Demonstration of isospecific free radical polymerization of acrylate controlled by conformation and chirality of monomer

Article abstract of DOI:10.1002/pola.27649

Radical polymerization of lactic acid-based chiral and achiral methylene dioxolanones, a model for conformationally s-cis locked acrylate, was carried out with AIBN to demonstrate an isospecific free radical polymerization controlled by chirality and conformation of monomer. Polymerization of the dioxolanones proceeded smoothly without ring opening to give a polymer with moderate molecular weight and 100% of maximum isotacticity. ESR spectrum indicated a twisted conformation of the growing poly(methylene dioxolanone) radical in contrast to an acyclic analogous radical, suggesting a restriction of the free rotation around main chain C_αC_β bond of the growing radical center. Chirality as well as the polarity and bulkiness of monomer affected the polymer tacticity, and chiral alkyl substituent would afford a high isotactic polymer, in which higher the enantiomeric excess of the monomer was, higher the isotacticity of the polymer was. While, achiral or polar substituents including dibenzyl and trichloromethyl groups would afford an atactic polymer. In addition, glass transition temperature (T_g) of the resulting polymers was significantly high, ranging from 172.2 to 229.8°C, and even for an isotactic polymer T_g was as high as 206.8°C.

Full text of DOI:10.1002/pola.27649

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
Demonstration of Isospecific Free Radical Polymerization of Acrylate
Controlled by Conformation and Chirality of Monomer
Hitoshi Tanaka,1 Yoshitaka Matsubara,2 Katsuhiko Kusunoki,2 Naoki Saito,2
Tatsuya Kibayashi2
1Advanced Materials, Institute of Technology and Science, University of Tokushima, 2-1 Minamijosanjima-cho
Tokushima 770–8506, Japan
2Systems Innovation Engineering, Graduate School of Advanced Technology and Science, University of Tokushima,
2-1 Minamijosanjima-cho, Tokushima 770–8506, Japan
Correspondence to: H. Tanaka (E-mail: tanaka@tokushima-u.ac.jp)
Received 3 February 2015; accepted 24 March 2015; published online 11 April 2015
DOI: 10.1002/pola.27649
ABSTRACT: Radical polymerization of lactic acid-based chiral
and achiral methylene dioxolanones, a model for conforma-
tionally s-cis locked acrylate, was carried out with AIBN to
demonstrate an isospecific free radical polymerization con-
trolled by chirality and conformation of monomer. Polymeriza-
tion of the dioxolanones proceeded smoothly without ring
opening to give a polymer with moderate molecular weight
and 100% of maximum isotacticity. ESR spectrum indicated a
twisted conformation of the growing poly(methylene dioxola-
none) radical in contrast to an acyclic analogous radical, sug-
gesting a restriction of the free rotation around main chain
CaACb bond of the growing radical center. Chirality as well as
the polarity and bulkiness of monomer affected the polymer
tacticity, and chiral alkyl substituent would afford a high isotac-
tic polymer, in which higher the enantiomeric excess of the
monomer was, higher the isotacticity of the polymer was.
While, achiral or polar substituents including dibenzyl and tri-
chloromethyl groups would afford an atactic polymer. In addi-
tion, glass transition temperature (Tg) of the resulting polymers
was significantly high, ranging from 172.2 to 229.8 ꢀC, and
even for an isotactic polymer Tg was as high as 206.8 ꢀC.
C
V
2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2015, 53, 2007–2016
KEYWORDS: radical polymerization; isotactic; stereospecific
polymers
cific reaction as is known in the polymerization of chiral
acrylamidosulfonic acids which generate essentially a ran-
dom polymer,11 a chiral moiety attached on the propagating
radical center can sometimes induce isospecific polymeriza-
tion indeed. In such a chiral system achiral sp2 planar configu-
lation of the radical center on the propagating species seems
to be locked into a sp3 pyramidal specific configulation after
the next monomer addition. This situation is in accord with
anionic or coordination polymerizations where the active cen-
ter is sp3 pyramidal and therefore has essentially chirality.
Participation of the chilarity in the stereoregulation has also
been known in the polymerization of achiral N-arylmethyl
methacrylamides12 and arylmethyl methacrylates13 in chiral
solvents, in which high isotactic polymers have successfully
been obtained in the presence of chiral menthols.
INTRODUCTION Stereocontrol including tacticity regulation
in free radical polymerization has particularly attracted
much attention not only in applications but also academic
aspects1–5 since most polymers formed by radical polymer-
ization have rather an atactic structure, for example, proba-
bility of mesoaddition (Pm) 5 0.4–0.5 for vinyl monomers
and Pm 5 0.3–0.5 for 1,1-disubstituted monomers,6 and has
some potential advantages, for example, high accessibility for
a number of monomers and solvents, high reproducibility,
and easy procedures. Unfortunately, however, it has not been
yet achieved to control precisely the polymer tacticity and
helicity in free radical polymerization in contrast to an ionic
polymerization. Porter and coworkers reported that the
penultimate unit of the radical center effects on the configu-
ration of the ultimate center in the telomerization giving
trimer and so forth.7,8 In addition, telomerization of chiral
pyrrolidine acrylamide would give two stereoisomers with S
and R configurations for the attack of amide radical in large
excess of one isomer to the other.9,10 Although the acryla-
mides with chiral center does not always lead to a stereospe-
Recently, we found that the acrylates including (2)-menthyl
2-acetamidoacrylate would polymerize stereospecifically
depending on the s-cis and s-trans conformations of the
monomers, so-called conformation controlled radical
C
V
2015 Wiley Periodicals, Inc.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 2007–2016
2007

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
polymerization (CCRP).14,15 In the radical polymerization of
the s-cis conformer would preferentially polymerize rather
than the s-trans one to give an isotactic helical polymer,15 that
is, the triad tacticity of the resulting polymer was mm/mr/
with pivaldehyde and following vinylization of resulting
cyclic dioxolanone according to the literature.23 Cyclization
was carried out by refluxing a mixture of lactic acid 23.5 g
(0.261 mol) and pivaldehyde 28.6 g (0.332 mol) in n-pentane
30 mL for 5 hr in the presence of p-toluenesulfonic acid
0.80 g (4.6 mmol) with removing water using Dean-Stark
apparatus. After reaction, the mixture was diluted with ether
and washed with H2O, dried over MgSO4, and the solvent
evaporated. This condensation reaction gave a mixture of cis
and trans dioxolanones, which are diastereomers each other,
with the cis isomer always predominating. Isolation by silica
gel column chromatography using n-hexane/ethylacetate (20/
1 vol.-ratio) as an eluant furnished the cis- and trans-
dioxolanones, that is, (S)- and (R)22-tert-butyl-(S)25-methyl-
1,3-dioxolan-4-ones which produced the S and R configula-
tional isomers (1S and 1R), respectively, after vinylization.
