1:2-sequence-regulated radical copolymerization of naturally occurring terpenes with maleimide derivatives in fluorinated alcohol

Article abstract of DOI:10.1002/pola.26556

Naturally occurring bulky terpenes, such as (+)- and (-)-limonene and (-)-β-pinene, were quantitatively copolymerized with maleimide (MI) derivatives (i.e., phenyl-, cyclohexyl-, ethyl-, and unsubstituted-MI) in PhC(CF₃)₂OH solvent via selective 1:2-alternating propagation governed by the penultimate effect, which resulted in 1:2-sequence regulated polymers with relatively high glass transition temperatures and optical activities. Similar petroleum-derived bulky olefins possessing cyclohexenyl, cyclohexyl, or additional α-methyl substituents were copolymerized with phenylmaleimide via preferential 1:2-alternating propagation with a slightly lower selectivity. A further decrease in the bulkiness of nonpolar olefins increased the 1:1-alternating sequence. The copolymerization of limonene and acrylonitrile also proceeded approximately via 1:1-alternating propagation, in which the penultimate effect was less observable. Furthermore, when methylene chloride was used instead of fluorinated alcohol for the copolymerization of limonene and phenylmaleimide, the length of the sequence of MI units increased. Thus, in addition to the characteristic MI skeleton, the bulky and rather specific structure of either limonene or β-pinene induces 1:2-selective propagation via the penultimate effect, whereas the fluoroalcohol diminishes the homopropagation of MI via a hydrogen bonding interaction with the MI unit. RAFT copolymerization of limonene and various MI derivatives in PhC(CF₃)₂OH successfully proceeded to give the end-to-end 1:2-sequence-regulated copolymer with a selective initiating sequence and predominant capping sequence using an appropriate RAFT agent.

Full text of DOI:10.1002/pola.26556

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1:2-Sequence-Regulated Radical Copolymerization of Naturally Occurring
Terpenes with Maleimide Derivatives in Fluorinated Alcohol
Masaru Matsuda, Kotaro Satoh, Masami Kamigaito
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8603, Japan
Correspondence to: M. Kamigaito (E-mail: kamigait@apchem.nagoya-u.ac.jp)
Received 6 December 2012; accepted 9 January 2013; published online 6 February 2013
DOI: 10.1002/pola.26556
ABSTRACT: Naturally occurring bulky terpenes, such as (þ)- and (–
)-limonene and (–)-b-pinene, were quantitatively copolymerized
with maleimide (MI) derivatives (i.e., phenyl-, cyclohexyl-, ethyl-,
and unsubstituted-MI) in PhC(CF3)2OH solvent via selective 1:2-
alternating propagation governed by the penultimate effect,
which resulted in 1:2-sequence regulated polymers with relatively
high glass transition temperatures and optical activities. Similar
petroleum-derived bulky olefins possessing cyclohexenyl, cyclo-
hexyl, or additional a-methyl substituents were copolymerized
with phenylmaleimide via preferential 1:2-alternating propagation
with a slightly lower selectivity. A further decrease in the bulki-
ness of nonpolar olefins increased the 1:1-alternating sequence.
The copolymerization of limonene and acrylonitrile also pro-
ceeded approximately via 1:1-alternating propagation, in which
the penultimate effect was less observable. Furthermore, when
methylene chloride was used instead of fluorinated alcohol for
the copolymerization of limonene and phenylmaleimide, the
length of the sequence of MI units increased. Thus, in addition to
the characteristic MI skeleton, the bulky and rather specific struc-
ture of either limonene or b-pinene induces 1:2-selective propaga-
tion via the penultimate effect, whereas the fluoroalcohol
diminishes the homopropagation of MI via a hydrogen bonding
interaction with the MI unit. RAFT copolymerization of limonene
and various MI derivatives in PhC(CF3)2OH successfully pro-
ceeded to give the end-to-end 1:2-sequence-regulated copolymer
with a selective initiating sequence and predominant capping
C
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sequence using an appropriate RAFT agent. 2013 Wiley Periodi-
cals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1774–1785
KEYWORDS: bio-based polymer; copolymerization; limonene;
monomer sequence control; pinene; radical polymerization;
RAFT; renewable resources
INTRODUCTION Monomer sequence control in synthetic
polymers is one of the most challenging topics in polymer
chemistry, especially in the area of chain-growth polymeriza-
tion.1–5 Radical copolymerization allows the incorporation of
nearly all types of vinyl compounds into copolymer products,
whereas monomers react statistically in accordance with
monomer reactivity ratios, which are determined by the elec-
tronic and steric factors of the substituents, producing statis-
tical or ‘‘random’’ copolymers.6,7 However, in some excep-
tional cases, a combination of monomers, the vinyl groups of
which are electron-deficient and donating, alternating
sequences can be obtained; this is particularly true when
both monomers are either non- or hardly homopolymeriz-
able. The former group includes 1,2-disubstituted monomers
possessing electron-withdrawing groups, such as maleic
anhydride,8–12 maleimides,13–22 fumarates,23,24 and fumaro-
nitrile,25,26 whereas the latter group includes vinyl
ethers,27,28 vinyl sulfides,29 and non-polar olefins.30 In addi-
tion to the occurrence of such spontaneous alternating
sequences governed by the inherent monomer reactivities,
the use of Lewis acid additives or polar solvents can enhance
the formation of alternating sequences. This process is the
result of coordination of the Lewis acid or the solvent to the
polar substituent of the former monomer or formation of a
charge-transfer complex between these components, even
when one or both of the monomers are homopolymeriz-
able.6,7,31–39 However, higher-ordered sequence control is
generally difficult in chain-growth copolymerization, espe-
cially in the radical copolymerization of vinyl monomers.
