
trolled tacticities.
More recent reports have established
the broad utility of RAFT/MADIX processes in mediating the
controlled homopolymerizations of VAc, vinyl pivalate (VPv),
vinyl benzoate (VBz), and vinyl neodecanoate (VNd) to pro-
duce narrow molecular weight distribution poly(vinyl esters)
1
6
abilities, and inherent degradability. Many efforts thus far
have focused on polymers containing cleavable linkages in
the main chain (e.g., poly(esters) and poly(amides)), which
1
7
10–13
25
degrade by hydrolysis into small molecules. In spite of
their all carbon backbone, poly(vinyl esters) degrade by
ester side chain hydrolysis to yield carboxylic acids and bio-
with variable architectures.
Yamago has demonstrated
the utility of organo-tellurium, -stibine, and -bismuthine
degenerate transfer reactions for the polymerization of styr-
enic, acrylic, and vinyl ester monomers, as well as N-vinyl
1
8,19
degradable poly(vinyl alcohol).
As a commodity polymer,
Correspondence to: M. K. Mahanthappa (E-mail: mahesh@chem.wisc.edu)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 242–249 (2011) V
C
2010 Wiley Periodicals, Inc.
242
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ARTICLE
SCHEME 1 Mechanism of cobalt-mediated radical polymerization of vinyl acetate (VAc).
3
9
pyrrolidone and other nonconjugated monomers. Cobalt-
mediated radical polymerization (CMRP) has emerged as
another convenient degenerate transfer polymerization meth-
odology for the controlled enchainment of VAc and acrylic
1.29), in accordance with prior work by Bryaskova et al.
This previously reported redox initiation scheme was chosen
to obviate the use of expensive, low temperature azo-initia-
tors for CMRP and the byproducts of CA oxidation were not
found to have deleterious effects on the course of the poly-
2
6–30
monomers (Scheme 1).
In these procedurally simple
polymerizations, a combination of a Co(II) complex, a free
radical initiator, and VAc generates alkyl-Co(III) species
in situ that mediate the controlled polymerization of VAc to
3
merization. We found that both Na CA and CA both furnish a
high level of control in bulk CMRP of VAc, even though these
reagents are not particularly soluble in the monomer used
for these reactions. PVAc molecular weights linearly increase
with monomer conversion at low conversions (ꢁ 5%), con-
sistent with a controlled polymerization process and the
3
1
26
PVAc (M /M ¼ 1.15–1.3). Debuigne et al. have exten-
w
n
sively explored the scope and utility of CMRP for the synthe-
sis of block copolymers of VAc with acrylates and acryloni-
trile, styrene, and olefin monomers through sequential chain
extension reactions and various chain-end transformations
that enable tandem copolymerizations of ‘‘mechanistically in-
compatible monomers.’’ It is worth noting that CMRP reactions
conducted in the absence of strongly coordinating axial ligands
for cobalt typically operate by a combination of a reversible
trapping and a degenerate chain transfer mechanism involving
3
9
aforementioned report by Bryaskova et al. In this low con-
version limit, we consequently observe that Mn monotoni-
cally increases with time in a linear fashion as illustrated in
a plot of M versus reaction time t (Fig. 1). Extrapolation of
n
the latter plot to the low molecular weight limit provides an
estimate of the induction period during which no polymer is
formed, which is ascribed to the time required for all of the
Co(II) precursor to be converted into alkyl-Co(III) species
32
a transient dialkyl-Co(IV) intermediate, whereas the addition
of coordinating ligands to the polymerization reaction (e.g.,
DMSO) typically biases the polymerization mechanism toward
3
2
that mediate the degenerate transfer polymerization. Our
observed polymerization induction periods are substantially
33–36
an organometallic radical polymerization reaction manifold.
shorter (t ¼ 1.5 h) than those previously reported (t
ind
¼
ind
3
8
Additionally, the polymerization behavior of cobalt complexes
depends sensitively on the ancillary ligation at the metal center
and may thus be tuned to control the reactivity of any given
3–4 h), which we attribute to differences in monomer
30,37,38
monomer.