Resulting dioxolanones were readily transformed to the
required olefins by vinylization, that is, bromination with N-
bromosuccinimide (NBS) and AIBN followed by dehydrobro-
mination with triethylamine. Concretely, vinylization was
achieved by dropping triethylamine 10.8 g (0.107 mol) diluted
in benzene 15 mL into brominated dioxolanone 15.1 g (63.4
mmol) dissolved in benzene 50 mL over 1 hr, and further
reaction at ambient temperature for 1.5 hr, in which bromi-
nated dioxolanone was independently obtained by refluxing a
mixture of cis- or trans-dioxolanone 11.5 g (72.5 mmol), NBS
14.1 g (79.2 mmol), and AIBN 43.7 mg (0.266 mmol) in CCl4
40 mL for 4 hr. After dehydrobromination, resulting salt and
benzene were removed by filtration and evaporation, respec-
tively, the crude methylene dioxolanone was purified by distil-
lation under reduced pressure: bp 41 ꢀC/1.5 mmHg. 1S was
obtained as a colorless liquid in 45.2% yield: [a]D 214.3ꢀ in
CHCl3. 1H NMR (400 MHz, CDCl3, d): 0.98 (s, 9H, C(CH3)3),
4.86 (d, J 5 2.8 Hz, 1H, CH@), 5.14 (d, J 5 2.8 Hz, 1H, CH@),
5.44 (s, 1H). IR (neat): m 5 1806 (C@O), 1669 (C@C).
ꢀ
rr5 28.3/49.2/22.5 at 30 C and 43.6/41.8/14.6 near Tc (70
ꢀC in benzene in [monomer] 5 2.0mol.L21).16 This can be
explained by an assumption of two Tc of the acetamidoacry-
late and in this case the homopolymerization is just like a
copolymerization of s-cis and s-trans conformers, in which s-
cis conformer has the Tc value higher than s-trans one. These
results obtained indicate that the conformation as well as the
configulation including chirality of monomer should also play
an important role in the stereoregulation.
On the other hand, bio-based material has recently been
interested as an alternative material of the petroleum-based
material because of the renewability, accessibility, and sus-
tainability.17,18 The chirality, in particular, is the most useful
characteristic of the bio-based materials, and, therefore, the
bio-products are not only a replacement of the petroleum-
based materials. Among bio-based materials, lactic acid is
favorable as a bio-based chiral resource because of easy
mass production, harmless, and low-cost material.19,20
The results so far suggest that both chirality and conformation
of acrylate monomer are key parameters to construct
a polymer at any desired tacticity although the achiral
methacrylates with bulky ester substituents including 1-
phenyldibenzosuberyl moieties produce high isotactic poly-
mers,21,22 and it is informative to consider how tacticity arizes
in terms of the mechanism for radical propagation. In this
article, we synthesized the chiral acrylates with s-cis locked
structure, that is, (S)- and (R)22-tert-butyl and 2-isopropyl-5-
methylene-1,3-dioxolan-4-ones (1S, 1R, and 2S, 2R, respec-
tively), as well as the achiral and chiral s-cis locked acrylates
3, 4, and 5 using a renewable lactic acid as a chiral resource,
and demonstrated an isospecific free radical polymerization in
conventional reaction conditions except near Tc.
Compound 2 was synthesized according to the manner similar
to 1. Cyclization was carried out by refluxing a mixture of lac-
tic acid 33.8 g (0.375 mol) and isobutylaldehyde 37.8 g
(0.524 mol) in benzene 50 mL for 8 hr in the presence of p-
toluenesulfonic acid 1.0 g (5.8 mmol) with removing water
using Dean-Stark apparatus. After reaction, the mixture was
neutralized with NaHSO3 aqueous solution, dried over MgSO4,
and the solvent evaporated. The reaction mixture was distilled
to give 2-isopropyl-(S)25-methyl-1ꢀ,3-dioxolan-4-one 49.3 g
(0.342 mol) in 91.2% yield: bp 55 C/3 mmHg. The product
contained both cis and trans dioxolanones with the cis isomer
always predominating. Isolation by silica gel column chroma-
tography using n-hexane/ethylacetate (20/1 vol.-ratio) as elu-
ent furnished the cis- and trans-dioxolanones, that is, (S)- and
(R)22-isopropyl-(S)25-methyl-1,3-dioxolan-4-ones which pro-
duced the S and R configulational isomers of methylene dioxo-
lanone, respectively, after vinylization. Resulting dioxolanones
were readily transformed into the required olefins by vinyliza-
tion, that is, bromination with NBS and AIBN followed by
dehydrobromination with triethylamine. Concretely, vinyliza-
tion was achieved by dropping triethylamine 14.1 g (0.139
EXPERIMENTAL
Materials
1 was synthesized mainly by two steps, that is, cyclization of
lactic acid, for example, L-lactic acid and ferment lactic acid,
2008
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 2007–2016

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
mol) diluted in benzene 20 mL into brominated dioxolanone
14.08 g (62.5 mmol) dissolved in benzene 50 mL over 1 hr,
and further reaction at ambient temperature for 1.5 hr, in
which brominated dioxolanone was independently obtained
by refluxing the dioxolanone 10.0 g (69.4 mmol), NBS 18.2 g
(0.102 mol), and AIBN 37.0 mg (0.226 mmol) in CCl4 100 mL
for 4 hr. After dehydrobromination, resulting salt and benzene
were removed by filtration and evaporation, respectively, then
the crude product was purified by distillation under reduced
pressure to give 2-isopropyl-5-methylene-1ꢀ,3-dioxolan-4-one
(2) 4.3 g (30.2 mmol) in 43.5% yield: bp 65 C/8 mmHg.