Recently, we reported a selective 1:2 sequence-regulated rad-
ical copolymerization of (þ)-d-limonene [(þ)-Lim] and phe-
nylmaleimide (PhMI) in a fluorinated alcohol,40 which is an
important result in the development of novel bio-based poly-
mers from renewable resources. Additionally, this result
offers unprecedented sequence control of polymers in chain-
growth polymerization reactions. In particular, we showed
not only the first quantitative polymerization of limonene,
which is one of the most abundant naturally occurring ter-
penes and is obtained from the nonedible parts of plants
such as lemon and orange peel41–44 but also the highly
Additional Supporting Information may be found in the online version of this article.
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analyzed copolymerization with the penultimate model with
the further goal of revealing the mechanism underlying
selective 1:2-copolymerization (Scheme 1). Additionally, the
polymer properties and reversible addition-fragmentation
chain-transfer (RAFT) copolymerization53–56 were examined;
in particular, combinations of Lim or b-Pin and MI derivatives
were used to generate the desired 1:2-sequence-regulated
structures.
CHART 1 Various non-polar olefins used in this study.
RESULTS AND DISCUSSION
Radical Copolymerization of Various Non-Polar
Olefins and Phenylmaleimide
selective 1:2-copolymerization of a range of chain-growth
polymerization reactions, including ionic and coordination
polymerizations, which is unprecedented. Mechanistically,
the copolymerization was well characterized by the penulti-
mate model,7 although the comonomer factors that induce
such selective 1:2-copolymerization remain unknown; specif-
ically, the comonomers used included non-polar olefins, such
as (þ)-Lim, and electron-deficient monomers such as PhMI.
Additionally, from the perspective of the development of
novel bio-based polymers,45–48 other possibilities for combi-
nation should be investigated, including other major
terpenes such as a- and b-pinenes (Pin)19,36,37,49 and other
maleimide (MI) derivatives.50–52
A series of non-polar olefins was copolymerized with PhMI,
which is a typical electron-deficient monomer. AIBN was
used as the initiator, and the reactions were conducted in
PhC(CF3)2OH at 60 ꢀC; the initial monomer feed ratio was
1:2 ([olefin]0/[PhMI]0 ¼ 1/2) (Table 1). The nonpolar olefins
studied included not only naturally occurring terpenes, such
as (þ)-Lim (1), (þ)-a-Pin (2), and (–)-b-Pin (3) but also pe-
troleum-derived isopropenyl (4–7) (a-methyl olefin) and
vinyl compounds (8–10), each with a different pendent
structure. As reported in our previous publication,40 at the
1:2 feed ratio, both 1 and PhMI achieved nearly quantitative
conversions to yield copolymers with an incorporation ratio
of approximately 1:2 [entry 1 in Table 1 and Fig. S1(A) in
Supporting Information]. Irrespective of the structures, all of
the terpenes (1–3) were copolymerized with PhMI in
PhC(CF3)2OH to afford various copolymers [entries 1–3 in
Table 1 and Supporting Information Fig. S1(A)]. In particular,
(–)-b-Pin (3) was consumed at almost the same rate as PhMI
to give the 1:2 copolymers, which suggests a 1:2-alternating
copolymerization similar to that of 1 and PhMI, whereas the
tri-substituted 2 exhibited a lower reactivity that resulted in
a low terpene conversion (<30%). With respect to the
In this study, we investigated radical copolymerization to
determine the effects of monomer structure on monomer reac-
tivity or the resultant polymer sequences. Copolymerization
reactions were conducted between various non-polar olefins
(including terpenes such as (þ)- and (–)-Lim and a- and b-Pin,
in addition to petroleum-derived olefins (with both similar and
dissimilar structures)) (Chart 1) and electron-deficient mono-
mers [such as various MI derivatives and acrylonitrile (AN)].
The reactions were conducted in PhC(CF3)2OH. This approach
SCHEME 1 Schematics of 1:2 alternating copolymerization of d-limonene (1: M1) and PhMI (M2).
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TABLE 1 Radical Copolymerization of Various Olefins (M1) and PhMI (M2)a
Conv. (%)b
Incorp. (%)d
M1/M2
c
c
Entry
Olefin (M1)
M1/M2
Mn
Mw/Mn
r12
19
1/r12
r22
1
2
3
4
5
6
7
8
9
10
a
(þ)ꢃ1
88/92
29/49
94/93
100/78
91/61
90/62
83/85
96/83
71/63
66/84
9,700
2,700
16,800
33,500
2,900
4,300
6,600
17,500
3,400
4,500
2.16
1.61
2.93
4.15
1.89
2.28
2.72
2.27
1.67
1.93
34/66
0.053
–
0.0042
–
(þ)ꢃ2
20/80
33/67
40/60
39/61
39/61
30/70
34/66
35/65
30/70
–
(ꢃ)ꢃ3
7.6
2.7
3.0
2.8
0.13
0.37
0.33
0.36
0.033
0.48
0.53
0.018
0.0011
0.0010
0.067
0.030
0.25
4
5
6
rac-7
8
30
2.1
1.9
0.11
9
0.21
rac-10
56
0.47
Polymerization condition: PhC(CF3)2OH, 60 ꢀC, [M1]0 þ [PhMI]0 ¼ 1,200 mM, [M1]0/[PhMI]0 ¼ 0.50, ([M1]0þ[PhMI]0)/[AIBN]0 ¼ 150.