Toward the goal of developing vinyl esters as a monomer
platform for the generation of new biodegradable polymeric
materials with widely tunable properties, we demonstrate
3
9
the viability of CMRP using redox initiation for the homo-
polymerization of VPv and VBz and for vinyl ester block
copolymerization to yield well-defined microphase separated
poly(vinyl ester) block copolymers with VAc.
RESULTS AND DISCUSSION
Bulk VAc homopolymerizations mediated by Co(acac)2 at
ꢀ
3
0 C using lauroyl peroxide (LPO) as an initiator and citric
FIGURE 1 Mn versus time (filled symbols) and M /M versus
w
n
acid (CA) or citric acid tribasic sodium salt (Na
reducing agent in ratio [VAc]:[Co]:[LPO]:[Na CA]
39:1.77:1.36:1.00 provide PVAc with M ¼ 2.5–37.5 kg/mol
3
CA) as a
time plots (open circles) for CMRP of VAc with [VAc]:[Co]:[L-
a
¼
PO]:[Na CA] ¼ 339:1.77:1.36:1.00. The dotted line is a linear
3
3
3
regression of the M
period tind ¼ 1.5 h.
n
versus time data indicating an induction
n
and narrow molecular weight distributions (M /M ¼ 1.17–
w
n
CMRP ROUTES TO POLY(VINYL ESTER) BLOCK COPOLYMERS, BUNCK, SORENSON, AND MAHANTHAPPA
243

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
merizations, reflecting the comparable reactivities of the
propagating chain ends in these reactions and the ability of
the Co(III) species to form and to mediate reactions of nucle-
ophilic a-acyloxy carbon-centered radicals. The near inde-
pendence of the polydispersity with increasing conversion
further suggests that polymeryl-Co(III) species are easily
generated and that a rapid degenerate chain transfer equilib-
rium is readily established, thus resulting in controlled
polymerizations.
Studies of bulk VBz homopolymerizations yielded notably
different results from those of the alkyl vinyl esters. Using
[
VBz]:[Co]:[LPO]:[Na CA] ¼ 340:1.77:1.36:1.00, bulk VBz poly-
3
ꢀ
merizations at 30 C proceed with lower levels of control
FIGURE 2 M
/M versus monomer conversion plots (open circles) for
CMRP of VPv with [VPv]:[Co]:[LPO]:[Na
n
versus monomer conversion (filled symbols) and
to produce polymers having M ¼ 4–16 kg/mol and M /M
n
w
n
M
w
n
¼
1.35–1.64 with much shorter induction periods (t
¼
ind
3
CA] ¼ 340:1.76:1.36:1.00.
0.25 h); reproducible polymerization kinetics were obtained
The dotted line reflects a best fit that is intended as a guide to
the eye; see text for details.
only after scrupulously purifying the VBz monomer by
sequential vacuum distillation from CaH and reactive distil-
2
lation from AIBN. While M increases linearly with monomer
n
conversion (Fig. 3), we note that the observed molecular
weights are much larger than those that are theoretically
anticipated based on the level of monomer conversion, initial
monomer concentration, and the Co(acac)2 concentration.