was easily isolated by recrystallization from n-hexane, and
it gave 5 with R configuration at the 2 position on the ring
by vinylization in 56.3% yield. The single isomer 5 with R
configuration was white solid and it was used for the poly-
merization: mp 62–63 ꢀC. 1H NMR (400 MHz, CDCl3, d):
5.16 (d, J 5 2.4 Hz, 1H, CH@), 5.33 (d, J 5 2.4 Hz, 1H,
CH@), 6.02 (s, 1H, CH). IR (KBr): m 5 1810 (C@O), 1673
(C@C). Methyl 2-methoxyacrylate (MMOA) was prepared
according to the previous method 25: bp 58–59 ꢀC/
13mmHg. 2,2’-Azobis(isobutyronitrile) (AIBN) used as an
initiator was purified by recrystallization from ethanol.
Deuterated solvents including D2O and CDCl3 used for
NMR measurement were purchased from Acros Organics.
Other basic chemicals were purchased from Wako Chemi-
cals and Aldrich.
Compound 2 was obtained as a colorless liquid: [a]D 213.2
and 113.8ꢀ for S- and R-forms, respectively, in CHCl3. 1H
NMR (400 MHz, CDCl3, d): 0.99 (d, J 5 0.8Hz, 3H), 1.01
(d, J 5 0.8Hz, 3H), 2.04 (m, 1H), 4.85 (d, J 5 2.8Hz, 1H, CH@),
5.14 (d, J 5 2.8Hz, 1H, CH@), 5.60 (d, J 5 4.4Hz, 1H). 13C
NMR (100 MHz, CDCl3, d): 14.97, 32.95, 90.93, 107.60,
144.10, 162.48. IR (neat): m 5 1806 (C@O), 1669 (C@C).
Polymerization
Polymerization was carried out in bulk in a sealed ampoule
with shaking at given temperature. The ampoule containing
required amounts of reagents including initiator and monomer
was degassed several times by a freeze-thaw method and then
sealed under reduced pressure and placed in a constant tem-
perature bath. The resulting polymer was isolated by pouring
the contents of the ampoule into a large amount of methanol.
Compound 3 was prepared by cyclization of b-chlorolactic
acid, which was accessible from the oxidation of 3-chloro-
1.2-propanediol using nitric acid, with cyclohexanone and
following dehydrochlorination of resulting cyclic dioxolanone
according to the procedure similar to Bailey.24 Cyclization
was carried out by refluxing a mixture of b-chlorolactic acid
6.0 g (48.4 mmol) and cyclohexanone 5.0 g (50.9 mmol) in
toluene 30 mL for 8 hr in the presence of p-toluenesulfonic
acid 0.2 g (1.2 mmol) with removing water using Dean-Stark
apparatus. After reaction, the mixture was neutralized with
NaHCO3 aqueous solution, extracted with toluene, dried over
MgSO4, and finally the solvent was evaporated. Distillation of
Hydrolysis
Polymer was hydrolyzed with KOH. A solution containing
polymer 0.2 g and KOH 1.0 g in tetrahydrofuran 30 mL was
stirred over 1 day at ambient temperature. The resultant
solution after removal of tetrahydrofuran by evaporation was
poured into methanol to remove unreacted KOH, and result-
ing hydrolyzed polymer, that is, poly(potassium 1-hydroxya-
crylate) was dried under vacuum.
this
condensed
residue
gave
5-chloromethyl-2,2-
pentamethylene-1,3-dioxolane-4-one (bp 84.0 ꢀC/1.5mmHg)
in 65.6% yield. Resulting dioxolanone 6.5 g (31.8 mmol) was
transformed to the required olefin by dehydrochlorination
using diisopropylamine 10.0 mL (71.2 mmol) in toluene
30 mL at 75 ꢀC for 5 hr. After reaction, the precipitated
ammonium salts were removed by filtration, and the solvent
in the remaining solution was evaporated, then the crude 3
was distilled under reduced pressure (bp 62 ꢀC/1.5mmHg)
to give pure 3 in 79.6% yield.
Measurements
1H NMR spectrum was recorded on JEOL EX-400 (400 MHz)
spectrometer in CDCl3 at 20 ꢀC for monomer and 40 ꢀC for
polymer using tetramethylsilane as an internal standard. 13C
NMR spectrum of poly(methylꢀene dioxolanone) at 100 MHz
was measured in CDCl3 at 40 C on JEOL EX-400 spectrome-
ter under conditions of full proton decoupling in 5 mm tube.
For a hydrolyzed polymer, the spectrum was measured in
KOH/D2O solution at 80 ꢀC using dimethylsulfoxide as an
internal standard. Typical conditions in 13C NMR spectrum
measurements were as follows: sweep width: 20,000 Hz, data
point: 32,000, pulse angle: 90ꢀ, pulse delay: 2.46 s, and accu-
mulation: 50,000 scans. ESR spectrum was measured using
1H NMR (400 MHz, CDCl3, d): 1.27–1.75 (m, 10H, CH2), 4.66
(d, J 5 2.4 Hz, 1H, CH@), 4.95 (d, J 5 2.4 Hz, 1H, CH@). IR
(neat): m 5 1800 (C@O), 1671 (C@C).
Compound 4 was synthesized using dibenzyl ketone instead
of cyclohexanone in the manner similar to 3 in 45.7% yield.
4 was white solid: mp 52–53 ꢀC. 1H NMR (400 MHz, CDCl3,
d): 3.18 (s, 4H, CH2), 4.51 (d, J 5 2.4 Hz, 1H, CH@), 4.67 (d,
J 5 2.4 Hz, 1H, CH@), 7.20–7.31 (m, 10H, C6H5). IR (KBr): m
5 1796 (C@O), 1665 (C@C).
