CEJC 3(1) 2005 40–52
Preliminary observations on the copolymerisation of
acceptor monomer:donor monomer systems under
microwave irradiation
Christopher M. Fellows∗
Key Centre for Polymer Colloids, School of Chemistry,
University of Sydney,
New South Wales 2006, Australia
School of Biological, Biomedical, and Molecular Sciences,
University of New England,
Armidale, New South Wales 2351, Australia
Received 16 July 2004; accepted 1 October 2004
Abstract: The mechanistic rationalisation of specific ’microwave effects’ previously reported
in a range of chemical reactions suggests that they may be observable in the freeradical copolymerisation of comonomer pairs capable of forming donor-acceptor complexes.
Polymerisation under microwave irradiation is carried out for several comonomer pairs with
weak donor-acceptor interactions, and no acceleration in rate or change in degree of alternation
attributable to changes in propagation are observed. An increase in reaction rate of 150-200
% is observed for all systems, with trends in molecular weight suggesting this was due to an
increase in radical flux. This is consistent with earlier reports of rate enhancement for freeradical polymerisations using the initiator 2,2’-azobis-isobutyronitrile, and it is proposed that
microwave irradiation may reduce the amount of geminate radical recombination.
c Central European Science Journals. All rights reserved.
°
Keywords: Free-radical polymerisation, copolymerisation, microwave effect
1
Introduction
Microwave heating has been widely used in organic synthesis over the last decade, where
it can greatly improve yields for small-scale reactions by efficient dielectric heating [1-3]
It has been suggested that there are specific ‘microwave effects’ in chemical reactions
heated by microwaves, over and above the benefits of rapid heating without heat ex∗
E-mail: [email protected]
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41
change surfaces [4] A large proportion of these apparent anomalies have been explicable by microwave-induced superheating [5-8], but it is not yet conclusively established
whether all microwave effects can be explained in the same way. This communication
reports on an investigation of possible microwave effects in free-radical copolymerisation.
An a priori rationale for specific effects of microwave irradiation on chemical reactions can be derived from a consideration of activated complex theory, by examining the
expression for the steric factor in the Arrhenius expression for the rate constant of any
bimolecular reaction of a molecule A containing nA atoms and molecule B containing nB
atoms:
P =
0
q(AB)‡
qA qB
.
(1)
0
Here qA and qB are the partition functions for molecules A and B, while q(AB)‡
is the
partition function for the activated complex with the contribution from the vibration
along the reaction coordinate removed.
qA = (qtrans )3 (qrot )3 (qvib )3nA −6
(2a)
qB = (qtrans )3 (qrot )3 (qvib )3nB −6
(2b)
0
q(AB)‡
= (qtrans )3 (qrot )3 (qvib )3(nB +nA )−7 .
(2c)
Where qtrans , qrot , and qvib are the appropriate translation, rotational, and vibrational partition functions respectively. The general expression for the rotational partition function
of a many-atom molecule with no plane or centre of symmetry is:
·
qrot
kT
=
hc
¸ 32 h
π i 12
ABC
(3)
where A, B, and C are the rotational constants of the molecule, inversely proportional to
the moments of inertia about its three principal axes, and k is the Boltzmann constant.
From this analysis it appears that that microwave radiation of an appropriate wavelength might populate higher rotational states to a higher degree than would be expected
for that temperature, giving an effective distribution corresponding to the use of a higher
T in expression (3) than the measured temperature of the reaction T.
If such enhancement was identical for the products and the transition state, this
would be expected to give a reduction in the steric factor and hence in the overall rate
of the reaction (by equation 1), due to the greater number of rotational terms appearing
on the reactant side of the ratio. However, if the activated complex were markedly
more sensitive to microwave radiation than the reactants, it is possible that a steric
factor greater than that found in the thermally-driven reaction might be found. This
enhancement of rate for reactions proceeding through such sensitive activated complexes
would be the ‘microwave effect.’ This argument provides a basis for assertions made in the
literature that microwave irradiation should favour reactions with more polar transition
states [4], but does not appear to have been previously expressed in such terms.
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It is clear that this argument is based on a sphexish [9] interpretation of activated
complex theory, as it implicitly assumes that the transition state for an arbitrary bimolecular reaction is an activated complex capable of sustained independent existence,
during which it can be partitioned between different energetic states by interaction with
microwave radiation. This is untrue for the majority of chemical reactions.