Examination of the ratio Mn,thy/Mn,expt ¼ 0.016–0.024 dem-
onstrates that the initiation efficiency of this system is quite
low and that side reactions shunt a majority of the cobalt(II)
complex into some inactive form as previously observed by
purification protocols and batch-wise variability in VAc
monomer purity. Due to the facile hydrolysis of VAc to acet-
aldehyde and acetic acid, VAc is inherently contaminated
2
1
with a potent aldehyde chain termination agent. Careful
purification of the VAc monomer results in removal of the
acetaldehyde and reduced induction periods. Consistent with
our earlier studies of RAFT polymerizations of vinyl esters,
this finding underscores the need to rigorously purify vinyl
ester monomers to achieve controlled and reproducible poly-
3
9
Bryaskova et al. The relatively higher values of M /Mn
w
1
2
indicate that propagating PVBz chains are less effectively
captured by Co(acac)2 and less well controlled by alkyl-
Co(III) species generated in situ when compared with PVAc
and PVPv chain ends. The observed effects may stem from
the presence of the less electron donating a-benzoyloxy sub-
stituent adjacent to the propagating carbon radical center in
PVBz that: (i) diminishes the efficiency with which poly-
meryl-Co(III) species are produced and (ii) decreases the
propensity of the propagating chain end to undergo nucleo-
philic degenerate transfer reactions with polymeryl-Co(III)
merization behavior.
ꢀ
Bulk CMRP of freshly distilled VPv at 30 C proceeds in
a manner similar to that of VAc. Using [VPv]:[Co]:[LPO]:
[
Na CA] ¼ 340:1.76:1.36:1.00, VPv homopolymerizations
3
yield polymers with M
n
¼ 2.5–27 kg/mol having narrow poly-
dispersities (M /M ¼ 1.20–1.30), the latter of which
w
n
appear to be independent of monomer conversion. Linearity
of the Mn versus monomer conversion plot indicates that
this polymerization is well controlled (Fig. 2). In spite of the
linearity of this plot, we note that the observed molecular
weights are much higher than those theoretically predicted
from the equation
Mn;thy ¼ p½Mꢂ =½CoðacacÞ ꢂ ;
0
2 0
where p is the monomer conversion, [M] is the initial mono-
0
mer concentration, and [Co(acac) ] is the initial cobalt com-
2
0
plex concentration, which assumes that every cobalt complex
regulates the growth of one polymer chain. By examining the
ratio Mn,thy/Mn,expt ¼ 0.030–0.050, we observed that the ini-
tiation efficiency in this system is low; this indicates that a
substantial amount of the cobalt complex may participate in
oxidative side reactions, such as trapping of the carboxy rad-
icals resulting from LPO decomposition to generate inactive
3
9
Co(III) species as previously suggested by Bryaskova et al.
Extrapolation of the corresponding M versus reaction time
FIGURE 3 Mn versus conversion (filled symbols) and M /M
w
n
n
versus monomer conversion plots (open circles) for CMRP of
plot for this polymerization reaction indicates an induction
period tinh ¼ 2.5 h (data not shown). These polymerizations
proceed with a comparable level of control to bulk VAc poly-
VBz with [VBz]:[Co]:[LPO]:[Na CA] ¼ 340:1.77:1.36:1.00. The
3
dotted line is reflects a best linear fit that is intended as a
guide to the eye; see text for details.
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ARTICLE
ester, we hypothesized that the reactivity of the propagating
radical toward the cobalt complex would increase to provide
better polymerization control and narrower polydispersities.
ꢀ
These polymerizations instead proceeded sluggishly at 30
C
and yielded only small amounts of polymer after 77.5 h with
Mn ¼ 2.3 kg/mol and M /M ¼ 2.11. We attribute the
w
n
observed reactivity to the previously observed slow homopoly-
ꢀ
merization of MeOVBz at 30 C and the dilution of the reaction
medium with C
44
6 6
H required for solution polymerization.
Sequential CMRP block copolymerizations of VAc with
2
7,33
45
45
acrylic,
styrene, and olefin monomers have been pre-
SCHEME 2 CMRP block copolymerization of VAc with VBz and
viously reported without chain end transformations, whereby
an air-sensitive PVAc-Co(acac)2 macroinitiator was isolated
by removal of excess monomer in vacuo, followed by intro-
duction of a second monomer to initiate polymerization of a
second block. Building on this foundation, we explored the
sequential block copolymerization of VAc with VPv and VBz
by CMRP as an alternative to RAFT polymerization syntheses
of reported poly(vinyl ester) block copolymers. Due to their
higher volatilities, either VAc or VPv may be used to synthe-
size an initial homopolymer block; however, the low volatil-
ity of VBz limits its ability to be removed in vacuo for chain
extension reactions. Consequently, VBz was typically incorpo-
rated into block copolymers by CMRP as the final block.