ꢀ
AIBN as an initiator at 60 C and recorded on JOEL JES-RE1X
spectrometer operating at X-band (9.5 GHz) with a TE mode
cavity. Splitting constant a and g value were determined by
comparison with those of Mn21. FTIR spectrum was recorded
in neat, KBr, or film at ambient temperature on JASCO FT/IR-
8900 spectrometer. Diffraction measurement was carried out
Compound 5 was prepared by the condensation of chloral
and lactic acid followed by vinylization of resulting cyclic
dioxolanone in the manner similar to 1. In the condensa-
tion, cis and trans diastereomers were obtained in the
ratio of cis/trans541.1/58.9. In this case, trans isomer
on
a
Rigaku Multiflex diffractometer with graphite-
monochromaꢀted CuKa (k 5 1.542 Å) radiation. Datum was col-
lected at 23 C. Specific rotation [a]D at k 5 589.3 nm (Na-D)
was measured on JASCO P-1030 digital polarimeter in CHCl3.
Refractive index of polymer film at k 5 589.3 nm was
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 2007–2016
2009

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 1 SEC curve of poly(2) prepared by polymerization of
2 (ee 5 0%) with AIBN at 60 ꢀC in bulk for 3 hr.
FIGURE 2 IR spectra of (A) monomer 2 (neat) and (B) poly(2)
(film) prepared by polymerization of 2 (ee 5 0%) with AIBN at
60ꢀC in bulk for 3 hr.
measured with an ATAGO Model-302 Abbe refractometer at
20 ꢀC using 1-bromonaphthalene as an intermediate solvent.
Number- and weight-average molecular weights (Mn and Mw)
of polymer were determined by size exclusion chromatogra-
ability of the acrylates.26,27 The dioxolanones, however,
would easily polymerize in spite of the sterically hindered
1,1-disubstituted acrylate probably because of a strong
enthalpic driving force in propagation due to the stabiliza-
tion of the resulting captodative radical. That is, the radical
substituted by both electron-donating alkoxy OACHR and
electron-withdrawing ester carbonyl COAO groups on the
same a-methylene carbon (captodative substitution) in poly
(dioxolanone) radical can expected to be stabilized by syner-
gistic push–pull resonance stabilization.28 All the dioxola-
nones used would also give a polymer spontaneously in bulk
without any initiator by standing for several hours even in
air at ambient temperature. The intermediate tetramethylene
radical feasibly generated, because of the captodative stabili-
zation,28 during the cyclodimerization of the dioxolanones
might initiate such a spontaneous thermal polymerization as
is known in other captodative acrylates including dehydroa-
lanines and acyloxy acrylates.29,30 The monomer diluted with
benzene and stored in a refrigerator below 25 ꢀC, however,
phy (SEC) using
a Tosoh HLC 8020 (Column: TSKgel
G7000HHR 1 G5000HHR1 G3000) in tetrahydrofuran at 35
ꢀC on the basis of standard polystyrene. Differential scanning
calorimetry was performed with a SII-DSC 22 at a heating
ꢀ
rate of 10 C/min under a nitrogen stream.
RESULTS AND DISCUSSION
Polymerization of Chiral Monomer
Radical polymerization of 1 and 2 with AIBN in bulk at 60
ꢀC proceeded smoothly to give a moderate molecular weight
polymer with single peak (Fig. 1) in good yield as shown in
Table 1. It has been known that a-substitution of acrylates
strongly affects the polymerizability because of the closer
proximity to the propagating radical center and even small
change in the size of the a-substituents including a-ethyl and
a-propyl groups much reduce or lose the radical polymeriz-
TABLE 1 Dependence of Tacticity of Poly(1) and Poly(2) on ee Value of Monomersa
Monomerb
Polymer
Hydrolyzed Polymer
Entry
R/S
ee (%)
Yield (%)
Mn (31024 c
)
[a]D (deg.)d
mm (%)
mr (%)
rr (%)
e
e
1
2
3
4
5
6
7
1
1
1
1
2
2
2
0/100
15/85
25/75
50/50
0/100
100/0
50/50
100
70
50
0
82.4
74.1
68.5
68.8
77.2
78.0
78.0
–
–
80.1f
50.1
11.7
10.3
100
14.4f
38.8
53.1
56.5
0
5.5f
11.1
35.2
33.2
0
10.1
11.2
6.7
223.9
217.6
0
e
e
100
100
0
–
–
e
e
–
–
100
0
0
7.8
0
29.7
54.6
15.7
a
d
Polymerization with AIBN ([AIBN] 5 1.0 3 1022 mol L21) at 60 ꢀC in
[Polymer] 5 0.2 g.dL21 in CHCl3 at 20 ꢀC.
Polymer is insoluble in tetrahydrofuran and CHCl3.
Tacticity is for the KOH/D2O-soluble fraction (ca. 20%) of all the poly-
e
f
bulk for 3 hr.
b
R/S: molar ratio of R and S configulations of monomer, ee: enantio-
meric excess. Identification of R and S configulations was followed to
mers obtained.
the literature.23
c
Mn: number-average molecular weight of polymer determined by
size-exclusion chromatography at 35oC in tetrahydrofuran using stand-
ard polystyrene.
2010
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 2007–2016

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
FIGURE 3 1H NMR spectra of (A) monomer 2 and (B) poly(2)
prepared by polymerization of 2 (ee 5 0%) with AIBN at 60 ꢀC
in bulk for 3 hr. Spectra were measured in CDCl3 at 20 ꢀC for
monomer and 40 ꢀC for polymer.
FIGURE 4 Expanded 13C NMR spectra in main chain quarter-
nary carbon region of hydrolyzed poly(1). Spectra were meas-
ured in KOH/D2O at 80 ꢀC, and the numbers 1–4 on each
spectrum correspond to the entry numbers 1–4 in Table 1.
is stable and cannot induce a spontaneous polymerization.