However, many reactions do proceed through activated complex intermediates that
have a discrete physical existence and may persist for some time. The sensitivity of
these species to excitation would be primarily dependent on the charge separation in
the chemical species, as the intensity of microwave absorption is proportional to the
square of the charge separation in the molecule. Hence, possible enhancement of rate
due to a ‘microwave effect’ might be seen where the activated complex shows a greater
charge separation than either of the reactant species. The possibility of microwave effects
may be of interest in free radical copolymerisations between an acceptor and a donor
species that proceed to some extent through formation of a donor:acceptor complex [10] In
such a copolymerisation, changing the amount and/or reactivity of the complex through
microwave excitation might also be expected to have effects on the overall composition
and the microstructure of the copolymer.
It is currently still debatable in many copolymerisation systems whether, and to what
extent, complexes do in fact participate in copolymerisation reactions [11, 12]. Radicalmonomer complexes at the growing chain end of polymers have also been suggested in
some systems [13]. Of further interest are systems where both alternating copolymerisation and Diels-Alder addition may occur– the first reaction proceeding through a relatively
polar transition state, the other through a non-polar one [14]. If non-thermal specific microwave effects were to be verified, microwave-initiated polymerisation would have broad
application to many copolymerisation systems.
As a first exploratory study, a number of copolymerisations between methyl methacrylate or butyl methacrylate (monomers bearing an electron acceptor group) and styrene or
isoprene (monomers bearing an electron donor group) were carried out using a commercial
research microwave system.
It must be remembered that in any condensed phase where kT À Emicrowave energy
will be very rapidly transferred between molecules by collisional processes, and specific
excitation of the donor-acceptor complex is likely to be extremely short-lived. This may
well be sufficient to effectively quench any specific microwave effects, even where a persistent activated complex is expected. For this reason it is also unlikely that the particular
wavelength of microwave radiation used will have a significant effect, as all absorption
lines will show an extreme degree of collisional broadening. This variable is inaccessible with commercial microwaves, which are restricted by law to wavelengths of 12.2 or
32.8 cm; while this is lower energy than the rotational transitions associated with small
diatomic molecules [8], it is of the same order of magnitude as the transitions between
rotational levels for large asymmetric top molecules.
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2
43
Materials and methods
All monomers used were supplied by Aldrich and purified by running through a column
of basic alumina. They were stored at 4 ◦ C. The initiator 2,20 -azo-bis-isobutyronitrile
(AIBN, Aldrich) was recrystallised from ethanol. Toluene (Aldrich) was distilled at reduced pressure. A single stock solution of AIBN in toluene (15.6 mM) was prepared,
stored at 4 ◦ C, and used for all experiments.
Polymerisations were carried out by combining monomers and initiator solution at
◦
50 C or 60 ◦ C in the 10 mL crimped glass vessels supplied with the CEM Discover
Focused Microwave Synthesis System operating at a wavelength of 12.2 cm (2.45 GHz).
Syntheses were either carried out by immersing these glass vessels, containing either 2.0
or 4.0 mL of the reagents, either in a thermostatted water bath (thermal) or by irradiation
in the Discover microwave. An important caveat to this work arises from the mode of
operation of the Discover microwave system; when set to maintain reaction temperature
at a particular value, the instrument exposes the sample to pulses of microwave radiation
at a fixed power level, rather than irradiating continually at lower power. Thus, the
results reported here are likely to underestimate any possible microwave effects.
In typical copolymerisations, solutions of monomers in toluene (20 - 40 % by weight)
were prepared with a final AIBN concentration of either 9.0 or 12 mM.
Full details of the copolymerisations carried out are given in the appendix (Table
A1). Yields were calculated by isolation of the polymer product by precipitation in a
non-solvent (methanol) and drying under vacuum.
Size Exclusion Chromatography (SEC) of selected products was carried out using
samples of 0.10 mgcm−3 concentration with THF as the mobile phase at room temperature in a system comprising Waters Styragel HR4, HR3, and HR1 columns in series,
with differential refractive index detection. SEC was calibrated using poly(styrene) standards, and relative molecular weight values quoted assume Mark-Houwink parameters of
K = 14.1 min−1 and a = 0.7 .
3
Results
The yields obtained and compositions determined by 1 H-NMR for all copolymers characterised are given in Table 1.
The relative rates of all thermally initiated reactions and all microwave initiated reactions, measured by total solids at the end of the reaction (conversions limited to < 20
%, with most reactions < 5 %) are given in Figure 1.
From observation of Figure 1, it appears that:
• The enhancement of rate is not a function of the microwave-initiated systems reaching the nominal reaction temperature in a shorter time, as the proportional enhancement of reaction rate is similar for systems polymerised over periods of 1 hr, 2 hr,
and 4 hr.