VPv.
2
6
chain transfer agents via a putative dialkyl-Co(IV) species.
The confluence of these effects reduces the observed induc-
tion times and gives rise to uneven initiation and propaga-
tion of the polymer chains, leading to broader polydisper-
sities. The lower degree of control observed in the CMRP of
the less electron donating VBz monomer is consistent with
previous reports of difficulties associated with the polymer-
4
0
ization of vinyl chloroacetate. In view of mechanistic stud-
3
2
ies of CMRP, these findings suggest that these relatively
electron poor vinyl ester monomers do not effectively
undergo controlled CMRP in the absence of coordinating
ligands for cobalt. Similar effects in RAFT homopolymeriza-
tions of VBz and VClAc have been observed and were attrib-
uted to the reduced electron donating character of the prop-
agating PVBz chain end that reduces the rate of chain
transfer by a combination of slow addition to the RAFT agent
and slow fragmentation of the tertiary carbon-centered radi-
PVAc-b-PVBz diblock copolymers may be synthesized by a
two-step sequential monomer addition polymerization proce-
dure (Scheme 2 and Table 1). Bulk homopolymerization of
VAc with [VAc]:[Co]:[LPO]:[Na CA] ¼ 339:1.77:1.36:1.00 at
3
ꢀ
30
C for 4.2 h followed by removal of the monomer
in vacuo yielded a PVAc-Co(acac)
transfer agent with M
Redissolution of the cobalt end-capped macromolecular
2
macromolecular chain
/M
n
¼ 8.79 kg/mol and M
w
n
¼ 1.22.
1
2,41
cal intermediate.
previously termed ‘‘hybrid behavior’’ in the RAFT literature
This type of behavior has also been
ꢀ
chain transfer agent in bulk VBz and warming to 30
C
4
2
as observed by Barner-Kowollik et al. in the dithioben-
zoate-mediated RAFT polymerization of methyl methacrylate
and by Klumperman and coworkers in xanthate-mediated
resulted in the formation of a PVAc-b-PVBz diblock copoly-
mer, which was quenched by addition of a solution of
TEMPO radical after 5.75 h to mitigate oxidative chain cou-
4
3
N-vinyl pyrrolidone RAFT polymerizations.
1
pling reactions upon exposure to air. Quantitative H NMR
In an attempt to gain further insights into the problems
encountered in the CMRP of VBz, we studied the homopoly-
merization of vinyl 4-methoxybenzoate (MeOVBz) with
spectroscopy of this PVAc-b-PVBz-1 using the PVBz methine
(d 5.20 ppm) and the PVAc methine (d 4.85 ppm) resonan-
ces indicates that the polymer composition is 48.9 mol %
VAc (Fig. 4). SEC analyses [Fig. 5(a) ] demonstrate that this
[
MeOVBz]:[Co]:[LPO]:[Na CA]
¼
341:1.78:1.37:1.00 with
3
[MeOVBz] ¼ 1.6 M in C H . Based on the electron donating
protocol yields a unimodal diblock copolymer with Mn,NMR
¼
6
6
character of the methoxy substitutent of the aromatic vinyl
24.6 kg/mol and M /Mn ¼ 1.74, based on the absolute
w
TABLE 1 Poly(vinyl ester) Block Copolymers Synthesized by Sequential CMRP
Initial Block
Diblock Copolymer
a
b
c
b
c
d
Entry
M
n
(kg/mol)
M
w
/M
n
Mn,total (kg/mol)
M
w
/M
n
Composition (mol % VAc)
Morphology
PVAc-b-PVBz-1
PVAc-b-PVBz-2
PVAc-b-PVPv-1
a
8.79
15.6
19.3
1.22
1.42
1.35
24.4
36.9
1.74
1.61
1.32
0.49
0.56
0.19
dis
dis
141
unknown
M
n
from SEC analysis with Mark-Houwink corrected polystyrene standards.