Determined by 1H NMR analysis of residual monomers in the reaction mixture.c The number-average molecular weight (Mn) and
b
molecular weight distribution (Mw/Mn) were determined by size-exclusion chromatography.d Determined by 1H NMR analysis of the
isolated copolymers.
petroleum-derived olefins, the consumption rate of both
monomers became more similar when the pendent groups of
the olefin were bulkier. The monomers with less hindered
linear aliphatic groups (4 and 8) were consumed faster than
PhMI to reach 100% conversion of the olefin; additionally,
this resulted in copolymers that contained higher olefin
amounts, which was probably the result of conventional 1:1-
alternating cross-propagation.39 However, the bulkier olefin
(7), which has a structure similar to that of 1, led to a 1:2
consumption of the olefin and PhMI of which the conver-
sions were 83 and 85%, respectively, and yielded copoly-
mers with an approximate 1:2 incorporation. These results
indicate that the bulkiness of the olefin pendent groups plays
a significant role in inducing the 1:2-alternating copolymer-
ization with PhMI. As the olefin bulkiness was increased, the
incorporation ratio of olefin to PhMI gradually changed from
1:1 to 1:2 and reached a final value of nearly 1:2 for the
bulky terpenes, 1 and 3.
monomer reactivity ratios were insufficiently described by
the ordinary chain-end model, which uses r1 and r2. How-
ever, the monomer reactivity ratios were best fit by the
penultimate model, which was used to calculate r12 and r22
based on the modified Kelen–Tu¨do˜s or curve-fitting method,
assuming that homopropagation of the unconjugated olefin
does not occur and then the r11 and r21 are 0. The r12 and
r22 values, which represent the relative reactivity of the
ꢁꢁM1M2ꢂ and ꢁꢁM2M2ꢂ growing termini to M2 in compari-
son to M1, respectively, are significantly different, which indi-
cates that the penultimate monomer unit (M1 or M2) signifi-
cantly affects the reactivity of the terminal ꢁꢁM2ꢂ radical, as
summarized in Table 1.
As previously reported,40 the use of fluorinated alcohol as
the solvent is critical for achieving the 1:2-alternating
copolymerization. The copolymerization of 1 or 9 with PhMI
was also performed in CH2Cl2 at varying comonomer feed
ratios [open circles in Fig. 2(A,H)]. In both cases, incorpora-
tion of the olefins in the produced copolymers was lower
than 0.33, even at a high olefin loading; additionally, the r22
values were significantly higher in CH2Cl2 [r22 ¼ 0.83 (for 1)
and 1.3 (9)] than in PhC(CF3)2OH [r22 ¼ 0.0042 (for 1) and
0.21 (9)]. These results indicate that PhC(CF3)2OH increased
the copolymerizability of these particular olefins in compari-
son to CH2Cl2, in which the lower copolymerizability of the
olefins resulted in longer homopolymerizable MI sequences
(Scheme 2; Path C). As reported previously,40 the higher
copolymerizability of non-polar olefins in PhC(CF3)2OH likely
can be attributed to hydrogen bonding interactions of
PhC(CF3)2OH with the carbonyl groups of the PhMI units,
which enhances the cross-propagation to the electron-donat-
ing unconjugated olefins.
For further investigation of the copolymerizability of the
unconjugated olefins (M1) and PhMI (M2), the respective
monomer reactivity ratios were determined by copolymeriza-
tion at various comonomer feed ratios ([olefin]0 þ [PhMI]0
¼ 1200 mM, [AIBN]0 ¼ 8 mM in PhC(CF3)2OH at 60 ꢀC).
Figure 1 shows copolymer composition curves in which the
olefin incorporation ratios in the resulting copolymers were
plotted as functions of the comonomer feed ratio. In particu-
lar, the curve should be constant at 0.33 for 1:2 copolymer-
ization; however, for the conventional 1:1-alternating
copolymerization, the curve should be 0.50, irrespective of
the comonomer feed ratio. Incorporation of the less hindered
olefins, such as 4, 5, 8, and 9, into the copolymers was appa-
rently higher than 0.33, with values approaching 0.50 at the
higher olefin feed ratios. This result also indicates that the
bulkiness of the pendent groups is a significant factor for
1:2 propagation. As in the case of 1 and PhMI in the previ-
ous study,40 for all of the copolymerization reactions, the
For all copolymerizations in PhC(CF3)2OH, the inverse of r12
(1/r12) and r22 values were less than 1.0. These results sug-
gest that ꢁꢁM1M2ꢂ apparently favors M2 in the formation of
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FIGURE 1 Copolymer composition curves for the copolymerization of various olefins and PhMI in PhC(CF3)2OH (l) and CH2Cl2 (*) at 60
ꢀC; [olefin]0 þ [PhMI]0 ¼ 1200 mM, [olefin]0/[PhMI]0 ¼ 1/7, 1/3, 1/1, 3/1, 7/1, [AIBN]0 ¼ 8.0 mM. The dotted lines were fitted by the modified
Kelen–Tudo˜ s method (for 1, 3, 5, 6, 8, and 9) or the curve fitting method (for 4, 7, and 10), assuming that the values of r11 and r21 are 0.