• The enhancement of rate appears to be independent of the monomers used for the
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Methyl Methacrylate: Styrene Copolymerisations
Conditions
1
2
3
5
6
7
8
9
10
11
12
13
14
15
16
17
19
60 ◦ C
Thermal
Microwave
50
◦C
Thermal
Microwave
S/MMA
(feed)
S/MMA
(polymer)
Yield
(%)
Mn
Mw
0.997
2.040
1.001
0.504
2.006
0.512
0.500
1.011
2.033
1.998
0.964
0.494
1.940
1.984
0.497
1.006
2.048
1.32
1.37
1.02
0.63
1.65
0.65
0.54
1.41
1.48
1.67
0.99
0.62
1.67
1.67
0.64
1.05
1.70
4.0
3.5
3.6
5.5
4.2
5.1
3.4
11.5
5.7
5.9
3.6
6.7
1.7
1.6
1.9
0.9
5.4
31382
32759
50668
64580
42304
75054
18185
26113
28146
48611
31454
55461
Butyl Methacrylate: Styrene Copolymerisations
Conditions
20
21
60 ◦ C
Thermal
Microwave
S/BMA
(feed)
S/BMA
(polymer)
Yield
(%)
Mn
Mw
1.006
1.023
0.94
0.91
3.4
7.1
45442
30514
78657
52308
Butyl Methacrylate: Isoprene Copolymerisations
Conditions
22
23
60 ◦ C
Thermal
Microwave
IP/BMA
(feed)
IP/BMA
(polymer)
Yield
(%)
1.000
1.041
0.89
0.75
3.4
10.1
Mn
Mw
Methyl Methacrylate: Isoprene Copolymerisations
Conditions
24
25
60 ◦ C
Thermal
Microwave
IP/MMA
(feed)
IP/MMA
(polymer)
Yield
(%)
Mn
Mw
1.029
1.009
1.30
1.20
0.9
1.2
1213
1633
2265
5657
Table 1 Copolymerisation yields and conditions.
limited number of pairs investigated, and is therefore unlikely to arise from the
propagation step of the polymerisation reaction.
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5
microwave rate (% / hr)
4
3
2
1
0
0
1
2
3
4
5
thermal rate (% / hr)
Fig. 1 Rate of polymerisation under microwave heating as a function of rate of polymerisation
under conventional heating. MMA:S copolymers ( ), BMA:S copolymers (¤), MMA:IP copolymers (#) and BMA:IP copolymers (¥). Each data point corresponds to a unique combination
of monomer feed ratios and reaction temperatures, with error bars derived from the maximum
and minimum gravimetry values obtained for samples prepared under those conditions. The line
shown is a least-squares fit to all data points.
In free-radical polymerisation reaction rate is directly proportional to the effective
radical flux, and typical activation energies for azo-initiator decomposition are much
higher than activation energies for the propagation step. The most probable cause of
this enhancement in reaction rate is thus enhanced decomposition of the initiator, AIBN.
Similar microwave-induced increases in free-radical polymerisation reaction rate under
AIBN initiation have been previously reported in homopolymerisation [15-18], with an
effect of a similar magnitude [15, 18] (× ∼ 2) to that observed here (×1.7 ). Higher degrees
of rate enhancement (× ∼ 6) have previously been observed for methyl methacrylate
homopolymerisation initiated by alkyl halides [19].
The molecular weight of selected polymers prepared thermally and by microwaveinitiated thermal polymerisation was determined by Size Exclusion Chromatography, and
the results are given in Table 1. According to the standard model for free-radical polymerisation, the molecular weight should be proportional to the –1/2 power of the radical
flux, so these results suggest a difference in effective radical flux of ∼1.7–1.8. If the rate
enhancement observed was due to an increased rate of propagation, higher molecular
weight polymers would be expected under microwave irradiation. This was only found
to be the case for the copolymerisation of MMA and IP (Table 1), where the microwaveinitiated polymerisation was distinctly bimodal with a prominent shoulder at a molecular
weight of approximately 15 000.
Given the conditions used, there appears to be no direct measurable effect of miUnauthenticated
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C.M. Fellows / Central European Journal of Chemistry 3(1) 2005 40–52
crowave initiation on the propagation reaction in MMA:S polymerisation, although an
effect on MMA:IP copolymerisation cannot be discounted.