Obtained from SEC analysis relative to polystyrene standards.
n,total was obtained using the degree of polymerization of the PVAc block from SEC analysis and the polymer composition determined from quan-
b
c
M
1
titative H NMR.
d
From SAXS measurements at 150 ꢀC; dis ¼ disordered.
CMRP ROUTES TO POLY(VINYL ESTER) BLOCK COPOLYMERS, BUNCK, SORENSON, AND MAHANTHAPPA
245

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1
FIGURE 4 H NMR spectrum of
PVAc-b-PVBz-1 with [VAc] ¼ 48.9
mol % synthesized by sequential
CMRP.
molecular weight of the PVAc starting block and the polymer
composition. The somewhat broad polydispersity of this
block copolymer likely originates in the low propensity of
the PVBz chain end to undergo degenerate transfer as
observed in VBz homopolymerizations (vide supra).
the PVPv block is synthesized first with subsequent VAc
chain extension to produce a diblock copolymer with narrow
4
6
polydispersity (M /M ¼ 1.35). These data taken together
w
n
demonstrate a high degree of blocking efficiency irrespective
of the order of monomer addition for these alkyl vinyl ester
CMRP block copolymerizations.
Sequential block copolymerizations of VAc and VPv similarly
furnish unimodal PVAc-b-PVPv diblock copolymers [Table 1
and Fig. 5(b)]. Molecular parameters for a representative
diblock copolymer produced by sequential CMRP are listed
in Table 1. Notably, very high molecular weight alkyl vinyl
ester block copolymers may be produced with relatively nar-
row polydispersities. Note that we have recently reported
the CMRP block copolymerization of PVPv-b-PVAc, in which
Block copolymers comprised of chemically incompatible
homopolymer segments microphase separate into ordered
nanostructures that balance polymer chain stretching en-
tropy against unfavorable interblock repulsions, the morphol-
ogies of which may be readily assigned by small-angle X-ray
4
7,48
scattering (SAXS).
Results of SAXS analyses of diblock
copolymers produced by sequential CMRP block copolymer-
izations are listed in Table 1. PVAc-b-PVPv-1 clearly micro-
phase separates with a modest degree of long range order at
ꢀ
1
50 C as discerned from the broad scattering maxima
(
Fig. 6), which likely originate from a combination of sample
polydispersity and poor ordering due to the high molecular
FIGURE 5 SEC trace overlays for poly(vinyl ester) diblock
copolymers synthesized by sequential CMRP reactions under
FIGURE 6 Azimuthally integrated intensity profile as a function
ꢃ1
redox initiation conditions: (a) PVAc-Co(acac)
chain transfer agent and the resulting diblock copolymer PVAc-
b-PVBz-1, and (b) PVAc-Co(acac) macromolecular chain trans-
fer agent and the resulting diblock copolymer PVPv-b-PVAc-1.
2
macromolecular
of scattering wavevector q ( A˚ ) of an isotropic two-dimensional
ꢀ
lab source SAXS pattern of PVAc-b-PVPv-1 at 150 C. Filled sym-
2
bols indicate the expected positions associated with a tenta-
tively assigned body-centered cubic spheres morphology.