¨
repeating MI sequences, irrespective of the penultimate
olefin structure. In contrast, the repeating ꢁꢁM2M2ꢂ domi-
nantly adds to the electron-rich olefin, M1, as in the conven-
tional 1:1 alternating copolymerization (Scheme 2). Among
the various olefins, the naturally occurring Lim and b-Pin
(1 and 3) gave particularly low 1/r12 and r22 values, which
resulted in the selective 1:2-alternating propagation (Scheme
2; Path B). This is likely because the incorporated structure
of 3 after the ring opening is quite similar to that of 1 in
terms of bulkiness and residual C¼¼C bonds, as shown in
Scheme 3. The slightly higher 1/r12 value for 3 (0.13) rela-
tive to 1 (0.053) likely resulted from the higher reactivity of
the strained bicyclic monomer, 3.
isopropenyl compounds (4–6) were lower than those of the
corresponding vinyl compounds (8 and 9), despite the
higher electron density of the unsaturated C¼¼C bonds, which
originated from the a-methyl substituent. This also indicates
that the steric hindrance results in a ꢁꢁM1M2M2 sequence
rather than the conventional ꢁꢁM1M2M1 alternating
sequence (Scheme 2; Path A). In addition to the bulkiness of
the pendent group, the cyclohexenyl group was roughly
effective to provide selective 1:2 cross-propagation. The 1/
r12 values of 7 and 10 were significantly lower than those of
the saturated cyclohexyl analogues (6 and 9), respectively,
although the r22 values were slightly increased. Thus, not
only the use of fluorinated alcohol as the solvent but also
the steric hindrance and unsaturated pendent ring structure
of the olefins was important for achieving unprecedented
1:2-alternating copolymerizability.
For the other petrochemical-derived olefins, the 1/r12 values
were lower than 1.0 but were not low enough to produce
the selective 1:2 copolymers. The 1/r12 values of the
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FIGURE 2 1H NMR spectra of (CDCl3, 55 ꢀC) of the copolymers of 1 (A), 3 (B), 4 (C), and 7 (D) with PhMI obtained in PhC(CF3)2OH
at 60 ꢀC; [olefin]0/[PhMI]0 ¼ 400/800 mM; [AIBN]0 ¼ 8.0 mM.
SCHEME 2 Mechanism for radical copolymerization of various unconjugated olefins and MI derivatives in various solvents.
1
The structures of the copolymers as obtained from the
various unconjugated olefins and PhMI in PhC(CF3)2OH were
analyzed by 1H NMR spectroscopy (Fig. 2 and Fig. S2 in
Supporting Information). Figure 2 shows the H NMR spectra
of poly(1-co-PhMI), poly(3-co-PhMI), poly(4-co-PhMI), and
poly(7-co-PhMI). In all spectra, the large signals were
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in citrus peel, leaf oil, and camphor oil.43,44 The effect of the
stereochemistry of 1 also was examined by the copolymer-
ization of (þ)-, (–)-, and rac-1 with PhMI (Fig. 3 and Table
S1 in Supporting Information). Irrespective of differences in
monomer stereochemistry, copolymerization proceeded in
the 1:2-alternating fashion. Copolymers obtained from chiral
(þ)- and (–)-1 exhibited mirror images of circular dichroism
(CD) and specific optical rotation, whereas no optical activity
was observed in the copolymer of rac-1. Additionally, the co-
polymer obtained from (–)-3 exhibited characteristic CD and
a specific optical rotation. These results indicate that the
configuration of the chiral centers in (þ)-1, (–)-1, and (–)-3
was retained during the copolymerization. The thermal prop-
erties of the obtained copolymers were evaluated by differ-
ential scanning calorimetry (DSC) under a nitrogen atmos-
phere. All of the obtained copolymers also exhibited
relatively high glass-transition temperatures (Tgs) because of
the rigid MI and the bulky terpene backbone.50
SCHEME 3 Propagation of limonene (1) and b-pinene (3)
during radical copolymerization.
assigned to the protons in the main chains of the olefins/
PhMI copolymers; the methine (i) and phenyl (j) protons in
the PhMI units absorbed at 1.2ꢃ4.5 and 6.5–7.6 ppm, respec-
tively, whereas the residual olefin (f for 1, b for 3 and e, f for
7), aliphatic (a, c-e, g, and h for 1, a, c–f for 3, a, c, and d for 4,
and a, c, d, g, and h for 7), and methyl (b for 1, g for 3, b, e for
4, and b for 7) protons in the olefin units absorbed at 5.0–6.2,
1.2–4.5, and 0.5–1.2 ppm, respectively. The copolymer compo-
sitions, that is, the PhMI/olefin ratios, were calculated from
the peak intensities of the phenyl (j for PhMI), the residual
olefin (f for 1, b for 3, and e, f for 7), and aliphatic and methyl
protons (a–e for 4, and a, c, d, g, and h for 7), which agreed
well with the values calculated from the initial charge ratio
and conversions of the two monomers. Thus, all of the con-
sumed monomers were incorporated into the copolymers.