The IR spectrum of the polymer obtained was still main-
tained a characteristic peak due to the lactic C@O group at
1811 cm21, but exhibited no absorption around 1720 cm21
due to the diketone C@O moiety which can be produced by
ring-opening polymerization as indicated in Figure 2. More-
over, the absorption at 1669 cm21 due to the C@C group in
monomer was not observed in the IR spectrum of the result-
ing polymer indicating a conventional vinyl polymerization
without ring opening. The 1H NMR spectra of monomer and
polymer were represented in Figure 3. Although the peaks of
the polymer is broadened, the vinyl protons at 4.85 and 5.14
ppm observed in monomer 2 in Figure 3(A), for instance,
convert into the polymer main-chain methylene protons
around 2.0 ppm in Figure 3(B) by polymerization. The NMR
spectra as well as the IR spectra observed in Figures 2 and
3 strongly suggest a vinyl polymerization. While, 1,3-dioxo-
lane derivatives31 have been reported to proceed through a
ring-opening radical polymerization to give a polyester in
contrast to the present case. In the polymerization of the
cyclic methylenes, propagation mode should be governed by
the stability of the resulting intermediate radical. In the pres-
ent case, beyond the 1,3-dioxolane derivatives, the growing
vinyl radical (A) is obviously more persistent than the ring-
opening radical (B) because of a resonance stabilization over
adjacent carbonyl group or captodative28 stabilization as
depicted in Scheme 1.
Characteristics of the resulting polymers as well as the poly-
merizability and enantiomer composition of the monomers
are also summarized in Table 1, in which the triad tacticity
(mm, mr, and rr) of polymer was estimated by 13C NMR
spectrum of the corresponding hydrolyzed polymer as shown
in Figures 4 and 5 according to the literature.32 As clearly
seen in Table 1, the isotacticity (mm) of both poly(1) and
poly(2) increases with increasing enantiomeric excess (ee) of
the monomer even at normal temperature (60 ꢀC) without
heating as high as a ceiling temperature (over ca. 120 ꢀC).
Poly(2), in particular, has finally absolute isotacticity
(mm 5 ca. 100%) in the polymerization using single isomer
(ee 5 100%). Such a stereochemical restriction is supported
by the ESR spectrum observed in the polymerization of 2
(ee 5 100%) with AIBN in benzene at 60 ꢀC as depicted in
FIGURE 5 Expanded 13C NMR spectra in main chain quarter-
nary carbon region of hydrolyzed poly(2). Spectra were meas-
ured in KOH/D2O at 80 ꢀC, and the numbers 5 and 7 on each
spectrum correspond to the entry numbers in Table 1.
SCHEME 1 Propagation routes of methylene dioxolanone.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 2007–2016
2011

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
plexed with bulky SnCl4 molecule, the dihedral angle was 63–
66ꢀ, that is, 3–6ꢀ deviation from the neutral position of
h 5 60ꢀ.33 Therefore, the angle 68.4ꢀ obtained for polymer
radical 2 is fairly high, and it seems that the rigid dioxolanone
ring of 2 strongly restricts the rotation between b hydrogen
and the carbon pz orbital to form a twisted structure. Stereo-
controlled polymerization can be depicted in Scheme 2, in
which the chiral site attached on the side chain of the propa-
gating radical center will control an attacking direction of
monomer as already seen in the polymerization of chiral men-
thyl acetamideacrylate.14,15 That is, the propagating radical P0
in Scheme 2 is generally possible to attack to a monomer
from both upper and back faces with nearly same probability
because of an essentially sp2 planar structure of the radical,
but in the present system the attacking face should be limited
and determined forcibly by the side chain chirality. At present,
it is not clear which enantiomer can attack preferentially to a
monomer from upper or back face, but for instance propagat-
ing radical with R configulational side chain will attack to a
monomer only from the upper face when the radical with S
side chain can attack from the back face. That is, 2S and 2R
monomers will never give the same isotactic sequence, for
example, PSS or PRR in Scheme 2, according to the one-sided
facial selectivity, as expected from the selective generation of
the light and left one-handed isotactic helical polymer from
the S and R chiral sites of menthyl acetamidoacrylate.14,15
High isotactic polymer was also obtained using 1 with
ee 5 100%, that is, mm 5 80.1% for KOH/D2O-soluble poly-
mer fraction as seen in Table 1. In the case of monomer 1, a
large amount (ca.80%) of KOH/D2O2 and organic solvent-
insoluble poly(1) was produced. As to other high isotactic
polymer,34–37 the isotacticity of the insoluble fraction of
poly(1) may be higher than that of the soluble fraction. There
are some possibilities about the deviation from mm5100%
for monomer 1. Irregular propagation as well as a configula-
tional disorder depending on the bulkiness of the tert-butyl
FIGURE 6 ESR spectra of propagating (A) poly(2) and (B) poly(-
methyl 2-methoxyacrylate) radicals.
Figure 6(A). It is clear that the spectrum mainly consists of
four lines and exhibits splitting by two unequivalent b-H’s
(CH2), ab-H 5 9.24 and 20.4G, g 5 2.0027. Contrarily, the ESR
spectrum derived from methyl 2-methoxyacrylate (MMOA),
acyclic analogy of 2, represents three lines with 1:2:1 inten-
sity, indicating two equivalent b-H’s (CH2), ab-H 5 12.6G,
g 5 2.0036 as shown in Figure 6(B). It is well-known that
the ab-H value is dependent on the time-average dihedral
angle h between b-hydrogen and the 2pz orbital on the car-
bon of the propagating radical center, thus the h was calcu-
lated to be 68.4 and 59.6ꢀ for 2 and MMOA, respectively,
based on the ab-H of 17.6G for 1-methyl-1-(methoxycarbony-
l)ethyl radical which was generated by the hydrogen abstrac-
tion from methyl 2-methoxypropionate by tert-butoxy radical
under UV irradiation. Even coordinated MMOA radical com-
SCHEME 2 Mechanism of stereospecific propagation of chiral methylene dioxolanone. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
2012
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 2007–2016

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
FIGURE 7 X-ray diffraction of powdered poly(1). The numbers
1 and 3 on each spectrum correspond to the entry numbers 1
and 3 in Table 1, in which number 1 spectrum contains both
solvent-soluble and -insoluble fractions. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
FIGURE 8 Expanded 13C NMR spectra in main chain quarter-
nary carbon region of hydrolyzed polymers. Spectra were
measured in KOH/D2O at 80 ꢀC, and the numbers 1, 3, and 4
on each spectrum correspond to the entry numbers 1, 3, and 4
in Table 2, that is, poly(3), poly(4), and poly(5), respectively.