246
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
weight of the polymer. Based on the position of the principal
scattering maxima observed in the azimuthally integrated
SAXS pattern, this poorly ordered microphase separated
morphology exhibits a principal spacing d* ¼ 2p/q* ¼
Synthesis of Vinyl 4-methoxybenzoate
This preparation is an optimized version of a previously
4
4
reported procedure: In a 500 mL three-necked flask fitted
with a reflux condenser and a thermometer, 4-methoxyben-
zoic acid (24.40 g, 0.160 mol), mercury(II) acetate (1.46 g,
4.58 mmol), and VAc (94 mL, 1.02 mol) were suspended in
36.5 nm. SAXS patterns for PVAc-b-PVBz diblock copolymers
with nearly symmetric molar compositions exhibit only low
ꢀ
ꢀ
intensity correlation-hole scattering at 150 C consistent
CHCl3 (80 mL) and refluxed at 75 C for 8 h. The reaction
4
9
with a homogeneous melt. These observations are consist-
ent with prior work on microphase separated poly(vinyl
ester) block copolymers, which indicate that PVAc-b-PVBz
block copolymers remain homogeneous in the melt at mod-
est molecular weights while PVAc/PVPv block copolymers
mixture was then diluted with hexanes (80 mL), cooled to
room temperature, and washed with a saturated NaHCO (aq)
3
(3 ꢅ 50 mL). The organic layer was dried over MgSO , and the
4
solvent was removed in vacuo. The resulting white solid was
recrystallized from anhydrous EtOH and dried. Yield: 30.1%
1
2
1
ꢀ
are quite strongly melt-segregated.
(8.64 g). H NMR: (300 MHz, CDCl , 22 C): d (ppm) 8.07 (dt,
3
3
3
JH-H ¼ 9.0, 2.8 Hz, 2H, ArAH), 7.51 (dd, J
¼ 13.9, 6.3 Hz,
HAH
3
EXPERIMENTAL
1H, H C ¼¼ CHA), 6.95 (dt, J
¼ 9.0, 2.8 Hz, 2H, ArAH), 5.04
2
3
HAH
3
(
dd, J
¼ 14.1, 1.7 Hz, 1H, H C ¼¼ CHA), 4.67 (dd, J ¼ 6.3,
HAH
2
H-H
Materials
1
.5 Hz, 1H, H C ¼¼ CHA), 3.88 (s, 3H, AOCH ).
2
3
All reagents were purchased from Sigma-Aldrich Chemical
Co. (Milwaukee, WI) and used as received unless otherwise
noted. Vinyl acetate (VAc) and VPv were distilled at ambient
Representative Synthesis of Poly(vinyl pivalate)
LPO (135.5 mg, 0.34 mmol) and Co(acac) (113.7 mg, 0.442
2
pressure from CaH
2
to remove inhibitors and other impur-
mmol) were combined in a dry Schlenk flask equipped with
ities. VBz was purified by the previously described two-step,
a stirbar under nitrogen. A combination of Na CA (73.5 mg,
3
1
2
0
2,2 -Azobis(2-methylpro-
reactive purification procedure.
0.25 mmol) and the monomer (12.6 mL, 85 mmol) were
pionitrile) (AIBN) was recrystallized from methanol. LPO
was recrystallized from hexanes and dried under vacuum.
Co(acac) ꢄ2H O (Alfa Aesar) was dehydrated by recrystalli-
degassed by three freeze-pump-thaw cycles in a second dry
Schlenk flask equipped with a stirbar. The monomer-citric
acid suspension was then cannula transferred to the flask
2
2
zation from acetone and subsequent drying in vacuo. Citric
2
containing the Co(acac) /LPO to initiate polymerization at
acid tribasic sodium salt (Na CA) and citric acid (CA) were
ꢀ
3
30 C. Reaction aliquots were withdrawn by gas-tight syringe
at regular intervals and diluted with hexanes, followed by
rotary evaporation of the solvent and monomer, and freeze-
drying the resulting solids from C H as previously
ꢀ
recrystallized from water and dried at 110 C. 2,2,6,6-Tetra-
methylpiperidine-N-oxyl (TEMPO) was purified by sublima-
tion at reduced pressure. Mercury(II) acetate (Fisher, 99.7%)
was used as received. C H was vacuum transferred from
6
6
12
6
6
reported. Typical monomer conversions were less than 5%.