The effects of the MI unit pendent groups on 1:2-alternating
copolymerization and polymer properties were also investi-
gated using various MI derivatives such as cyclohexylmalei-
mide (CyMI), ethylmaleimide (EtMI), and unsubstituted MI
rather than PhMI. The MI derivatives also were copolymerized
with Lim (1) or b-Pin (3) ([terpene]0/[MI]0 ¼ 1/2) using
AIBN as the initiator in PhC(CF3)2OH at 60 ꢀC. Similar to PhMI,
the copolymerization of these maleimides proceeded smoothly
irrespective of their structure: both monomers were simulta-
neously and quantitatively consumed at the same rate with a
consumption ratio of approximately 1:2 (Fig. S3 in Supporting
Information). Table 2 summarizes the copolymerizations with
various MI derivatives as well as AN, which is an electron-defi-
cient vinyl monomer that is often used in conventional 1:1-
alternating copolymerization with non-polar olefins such as
propylene.31,32 Irrespective of the substituents, the monomer
compositions in the copolymers obtained from the MI
1
These H NMR analyses further confirmed the 1:2 alternating
copolymerizations of 1 or 3 and PhMI.
1:2 Radical Copolymerization of Terpenes and Various
MI Derivatives
Optically active Lim (1) can be found in nature as (þ)-, (–)-,
and racemic forms, although (þ)-1 occurs most abundantly
FIGURE 3 CD spectra (A) and SEC curves (B) of poly(terpene-co-PhMI) obtained in the radical copolymerization of chiral or race-
mic terpene and PhMI in PhC(CF3)2OH at 60 ꢀC; [terpene]0/[PhMI]0 ¼ 400/800 mM, [AIBN]0 ¼ 8.0 mM.
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TABLE 2 Radical Copolymerization of Terpenes (M1) and Polar Monomers (M2)a
Conv. (%)b
Incorp. (%)d
M1/M2
Entry
M1
M2
M1/M2
Mn
Mw/Mn
Tg (ꢀC)e
c
c
1
2
3
4
5
6
7
8
9
a
1
AN
PhMI
CyMI
EtMI
MI
34/39
4,300
9,700
1.99
2.16
1.96
1.99
2.79
2.93
2.57
3.21
2.64
29/71
–
243
226
182
275
197
181
152
–
88/92
85/88
77/81
85/86
94/93
91/92
92/93
89/76
34/66
33/67
35/65
39/61
33/67
34/66
35/65
35/65
8,700
7,000
10,700
16,800
15,800
12,600
25,700
3
PhMI
CyMI
EtMI
MI
Polymerization condition: PhC(CF3)2OH, 60 ꢀC, [M1]0 þ [M2]0 ¼ 1,200 mM (for PhMI) or 3,000 mM (for AN), [M1]0/[M2]0 ¼ 0.50,
([M1]0 þ [M2]0)/[AIBN]0 ¼ 150.
b
Determined by 1H NMR analysis of residual monomers in the reaction mixture.
c
The number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were determined by size-exclusion chro-
matography.
d
Determined by 1H NMR analysis of the isolated copolymers.e Evaluated by a differential scanning calorimetry.
derivatives were nearly 1:2, which agrees with the values cal-
culated from the initial charge ratio and conversions. In con-
trast, AN afforded copolymers with smaller amounts of ter-
pene units. Additionally, all maleimides and Lim or b-Pin
afforded copolymers with relatively high Tg due to the rigid
backbone chains although it slightly decreased in the order of
R ¼ Ph > Cy > Et. All these copolymers of maleimides and
Lim or b-Pin in Table 2 showed only one thermal transitions
(Tg) and no decompositions between 40 and 350 ꢀC.
r12 and r22 were calculated as 21 and 0.0059, respectively, for
CyMI and as 17 and 0.029, respectively, for EtMI, based on the
modified Kelen–Tu¨do˜s method. These results again indicate that
the penultimate model governs the selective 1:2-alternating
copolymerizations and that the side groups of the MI derivatives
did not significantly affect the 1:2 selectivity.
However, the r12 and r22 values for the copolymerization of 1
and AN (M2) were 0.72 and 0.61, respectively; these values are
both less than 1.0 and are almost the same, which suggests that
the reactivity of the terminal AN radical is nearly independent of
the penultimate monomer unit and that no 1:2 selectivity
occurred for AN.39 Thus, the MI structure was also crucial for the
1:2-alternating copolymerization of terpene, whereas the effect of
the MI derivative pendent group on selectivity was small.
The 1:2-alternating copolymerization of the maleimides (M2) with
1 (M1) was further addressed in terms of the monomer reactivity
ratio determined for copolymerization at varying comonomer
ꢀ
feed ratios in PhC(CF3)2OH at 60 C: [1]0 þ [MI]0 ¼ 1200 mM,
[AIBN]0 ¼ 8.0 mM (Fig. 4). The copolymer composition curves for
the MI copolymerization show a characteristic plateau in which
the content of 1 (M1) in the obtained copolymers was constant at
approximately one-third (0.33) irrespective of the feed ratio and
the MI structure. Similar to PhMI, by using the penultimate model,
RAFT Copolymerization of Limonene and MI Derivatives
The copolymerization of 1 and PhMI in fluoroalcohol pro-
duced not only 1:2-alternating copolymers with quantitative
FIGURE 4 Copolymer composition curves for the copolymerization of 1 and CyMI (A), EtMI (B), and AN (C) in PhC(CF3)2OH at 60
ꢀC; [Mtotal 0
]
¼ 1200 mM (for CyMI and EtMI), [Mtotal 0 ¼ 3000 mM (for AN), [1]0/([MI]0 or [AN]0) ¼ 1/7, 1/3, 1/1, 3/1, 7/1, [AIBN]0 ¼
]
8.0 mM (for CyMI and EtMI), 20 mM (for AN). The dotted lines were fitted by the modified Kelen–Tudo˜ s method, assuming that
¨
the values of r11 and r21 are 0.