substituent will be considered in this case. In fact, N-benzoyl-
4-methylene-2-alkyloxazolidin-5-one which have an analogous
structure to the dioxolanones was much affected by the alkyl
substituent attached at 2-position, and tert-butyl substitution
prohibited a homopolymerization in contrast to the isopropyl
substitution.38 It is noted in monomer synthesis that if the
monomer with less than ee5ca.70% is enough it can be easily
prepared from a ferment lactic acid without optical resolution,
that is, without separation of the cis and trans dioxolanone
precursors by column chromatography, and even such a
monomer, in the case of 1 for instance, produces a polymer
(mm 5 50.1%) with the isotacticity higher than the acrylate
polymers of methyl methacrylate (mm 5 6%),39 (2)-menthyl
methacrylate (mm 5 13%),40–42 and racemic 1 (mm 5 10.4%).
For the solubility, the polymer with the isotacticity (mm) of
10–30% was soluble in most of the organic solvents including
tetrahydrofuran, toluene, and CHCl3. Contrarily, poly(2) with
mm5 100% and poly(1) with mm 5 80.1% are insoluble in
most of the organic solvents, and the former is soluble only in
the mixed solvent of CHCl3/CF3COOH (7/3) and the latter
swells only in CHCl3 and CH2Cl2.
Figure 7 shows the X-ray diffractions of the powdered
poly(1) which corresponds to the entries 1 and 3 in Table 1,
where the sample used in the measurement of spectrum 1
in Figure 7 contains both organic solvent-soluble and -insol-
uble fractions. The line observed at 2h 5 8.3ꢀ (1.06539 nm)
for the spectrum 1 is sharp compared with that at 2h 5 7.7o
(1.14827 nm) for the spectrum 3, that is, half line widths of
0.18921 and 0.29708 nm for the spectra 1 and 3, respec-
tively, and the line intensity observed at 2h 5 15.5ꢀ
(0.57174 nm) for the spectrum 1 is much stronger than that
(2h 5 15.4ꢀ) for the spectrum 3, suggesting that the polymer
generated from the ee 5 100% monomer has high crystallin-
ity. It is obvious in Table 1 that the levorotation of polymer
increases with increasing ee value of the feed monomer, that
is, the specific rotation of the polymers shown by entry num-
bers 4, 3, and 2 in Table 1 was [a]D50, 217.6, and 223.9ꢀ,
respectively, in chloroform at 20 ꢀC, and they are not much
deviated from [a]D 5 214.3ꢀ of the monomer. This implies
that the chirality observed for these polymers reflects only
the side chain chirality, and it is suggestive of a nonhelical
conformation of these polymers.
TABLE 2 Polymerization of 3, 4, 5 and Polymer Tacticitya
Polymerization
Polymerb
Mn (31024
Hydrolyzed Polymer
Entry
Monomer
Additive
Temp (ꢀC)
Time (hr)
Yield (%)
)
Mw/Mn
mm (%)
mr (%)
rr (%)
1
2
3
4
3
nonec
L-menthold
nonec
60
60
60
70
10
10
10
6
77.6
91.3
28.4
84.1
20
11
39
16
2.0
2.3
2.0
3.2
8.6
50.3
50.2
47.3
48.8
41.1
40.9
41.5
34.5
3
8.9
4
5e
11.2
16.7
nonef
a
d
Polymerization in [AIBN] 5 1.0 3 1022 mol L21
Mn and Mw: number- and weight-average molecular weights of poly-
.
Polymerization in [3] 5 1.0 mol L21 and [L-menthol]5 2.0 mol L21 in
b
benzene.
e
mer, respectively, determined by size-exclusion chromatography using
standard polystyrene in tetrahydrofuran at 35 ꢀC.
Enantiomeric excess of 5 5 100%.
Polymerization in [5] 5 2.0 mol L21 in CHCl3.
f
c
Polymerization in bulk.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 2007–2016
2013

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
tactic polymers. While, polarity is much different from each
other, and dipole moment is 0, 0.12, and 1.15 debyes45 and
Taft’s polar substituent constant is 20.30, 20.19, and 12.65
for tert-butyl, isopropyl, and trichloromethyl groups, respec-
tively.47 Therefore, the dipole–dipole repulsion between
neighboring trichloromethyl groups in the side chain of poly-
mer seems particularly to disturb an isospecific propagation.
That is, the tacticity is much sensitive not only to the chiral-
ity but also to the polarity of the substituent of monomer in
the present polymerization system. In addition, chiral solvent
like L-menthol have been reported to induce a one-handed
helical and isospecific radical polymerization of bulky meth-
acrylate.34,46 In this system, however, the tacticity of poly(3)
was little changed by L-menthol as seen in Table 2.