Na/benzophenone and stored under nitrogen.
PVBz polymers were precipitated into hexanes at least once
to remove residual monomer and were subsequently freeze-
All 1H NMR spectra were acquired in CDCl3 on a Bruker
ACþ 300 MHz spectrometer and were referenced to the re-
sidual protiated solvent peak.
dried from C
6 6
H . Typical monomer conversion was limited to
less than 2%.
Size Exclusion Chromatography (SEC)
Representative Diblock Copolymer Synthesis:
PVAc-b-PVBz-1
Co(acac)2 (113.8 mg, 0.442 mmol) was combined with LPO
SEC analyses were performed on a Viscotek GPCMax System
equipped with two Polymer Labs Resipore columns (300 ꢅ
7
.5 mm) and a differential refractometer, light scattering
(
136 mg, 0.341 mmol) in a Schlenk flask under nitrogen. In
ꢀ
ꢀ
module (7 - and 90 -angle detection), and a differential vis-
cometer; tetrahydrofuran was used as a mobile phase at a
elution rate of 1.0 mL/min at 30 C. For PVAc, number aver-
a separate flask, a suspension of CA (65.3 mg, 0.340 mmol)
in freshly distilled VAc (7.9 mL, 85.7 mmol) was prepared
and degassed by three freeze-pump-thaw cycles. The CA/VAc
slurry was cannula transferred onto the Co(acac) /LPO mix-
ture, and this reaction mixture was heated to 30 C to initiate
ꢀ
n w n
age molecular weights (M ) and polydispersities (M /M )
2
were obtained using a Mark-Houwink corrected poly(sty-
ꢀ
1
2
rene) calibration curve as previously described. Absolute
molecular weights for PVPv and PVBz were determined by
SEC with light scattering detection using previously reported
the polymerization. After 5.8 h, the reaction was removed from
the heating bath and cooled under nitrogen. The cobalt-end-
capped PVAc was isolated by removal of the excess VAc in vacuo
to yield a solid. Size exclusion chromatography analysis of a
reaction aliquot showed M ¼ 8.7 kg/mol and M /M ¼ 1.21.
dn/dc values, while M /M for these samples are reported
w
n
1
2
relative to PS Standards.
n
w
n
SAXS
The resulting PVAc-Co(acac) macromolecular chain transfer
2
Laboratory source SAXS patterns were acquired in the Mate-
rials Science Center at the University of Wisconsin–Madison
agent was redissolved in degassed, freshly distilled VBz
(10.3 mL, 74.4 mmol), and the reaction was re-heated to
30 C for 11.8 h. The polymerization was cooled to room
ꢀ
using a Rigaku SMAX-3000 employing Cu K
a
X-rays and a
Gabriel X-ray detector with a 2.015 m sample-to-detector
distance. SAXS patterns acquired at elevated temperatures
employed a Linkam temperature stage.
temperature, stirred with TEMPO (181 mg, 1.16 mmol) in
THF (17 mL) and exposed to air. The resulting copolymer
was then precipitated into cold hexanes (300 mL) to yield
CMRP ROUTES TO POLY(VINYL ESTER) BLOCK COPOLYMERS, BUNCK, SORENSON, AND MAHANTHAPPA
247

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
a PVAc-PVBz copolymer. Using SEC, the polydispersity of
the resultant PVAc-PVBz copolymer was determined to be
8 Moad, G.; Rizzardo, E.; Thang, S. H. Polymer 2008, 49,
1079–1131.
1
M /M ¼ 1.74 (against PS Standards). Quantitative H NMR
w
n
9 Jitchum, V.; Perrier, S. Macromolecules 2007, 40, 1408–1412.
analysis shows that this polymer contains 51.1 mol % VBz
1
0 Destarac, M.; Charmot, D.; Franck, X.; Zard, S. Z. Macromol
and the polymer has a calculated Mn,total ¼ 24.4 kg/mol.