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(Fig. 6). The Mn of the obtained copolymers (the circles in
Fig. 6), which were measured by SEC on PMMA standards,
was found to increase similarly in direct proportion to the
monomer conversion, although the obtained values were
lower than the calculated values, as in the case of 1 and
PhMI. The copolymer of 1 and CyMI obtained with CBTC
1
was analyzed by H NMR spectroscopy (Fig. S6 in Supporting
Information). In addition to the characteristic main-chain sig-
nals of poly(1-co-CyMI), the polymer gave small signals of
cumyl protons at the a-end that originated from CBTC at
7.0–7.5 ppm. The Mn values calculated from those peaks
were in good agreement with the calculated values irrespec-
tive of the monomer conversion (the open squares in Fig. 6).
Thus, the controlled radical copolymerization of maleimide
derivatives was achieved by using CBTC, which served as the
RAFT agent in the formation of one polymer chain.
FIGURE 5 Mn, Mw/Mn, and SEC curves of poly(1-co-PhMI)
obtained with varying the [Mtotal]0/[CBTC]0 ratio in PhC(CF3)2OH
at 60 ꢀC; [1]0/[PhMI]0 ¼ 1/2, [Mtotal 0
¼ [1]0 þ [PhMI]0 ¼ 1200
]
mM, [Mtotal]0/[AIBN]0 ¼ 240, [Mtotal]0/[CBTC]0 ¼ 20, 60, or 120.
The diagonal line indicates the calculated Mn assuming the
formation of one living polymer per one CBTC molecule.
The copolymers of 1 and various MI derivatives obtained
with CBTC were further analyzed by matrix-assisted laser-
desorption-ionization time-of-flight mass spectroscopy
(MALDI-TOF-MS) in the presence of trans-2-[3-(4-tert-butyl-
phenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as
the matrix and CF3CO2Na as the ionizing agent (Fig. 7). The
spectra show a series of the selective AMAMALA sequence
(M: MI derivatives, L: limonene) in which each peak is sepa-
rated by intervals of 173 Da for PhMI, 179 Da for CyMI, 125
Da for EtMI, and 136 Da for 1. Although a few series of
peaks that did not have the ideal structure were observed
because of the loss of the chain-end S [S: SAC(¼¼S)ASBu]
group, which was derived from the RAFT agent during the
ionization process, the molecular weights of each individual
peak are in good agreement with the calculated values: that
is, the molecular weight of both monomers plus the sodium
ion from the salt for the MS analysis.
conversions of both monomers but also a regulated sequence
from the initiating end to the growing end when coupled
with an appropriate RAFT agent, as reported previously.40
For simultaneous control of molecular weight and monomer
sequence during copolymerization of 1 and the various
maleimides in PhC(CF3)2OH, RAFT copolymerization was
investigated with S-cumyl S0-buthyl trithiocarbonate
(CBTC)57 as the RAFT agent by varying the ([1]0
[PhMI]0)/[CBTC]0 ratio from 20 to 120. The monomer feed
þ
ratio of 1 and PhMI was constant at 1:2.
The copolymerization proceeded smoothly irrespective of the
monomer-to-RAFT agent ratios, and the two monomers were
simultaneously consumed at approximately the same rate, as
observed in the above free radical copolymerizations without
RAFT agents. Figure 5 shows the Mn, Mw/Mn, and size-exclusion
chromatography (SEC) curves of the resultant copolymers
obtained from the RAFT copolymerization of 1 and PhMI by
varying the monomer-to-RAFT agent ratio from 20 to 120. The
Mn values of the obtained copolymers (the circles in Fig. 5) were
measured by SEC, based on the standard poly(methyl methacry-
late) (PMMA) calibration, and were found to increase in direct
proportion to the monomer conversion. However, the Mn values
were lower than the calculated values, which were generated by
assuming that one CBTC molecule generates one living polymer
chain, mainly because of the difference in hydrodynamic volume
with respect to the PMMA standards. The absolute Mn value was
then determined using multiangle laser light scattering (MALLS),
which yielded a value of Mn(MALLS) ¼ 8400 that was similar to
the value calculated from the monomer-to-RAFT agent ratio and
the monomer conversion, Mn(calcd) ¼ 9500. Although the mo-
lecular weight distribution (MWD) became broader as the con-
centration of the RAFT agent was decreased, a copolymer with a
relatively high molecular weight was obtained at a high ([1]0 þ
[PhMI]0)/[CBTC]0 ratio. Thus, the molecular weight of the 1:2
sequence-regulated copolymer of limonene and PhMI can be
controlled using the RAFT system.