Polymer Properties
High isotactic acrylate polymer has already been known in
the radical polymerization of the methacrylates bearing
bulky substituents including trityl groups (mm 5 52–92%)47
at ester moiety. These polymers, however, were unstable and
ꢀ
easily decomposed, for instance, at 46 and 54 C in the solid
state44 and at 50–100 ꢀC in solution.48 Contrarily, poly(1)
and poly(2) were stable even by heating for 1 hr over 200
and 250 ꢀC in the solid state for isotactic and atactic poly-
mers, respectively. Glass transition temperature (Tg) is much
affected by the tacticity of polymer as seen in the DSC curves
in Figureꢀ9. Tg is obviously reduced, for instance, from 229.8
ꢀ
to 206.8 C and from 213.2 to 172.2 C with increasing iso-
tacticity of poly(1) and poly(2), respectively, as listed in
Table
3
even though Tg of 206.8 ꢀC for isotactic
(mm 5 80.1%) poly(1) is still significantly high in compari-
son with other acyclic analogies including atactic and isotac-
tic poly(MMA) (Tg 5 105.0 and 45 ꢀC) and AIBN-initiated
poly(methyl 2-methoxyacrylate) (Tg 5 117.1 ꢀC) as depicted
in Figure 9. Such a decreasing tendency of Tg with increasing
isotacticity has also been observed in other polymers,49 and
FIGURE 9 DSC curves of poly(1) and poly(methyl 2-methoxya-
crylate). Chart numbers 1, 4, and poly(MMOA) correspond to
the entry numbers 1 and 4 in Table 1, and poly(methyl 2-
methoxyacrylate), respectively. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
TABLE 3 Dependence of Glass Transition Temperature on Tac-
ticity of Poly(1) and poly(2)a
Polymer Tacticity and Glass
Monomerb
Transition Temperature
ee
mm
mr
rr
Tg
(ꢀC)c
Polymerization of Achiral or Polar Monomer
Entry
R/S
(%)
(%)
(%)
(%)
Table 2 represents the polymerization of 3, 4 and 5, and
polymer tacticity which is estimated from the NMR spectra
in Figure 8. The tacticity of the polymers obtained from 3
and 4 is around mm/mr/rr 5 10/50/40 which is similar to
that of the atactic poly(1). Even chiral monomer 5 with
ee 5 100%, the mm value of the polymer was only 16.7%,
which is much lower than that of isotactic poly(1) and
poly(2). Meyer’s steric parameter (V)43 is 0,0716, 0.0574,
and 0.0643, and Taft’s steric substituent constant (Es)44 is
23.8, 20.47, and 22.06 for tert-butyl, isopropyl, and tri-
chloromethyl groups, respectively. Therefore, a steric hin-
drance cannot explain enough the low isotacticity of poly(5)
since V and Es of trichloromethyl group lies between those
of tert-butyl and isopropyl groups which can afford high iso-
1
2
3
4
5
1
1
1
2
2
50/50
25/75
0/100
50/50
0/100
0
10.3
11.7
80.1
29.7
100
56.5
53.1
14.4
54.6
0
33.2
35.2
5.5
229.8
226.5
206.8d
213.2
172.2
50
100
0
15.7
0
100
a
Polymerization with AIBN ([AIBN] 5 1.0 3 1022 mol L21) at 60 ꢀC in
bulk for 3 hr.
b
R/S: molar ratio of R and S configulations of monomer, ee: enantio-
meric excess. Identification of R and S configulations was followed to
the literature.23
c
Tg: glass transition temperature.
Polymer sample used contains both solvent-soluble and -insoluble
d
fractions.
2014
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 2007–2016

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
REFERENCES AND NOTES
1 M. P. Stevens, In Polymer Chemistry: An Introduction; Oxford
University Press: New York, 1990.
2 J. W. Nicholson, In The Chemistry of Polymers; Royal Society
of Chemistry: Cambridge, 1991.
3 N. A. Porter, T. Allen, R. A. Breyer, J. Am. Chem. Soc. 1992,
114, 7676–7683.
4 B. Giese, M. Zehnder, M. Roth, H-G. Zeitz, J. Am. Chem. Soc.
1990, 112, 6741–6742.
5 Y. Okamoto, T. Nakano, Chem. Rev. 1994, 94, 349–372.
6 G. Moad, D. H. Solomon, In The Chemistry of Free Radical
Polymerization; Pergamon: Oxford, 1995; pp 151.
7 W-X. Wu, A. T. McPhail, N. A. Porter, J. Org. Chem. 1994, 59,
SCHEME 3 Sustainable synthesis of new type of poly(lactic
acid), that is, tactic poly(a-hydroxy acrylate), using ferment lac-
tic acid as a raw material.
1302–1308.
8 N. A. Porter, R. L. Carter, C. L. Mero, Tetrahedron 1996, 52,
4181–4194.
9 N. A. Porter, E. Swann, J. Nally, A. T. McPhail, J. Am. Chem.
Soc. 1990, 112, 6740–6741.
it has been believed to be the effect of the higher stress-
relaxation of the chain segments of sterically unstable isotac-
tic structure. In addition, these polymers are transparent and
the refractive index (nD) measured at k 5 589.3 nm (Na-D)
was 1.5281 and 1.5257 for poly(5) and poly(3), respectively,
which are higher than that for poly(MMA) (nD 5 1.4910)50
probably because of the captodative polarity and high den-
sity of monomer unit due to the complicated 1,1-
disubstitution.
10 N. A. Porter, R. A. Breyer, E. Swann, J. Nally, J. Predhan, T.
Allen, A. T. McPhail, J. Am. Chem. Soc. 1991, 113, 7002–7010.
11 S. A. Ashraf, L. A. P. Kane-Maguire, S. G. Pyne, G. Wallac,
Macromolecules 1998, 31, 8737–8743.
12 A. K. M. F. Azam, M. Kamigaito, Y. Okamoto, Polym. J.
2006, 38, 1035–1042.
13 T. Nakano, K. Ueda, Y. Okamoto, J. Polym. Sci., Part A:
Polym. Chem. 2001, 39, 1610–1614.
14 H. Tanaka, M. Niwa, Polymer 2008, 49, 3693–3701.
15 H. Tanaka, M. Niwa, Polymer 2005, 46, 4635–4639.
CONCLUSIONS
16 These polymers were obtained in the conditions of
[monomer] 5 2.0 mol.L21 in benzene at normal temperature (30 ꢀC)
and near Tc (70 ꢀC) with 2,2’-azobis(4-methoxy-2,4-dimethylvalero-
nitrile) and AIBN as a radical initiator, respectively. Specific rotation
[a] was also varied with the polymerization temperature from
[a]587 5 294.4ꢀ in CHCl3 ([a]302 5 2405.7ꢀ in tetrahydrofuran) to
[a]587 5 210.6ꢀ in chloroform ([a]302 5 190.1ꢀ in tetrahydrofuran)
for the polymers obtained at 30 and 70 ꢀC, respectively.