The composition of the copolymers was examined in order to establish if more subtle effects were occurring. If a donor-acceptor complex is present, the effectively higher
temperature it may experience under microwave irradiation would be expected to reduce
its relative concentration- i.e., it is more likely to dissociate at higher effective temperatures. If the complex polymerises in a concerted fashion, any changes observed would
thus most likely in the direction of a reduced tendency toward alternation. However, a
trend to reduced alternation would also be consistent with the case where the effective
temperature of the polymerisation was greater than the measured temperature [20]. In
previous reports on the microwave-initiated polymerisation of acrylonitrile and styrene,
a reduction in the alternating tendency of copolymerisation was observed (rAcrylonitrile =
0.08 cf. 0.05 and rStyrene = 0.59 cf. 0.33) [21, 22].
Conversely, if radical-complexes at the chain end are participating in the reaction, it
appears probable that the cross-propagation reaction would be favoured by microwave irradiation, according to the mechanism outlined above, and an enhanced tendency toward
alternation exhibited. These changes in microstructure would be expected to be more
evident than minor changes in rate that may lie within the scatter in the data.
Accordingly, the polymers prepared were characterised by 1 H and 13 C NMR spectroscopy to measure the proportion of donor and acceptor residues along the chain, and
estimate any changes in triad distribution. These triad distributions are sensitive to the
details of copolymerisation mechanism and have frequently been used to differentiate
between different mechanisms [23]. Typical spectra are shown in Figure 2. No significant changes in microstructure indicative of markedly different distribution of monomer
residues along the chain were observed for any poly(S-co-MMA) polymers.
The relation between the composition of poly(S-co-MMA) copolymers (at <20 %
conversion) and the feed composition used to prepare them is shown in Figure 3. No
significant differences in composition for polymers prepared by microwave irradiation can
be seen. For the two BMA:S copolymers obtained, indistinguishable compositions were
also obtained. This preliminary study gives no indication that the propagation behaviour
of MMA:S and BMA:S under microwave irradiation is any different from their propagation
behaviour under thermal irradiation.
These results are consistent with previous reports of an enhancement in the decomposition rate of AIBN under microwave irradiation. This may arise from deficiencies in
measuring the actual reaction temperature [5], giving an elevated temperature that gave
similar overproduction of radicals under the various conditions employed in these investigations; an approximate increase in temperature of ∼6 ◦ C would be sufficient to generate
the rate enhancement observed [20]. This temperature variation is much greater than the
precision claimed by the manufacturers of the instrument but cannot be discounted.
Alternatively, the decomposition of AIBN may be subject to a specific microwave
effect. A number of possibilities can be considered:
(i) The decomposition of AIBN generates two polar radical species from an apolar pre-
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(a)
(b)
8
7
6
5
4
ppm
3
2
1
0
Fig. 2 1 H NMR spectra of copolymers prepared from a 1:1 feed ratio at 60 ◦ C over two hours,
(a) using microwaves, sample 9, and (b) using conventional heating, sample 1.
polymer composition (S:MMA)
2.5
2
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
feed composition (S:MMA)
Fig. 3 Copolymer composition as a function of feed composition for MMA:S copolymerisations,
at 50 ◦ C (thermal ; microwave # ) and 60 ◦ C (thermal ¥ ; microwave ¤). The solid line is the
composition predicted by the terminal model with rStyrene = 0.52 and rM M A = 0.46.
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C.M. Fellows / Central European Journal of Chemistry 3(1) 2005 40–52
cursor, which might be expected, according to the analysis of Loupy and Perreux
[4], to display a specific microwave effect. This model is however predicated on the
stabilisation of a transition state more polar than the ground state by microwave
radiation, which seems to be inconsistent with activated complex theory as discussed
above.
(ii) The influence of the rapidly alternating electric field on the solvent cage surrounding
AIBN may enable the radical fragments to escape the cage more readily, reducing
the effects of geminate recombination; however, this effect would be expected to be
strongly solvent dependent. Rate enhancement effects of a similar magnitude have
been observed for MMA in dimethyl formamide solution [15], for bulk styrene and
MMA [16, 17], and toluene solution (this work).
(iii) The influence of the rapidly alternating electric field may orient the radical fragments
generated by decomposition of AIBN so that their dipoles are aligned, separating
the two radicals more rapidly than they would be expected to diffuse apart and
thus reducing the amount of geminate recombination (Figure 4). The efficiency of
AIBN at 60 ◦ C is approximately 0.6 [24], so the maximum possible enhancement in
rate attributable to this mechanism would be ∼1.5, which would be expected to be
further reduced by the non-continuous nature of the microwave irradiation.