Additionally, irrespective of the substituents in the MI units,
the highest series of peaks are separated by the total masses of
one limonene and two MI units, which can be assigned to a
FIGURE 6 Mn, Mw/Mn, and SEC curves of copolymer obtained
in the radical copolymerization of
1 and various MI in
PhC(CF3)2OH at 60 ꢀC; [1]0/[MI]0 ¼ 1/2, [1]0 þ [MI]0 ¼ 1200 mM,
[CBTC]0 ¼ 60 mM, [AIBN]0 ¼ 5.0 mM. The open square indicate
the molecular weights of poly(1-co-CyMI) calculated by 1H
NMR from the a-end peaks.
RAFT copolymerizations also were conducted for other com-
binations with 1 and various MI derivatives, CyMI and EtMI
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FIGURE 7 MALDI-TOF-MS spectra of poly(1-co-MI) in PhC(CF3)2OH at 60 ꢀC; [1]0/[MI]0 ¼ 400/800 mM, [CBTC]0 ¼ 60 mM, [AIBN]0
¼
5.0 mM.
series
of
end-to-end
sequenced
copolymers
selective 1:2-alternating copolymerization; hydrogen-bonding
[CA(MAMAL)nAMAS] (C: cumyl group; S: trithiocarbonyl
group) starting with the CAMAMALAunit at the initiating ter-
minal and the AMAMALAMAS unit at the capping terminal
in addition to the AMAMALA main-chain repeating sequence.
These results again indicate that the copolymerization of limo-
nene and various MI derivatives produces copolymers with
1:2-alternating sequences irrespective of the MI structure.
interactions of the PhC(CF3)2OH solvent with the MI units
also proved to be important. Thus, these various factors
work synergistically to produce an unprecedented 1:2-alter-
nating copolymerization of naturally occurring bulky ter-
penes and MI derivatives. A special function can be imparted
to the novel bio-based sequence-regulated copolymers by
functionalizing the residual double bonds in the terpene
units or by introducing functional groups in the MI units.
CONCLUSIONS
EXPERIMENTAL
In conclusion, this study revealed the effects of the structure
of various unconjugated olefins and MI derivatives on 1:2-
alternating copolymerization in PhC(CF3)2OH. The bulky and
specific structures originating from naturally occurring limo-
nene and b-pinene proved to be crucial factors for inducing
Materials
a,a-Azobisisobutyronitrile (AIBN) (Kishida, >99%) was puri-
fied by recrystallization from methanol. N-Phenylmaleimide
(PhMI) (Aldrich, 97%), N-CyMI (Aldrich, 97%), N-EtMI (Tokyo
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Kasei, >98%) and unsubstituted MI (Aldrich, 97%) were puri-
fied by recrystallization from acetone, toluene, or methanol.
(þ)-d-Limonene ((þ)-1) (Aldrich, 97%), (–)-l-limonene ((–)-1)
(Aldrich, 96%), (6)-limonene (rac-1) (Tokyo Kasei, >95%),
(þ)-a-pinene (2) (Aldrich, ꢄ99%), (–)-b-pinene (3) (Aldrich,
ꢄ99%), 2-methyl-1-pentene (4) (Tokyo Kasei, 98%), 2,3-di-
methyl-1-butene (5) (Aldrich, 97%), 1-hexene (8) (Tokyo
Kasei, >95%), vinylcyclohexane (9) (Aldrich, ꢄ99%), 4-vinyl-
1-cyclohexene (10) (Aldrich, 97%), PhC(CF3)2OH (Wako,
>99%), and AN (Aldrich, ꢄ99%) were distilled from calcium
hydride under reduced pressure before use. CBTC was synthe-
sized according to the literature.40
1H NMR (CDCl3, r.t.): d 1.41–2.63 (m, 7H), 1.70 (m, 3H,
CH2¼¼CACH3), 4.71 (m, 1H, cis-CH2¼¼CACH3), 4.72 (m, 1H,
trans-CH2¼¼CACH3), 5.69 (m, 2H, ACH2ACH¼¼CHACH2A).
Free Radical 1:2 Alternating Copolymerization
Copolymerization was carried out by the syringe technique
under dry nitrogen in sealed glass tubes. A typical example
for Lim and PhMI copolymerization with AIBN in
PhC(CF3)2OH is given below. In a 50 mL round-bottomed
flask were placed PhC(CF3)2OH (0.85 mL), PhC(CF3)2OH
solution of PhMI (3.0 mL of 1200 mM solution, 3.6 mmol),
Lim (0.29 mL, 1.8 mmol), and PhC(CF3)2OH solution of AIBN
(0.36 mL, 0.15 mmol) at room temperature. The total vol-
ume of the reaction mixture was 4.5 mL. Immediately after
mixing, aliquots (0.6 mL each) of the solution were distrib-
uted via a syringe into baked glass tubes, which were then
sealed by flame under nitrogen atmosphere. The tubes were
immersed in thermostatic oil bath at 60 ꢀC. In predeter-
mined intervals, the polymerization waꢀs terminated by the
cooling of the reaction mixtures to ꢃ78 C. Monomer conver-
sion was determined from the concentration of residual
monomer measured by 1H NMR with PhC(CF3)2OH as an
internal standard (106 h, 88% for Limonene and 92% for
PhMI, respectively). The quenched reaction solutions were
evaporated to dry to give poly(Lim-co-PhMI).
Synthesis of Isopropenylcyclohexane (6)
The monomer syntheses were carried out by a syringe tech-
nique under a dry argon atmosphere in an oven-dried glass
tube equipped with three-way stopcocks. Isopropenylcyclo-
hexane (6) was synthesized by a Wittig reaction as follows.