High isotactic heat-resistant acrylate polymer has not so far
been prepared in free radical polymerization since a propa-
gating radical has not a counterpart and takes essentially sp2
planar structure in contrast to an anionic polymerization. In
this work, however, high isotactic polymer was obtained
through a conventional free radical polymerization of bio-
based acrylate by utilizing effectively the monomer chirality
and conformation. Demonstration of the importance of chir-
ality and conformation of monomer may give a clue to
understand a stereochemistry in free radical polymerization
and construct a tailor-maid polymer with desired tacticity
and functionality. This method is superior to an anionic poly-
merization where the reaction is generally carried out at
very low temperature, for example, 220–270 ꢀC under a
dry medium using hardly handleable air or water-sensitive
metal catalyst.51,52 It is noteworthy that poly(a-hydroxy acry-
late) which is obtained by the hydrolysis of these dioxola-
none polymers, so-called new type of poly(lactic acid), is
conveniently produced only from a lactic acid as a base
material since the feed aldehyde is recovered by the hydroly-
sis of dioxolanone polymers as shown in Scheme 3. More-
over, isotactic dioxolanone polymers and poly(a-hydroxy
acrylate) represent high chemical and heat resistances for
the former and high hydrophilicity for the latter.
17 E. S. Beach, B. R. Weeks, R. Stem, P. Anastas, Pure Appl.
Chem. 2013, 85, 1611–1624.
18 R. Mulhaupt, Macromol. Chem. Phys. 2013, 214, 159–174.
19 C. Paul, Macromol. Speciality Chem. Mag. 2009, 29, 42–44.
20 I. S. M. A. Tawakkai, M. J. Cran, J. Miltz, S. W. Bigger, J.
Food Sci. 2014, 79, 1477–1490.
21 T. Nakano, M. Mori, Y. Okamoto, Macromolecules 1993, 26,
867–868.
22 T. Nakano, N. Kinjo, Y. Hidaka, Y. Okamoto, Polym. J. 2001,
33, 306–309.
23 D. I. Magee, T. C. Mallais, P. D. M. Mayo, G. M. Strunz, Tet-
rahedron 2006, 62, 4153–4161.
24 W. J. Bailey, P. Feng, Polym. Prepr. (Am. Chem. Soc., Div.
Polym. Chem.) 1987, 28, 154–155.
25 H. Tanaka, T. Kameshima, K. Sasai, T. Sato, T. Ota, Makro-
mol. Chem. 1991, 192, 427–435.
26 J. Penelle, J. Collot, G. Rufflard, J. Polym. Sci., Part A:
Polym. Chem. 1993, 31, 2407–2412.
27 J. Rzayev, J. Penelle, J. Polym. Sci., Part A: Polym. Chem.
2002, 40, 836–843.
ACKNOWLEDGMENT
The authors are grateful to Dr. S. Yanagiya (Institute of Technol-
ogy and Science, University of Tokushima) for help with X-ray
measurement.
28 H. Tanaka, Prog. Polym. Sci. 2003, 28, 1171–1203.
29 I. J. Mathias, R. E. Hermes, Macromolecules 1986, 19, 1536–
1542.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 2007–2016
2015

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
30 H. Tanaka, S. Yoshida, Macromolecules 1995, 28, 8117–
8121.
41 K. Matsuzaki, A. Ishida, N. Tateno, J. Polym. Sci. Part C:
Polym. Symp. 1967, 16, 2111–2120.
42 S. Robert, A. Angelina, Polym. J. 1990, 22, 497–501.
31 G. C. Eastmond, A. Ledewith, S. Russo, P. Sigwalt, In Com-
prehensive Polymer Science; Pergamon: London, 1989.
43 A. Y. Meyer, J. Chem. Soc. Perkin Trans. 2 1986, 1567–1572.
44 M. S. Newman, Steric Effect in Organic Chemistry; Wiley:
New York, 1956.
32 K. Yamazawa, S. Kawauchi, M. Satoh, J. Polym. Sci., Part B:
Polym. Phys. 2002, 40, 1400–1405.
45 A. Weissberger, In Organic Solvents, 3rd ed.; Wiley Inter-
sience: New York, 1979.
33 H. Tanaka, I. Sakai, T. Ota, J. Am. Chem. Soc. 1986, 108,
2208–2211.
46 T. Nakano, Y. Shikisai, Y. Okamoto, Polym. J. 1996, 28, 51–
60.
34 N. Hoshikawa, Y. Hotta, Y. Okamoto, J. Am. Chem. Soc.
2003, 125, 12380–12381.
47 Y. Kawakami, Y. Sakai, A. Okada, Polym. J. 1990, 22, 705–
718.
35 N. Hoshikawa, C. Yamamoto, Y. Hotta, Y. Okamoto, Polym.
J. 2006, 38, 1258–1266.
48 M. Hosoda, U. Schoenhausen, P. Pino, Makromol. Chem.
1993, 194, 223–232.
36 V. B. Edith, K. Quintin, F. Genoveva, B. Hugo, R. Harry R,
Macromolecules 2004, 37, 9673–9675.
49 J. H. Gibbs, E. A. Di. Marzio, J. Chem. Phys. 1958, 28, 373–
383.
37 N. Markus, K. Katrina, A. Vladimir, T. Heikki, Macromole-
cules 2008, 41, 4881–4886.
50 T. C. Gilmer, H. Ehrenfeld, K. Wilson, T. Bivens, D. Clay, C.
Endreszl, J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 1025–
1037.
38 M. Niwa, H. Norisada, H. Tanaka, Polym. Prep. Jpn. 2014,
63, 3920–3921.
39 F. A. Bovey, G. V. D. Tiers GVD, J. Polym. Sci. 1960, 44,
173–175.
51 H. Yuki, K. Hatada, K. Ohta, Y. Okamoto, Appl. Polym.
Symp. 1975, 26, 39–50.
40 H. Sobue, K. Matsuzaki, S. Nakano, J. Polym. Sci., Part A:
Polym. Chem. 1964, 2, 3339–3346.
52 R. P. Quirk, Q. Zhuo, S. H. Jang, Y. Lee, G. Lizarraga, Am.
Chem. Soc. Symp. 1998, Ser 696, 2–27.
2016
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 2007–2016