Rapid Commun 2000, 21, 1035–1039.
Synthesis of Poly(vinyl 4-methoxybenzoate)
1
1 Stenzel, M. H.; Cummins, L.; Roberts, G. E.; Davis, T. P.; Vana,
To a Schlenk flask equipped with a stirbar, vinyl 4-methoxy-
benzoate (2.51 g, 14.0 mmol), Co(acac)2 (18.6 mg, 0.073
mmol), lauoryl peroxide (22.2 mg, 0.056 mmol), and citric
acid tribasic sodium salt (10.7 mg, 0.041 mmol) were added.
On a vacuum line, anhydrous C H (8.8 mL) was added to
P.; Barner-Kowollik, C. Macromol Chem Phys 2003, 204,
1160–1168.
1
2 Lipscomb, C. E.; Mahanthappa, M. K. Macromolecules 2009,
4
2, 4571–4579.
6
6
1
3 Gu, Y.; He, J.; Li, C.; Zhou, C.; Song, S.; Yang, Y. Macromo-
the reaction mixture under nitrogen and it was stirred at
ꢀ
lecules 2010, 43, 4500–4510.
30
C. After 77.5 h, the reaction was exposed to air and
poured into hexanes to yield a small amount of solid. M ¼
14 Wakioka, M.; Baek, K.-Y.; Ando, T.; Kamigaito, M.; Sawa-
n
2
.3 kg/mol, M /M ¼ 2.11.
moto, M. Macromolecules 2002, 35, 330–333.
w
n
1
5 Koumura, K.; Satoh, K.; Kamigaito, M. Macromolecules
CONCLUSIONS
2
008, 41, 7359–7367.
CMRPs of VPv and VBz were explored to extend this useful
degenerate chain transfer polymerization methodology to the
synthesis of well-defined vinyl ester block copolymers by
sequential chain extension reactions. Bulk homopolymeriza-
tion studies revealed that alkyl vinyl esters such as VAc and
VPv have comparable reactivities and their CMRP proceeds
with a high degree of control to yield polymers with Mw/
1
6 Ober, C. K.; Cheng, S. Z. D.; Hammond, P. T.; Muthukumar,
M.; Reichmanis, E.; Wooley, K. L.; Lodge, T. P. Macromolecules
2
009, 42, 465–471.
1
7 Schreck, K. M.; Hillmyer, M. A. J Biotechnol 2007, 132,
2
87–295.
18 Chiellini, E.; Corti, A.; D’Antone, S.; Solaro, R. Prog Polym
M ꢁ 1.3, whereas CMRP of VBz resulted in less controlled
Sci 2003, 28, 963–1014.
n
polymerization with higher polydispersities M
w
/M
n
ꢁ 1.7.
1
9 Matsumura, S. Biopolymers 2003, 9, 329–361.
Sequential block copolymerizations of VAc with either VPv
or VBz yield unimodal block copolymers with variable mo-
lecular weights that may microphase separate in the melt.
These findings demonstrate that CMRP is a convenient and
complementary synthetic method to RAFT for the (co)poly-
merization of commercially available vinyl ester monomers.
Investigations of the mechanical properties of these new
block copolymers are ongoing and will be reported in due
course.
2
0 Heller, C.; Schwentenwein, M.; Russmueller, G.; Varga, F.;
Stampfl, J.; Liska, R. J Polym Sci Part A: Polym Chem 2009, 47,
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Financial support from the University of Wisconsin–Madison
and from a National Science Foundation CAREER Award (DMR-
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006, 44, 6147–6158.
0748503) is gratefully acknowledged. This research also made
24 Koumura, K.; Satoh, K.; Kamigaito, M.; Okamoto, Y. Macro-
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