Methyltriphenylphosphonium iodide (133.4 g, 0.33 mol) was
dispersed in dry THF (1200 mL). In to the suspension, nBuLi
ꢀ
(204 mL of 1.62 M solution in n-hexane) was added at 0 C.
ꢀ
After stirring at 0 C for 30 min, the solution was cooled to
ꢃ78 ꢀC, and then cyclohexylmethylketone (0.33 mol in THF,
150 mL) was added dropwise. After stirring at ambient tem-
perature for additional 20 h, the aqueous solution of NH4Cl
was added. The aqueous phase was extracted with Et2O and
then all the organic phase was corrected. The crude product
was purified by repeating fractional distillation to give 6
(10.9 g, 0.088 mol; yield ¼ 27%, purity >99%). 1H NMR
(CDCl3, r.t.): d 1.01–1.34 (m, 5H), 1.65–1.89 (m, 6H), 1.70
(m, 3H, CH2¼¼CACH3), 4.64 (m, 1H, cisACH2¼¼CACH3), 4.64
(m, 1H, trans-CH2¼¼CACH3).
RAFT 1:2 Alternating Copolymerization
RAFT copolymerization was carried out by the syringe tech-
nique under dry nitrogen in sealed glass tube. A typical
example for (þ)-Lim (1) and PhMI copolymerization with
CBTC/AIBN in PhC(CF3)2OH is given below. In a 50 mL
round-bottomed flask were placed PhC(CF3)2OH (1.2 mL),
PhC(CF3)2OH solution of PhMI (5.0 mL of 1200 mM solution,
6.0 mmol), 1 (0.49 mL, 3.0 mmol), and PhC(CF3)2OH solution
of AIBN (0.25 mL, 0.15 mmol) and CBTC (0.56 mL of 134
mM solution in PhC(CF3)2OH, 0.075 mmol) at room tempera-
ture. The total volume of the reaction mixture was 7.5 mL.
Immediately after mixing, aliquots (1.0 mL each) of the solu-
tion were distributed via a syringe into baked glass tubes,
which were then sealed by flame under nitrogen atmos-
phere. The tubes were immersed in thermostatic oil bath at
60 ꢀC. In predetermined intervals, the polymerization was
terminated by the cooling of the reaction mixtures to ꢃ78
ꢀC. Monomer conversion was determined from the concen-
tration of residual monomer measured by 1H NMR with
PhC(CF3)2OH as an internal standard (108 h, 85% for 1 and
86% for PhMI, respectively). The quenched reaction
solutions were evaporated to dry to give poly(1-co-PhMI).
Synthesis of 4-Isopropenylcyclohexene (7)
4-Isopropenylcyclohexene (7) was synthesized by two-step
reactions. First, the corresponding ketone was synthesized
from 3-cyclohexenecarboxylic acid, and then 7 was obtained
by Wittig reaction of the ketone. MeLi (140 mL of 1.07 M
solution in n-hexane) was added dropwise to a solution of
3-cyclohexenecarboxylic acid (37 mL, 0.317 mol) in dry Et2O
at 0 ꢀC under stirring. After 14 h at ambient temperature,
the mixture was added dropwise to HCl aqueous solution.
The mixture was neutralized and washed with the aqueous
solution of NaHCO3 and then NaCl. The crude product was
purified by fractional distillation to give 4-cyclohexenylme-
thylketone (28.9 g, 0.23 mol; yield ¼ 73%, purity >98%).
nBuLi (84 mL of 2.62 M solution in n-hexane) was added
dropwise to a suspension of methyltriphenylphosphonium
Measurements
ꢀ
iodide (89 g, 0.22 mol) in dry THF at 0 C. After stirring at 0
Monomer conversion was determined from the concentration
of residual monomer measured by 1H NMR spectroscopy
ꢀC for 30 min, a solution of 3-cyclohexenylmethylketone
(0.33 mol in THF, 150 mL) was added dropwise at ꢃ78 ꢀC.
After stirring at ambient temperature for 20 h, the aqueous
solution of NH4Cl was added at 0 ꢀC. The aqueous phase
was extracted with Et2O and then all the organic phase were
corrected. The crude product was purified by repeating frac-
tional distillation to give 7 (10.9 g, 0.089 mol; yield ¼ 41%,
purity >98%).
1
with reaction solvent as an internal standard. H NMR spec-
tra for monomer conversion were recorded in CDCl3 at 25
ꢀC on a Varian Mercury 300 spectrometer, operating at 300
MHz, and 1H NMR spectra for the product copolymer were
ꢀ
ꢀ
recorded in CDCl3 at a 55 C DMSO-d6 at 80 C, or acetone-
d6 at 50 ꢀC on a JEOL ECS-400 spectrometer, operating at
400 MHz. MALDI-TOF-MS spectra were measured on a
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This work was supported in part by the Funding Program
(Green Innovation GR051; Precision Polymerization of Plant-
Derived Vinyl Monomers for Novel Bio-Based Polymers) for
Next-Generation World-Leading Researchers from the Cabinet
Office, Government of Japan and Program for Leading Graduate
Schools ‘‘Integrative Graduate Education and Research Pro-
gram in Green Natural Sciences’’. The authors thank Dr. Daisuke
Taura, Dr. Hiroki Iida, and Professor Eiji Yashima (Nagoya Uni-
versity) for their technical support and useful suggestions on
CD measurement.
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