CH 3
H 3C
NC
H 3C
H 3C
CH 3
NC
NC
N
N
CH 3
N
H 3C
N
CN
CH 3
CN
H 3C
CH 3
CH 3
CN
H 3C
CH 3
H 3C
CN
CH 3
H 3C
CN
Direction of Electric Field
Fig. 4 Possible mechanism for enhanced radical generation from AIBN decomposition under
microwave irradiation.
One way to test this hypothesis would be to carry out polymerisations using initiators where geminate recombination is of reduced importance, such as benzoyl peroxide.
Direct investigation of AIBN decomposition under microwave irradiation in the absence
of monomer would also be of value.
This note has reported on an exploratory investigation of donor-acceptor comonomer
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systems for which little participation of charge-transfer complexes has been suggested in
the literature, carried out in order to explore the slim possibility that microwave effects
could encourage the reaction of complexes at the radical chain end and lead to increased
propagation rates and degrees of alternation. When facilities become available we plan to
carry out a similar series of reactions with comonomer systems for which charge-transfer
complexes have been reported to contribute significantly (e.g., maleic anhydride and vinyl
acetate [25]).
4
Conclusions
No specific microwave effects were observed in the propagation steps of the free radical copolymerisation of methyl methacrylate with styrene, or of butyl methacrylate with
styrene and isoprene. Such effects cannot yet be ruled out in the free radical copolymerisation of methyl methacrylate and isoprene. These results are consistent with the
absence of specific microwave effects either generally or in these particular systems, or
with a very limited participation of the donor-acceptor charge-transfer complex in these
systems.
An acceleration of approximately ×1.7 in the effective rate of decomposition of the
radical source, 2,20 -azo-bis-isobutyronitrile (AIBN), consistent with the acceleration under microwave irradiation reported by other researchers for homopolymerisations, was
observed. This may be attributable to rapid orientation of the dipoles of the radical
fragments under the applied alternating magnetic field, leading to a reduction of geminate recombination effects. Further work is clearly required in order to establish the
mechanism of the rate enhancement observed in these systems.
Acknowledgment
The CEM Discover Microwave system used was generously loaned by AI Scientific. The
Key Centre for Polymer Colloids is established and supported by the Australian Research
Council’s Key Centres program. The assistance of Dr Stuart Prescott, Dr In-Woo Cheong,
Ms Narimi Kubota, Mrs Jelica Strauch, and Mr Herbert Chiou in this work is gratefully
acknowledged.
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Appendix
Methyl Methacrylate: Styrene Copolymerisations
Conditions
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
60 ◦ C
Thermal
Microwave
50 ◦ C
Thermal
Microwave
MMA (M)
S (M)
AIBN (M)
S/MMA
0.979
1.262
1.899
1.881
2.572
1.251
2.538
2.549
1.000
1.249
1.267
1.962
2.587
1.294
1.268
2.546
1.904
2.546
1.247
0.976
2.574
1.902
1.886
1.297
2.509
1.300
1.274
1.011
2.539
2.531
1.892
1.278
2.510
2.516
1.266
1.916
1.351
2.555
0.01225
0.00891
0.00906
0.00912
0.00903
0.00904
0.00908
0.00911
0.01215
0.00900
0.00898
0.00898
0.00904
0.00897
0.00901
0.00913
0.00903
0.00898
0.00897
0.997
2.040
1.001
1.003
0.504
2.006
0.512
0.500
1.011
2.033
1.998
0.964
0.494
1.940
1.984
0.497
1.006
0.531
2.048
Butyl Methacrylate: Styrene Copolymerisations
Conditions
20
21
60 ◦ C
Thermal
Microwave
BMA (M)
S (M)
AIBN (M)
S/BMA
1.553
1.542
1.562
1.576
0.00897
0.00897
1.006
1.023
Butyl Methacrylate: Isoprene Copolymerisations
Conditions
22
23
60 ◦ C
Thermal
Microwave
BMA (M)
IP (M)
AIBN (M)
IP/BMA
1.759
1.751
1.759
1.823
0.00856
0.00848
1.000
1.041
Methyl Methacrylate: Isoprene Copolymerisations
Conditions
24
25
60 ◦ C
Thermal
Microwave
BMA (M)
IP (M)
AIBN (M)
IP/MMA
2.182
2.256
2.244
2.276
0.00855
0.00838
1.029
1.009
Table A1 Monomer and initiator concentrations employed for copolymerisations (AIBN = 2,20
azo-bis-isobutyronitrile).
Unauthenticated
Download Date | 6/16/17 11:59 AM