ESR Studies of Vinyl Polymerization in Solution

7 ESR Studies of Vinyl Polymerization in Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 29, 2018 | https://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch007

Solution KOICHI TAKAKURA and BENGT RÅNBY The Royal Institute of Technology, Stockholm, Sweden

The radical polymerization in aqueous solution of a series of monomers—e.g., vinyl esters, acrylic and methacrylic acids, amides, nitriles, and esters, dicarboxylic acids, and butadiene—have been studied in a flow system using ESR spectrometry. Monomer and polymer radicals have been identified from their ESR spectra. β-Coupling constants of vinyl ester radicals are low (12-13 gauss) and independent of temperature, tentatively indicating that the β-CΗ group is locked with respect to the α-carbon group. In copolymeri zation studies, the low reactivity of vinyl acetate has been confirmed, and increasing reactivity for maleic acid, acrylic acid, acrylonitrile, and fumaric acid in this order has been established by quantitative evaluation of the ESR spectra. This method offers a new approach to studies of free radical polymerization. 2

^ V n e of the most promising advances in the application of electron spin resonance ( E S R ) studies to polymer chemistry has been the direct observation of transient free radicals involved in radical polymerization. U n t i l recently, E S R studies of radical polymerization have been limited to solid-state systems or stiff gels i n which the free radicals are more or less stabilized or trapped. The very short lifetime and the low concen tration of the radicals i n liquid systems precluded their analysis i n liquids. Recently, however, Dixon and Norman (7, 8, 9) developed a rapidmixing flow method which made it possible to observe well-resolved E S R spectra of short lived free radicals, using redox reactions for initiation. The redox system they used consists of acidified aqueous solutions of titanous chloride ( T i C l 3 ) and hydrogen peroxide ( H 2 0 2 ) . O n mixing the two reactants, hydroxyl radicals are formed rapidly. If a suitable 125 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 29, 2018 | https://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch007

126

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

substrate is present i n the system, the H O radicals react with it to produce the substrate radical which can be observed by E S R . The application of the flow method to investigate radical polymerization was first made by Fischer (11, 12, 13, 14, 15) mainly on acrylic and methacrylic monomers, while vinyl esters and butadiene were studied i n this laboratory (33, 36). More recently, T o d d et al. (6, 21) studied alkenes, vinyl fluoride, and vinyl chloride using the flow method. For all the monomer systems reported so far, E S R spectra attributed to monomer radicals formed by addition of an initiating radical to a monomer molecule, have been recorded. For certain monomers, the E S R spectra caused by growing polymer radicals have been observed, usually at higher monomer concentration. O n the other hand, Livingston and Zeldes (25, 26) have recently developed a flow technique combined with ultraviolet irradiation to study short-lived radicals in solution. They recorded well resolved E S R spectra of radicals obtained from a number of alcohols by hydrogen abstraction using H 2 0 2 as photosensitizer. No study, however, on the addition reaction to vinyl compounds has yet been reported. The E S R method combined with a flow system should be very powerful for studying short lived transient radicals during vinyl polymerization i n solutions. As w i l l be made clear later, however, the conditions for the reaction occurring i n the flow cell are quite different from the conventional solution polymerization studied during a steady-state process. Nevertheless, the hyperfine structure of the E S R spectra observed by the flow technique, can provide straightforward information on the structure, concentration, reactivity, and even the steric conformation of the transient radicals involved, particularly at the initial stage of the polymerization. As a new approach, we have made E S R studies of polymerization and copolymerization mainly of vinyl esters using the rapid-mixing flow method. In addition to our work, studies by several workers i n the same field are reviewed. Experimental

A n aqueous flow system of the type described by Dixon and Norman (7,8) was used. T w o aqueous solutions acidified with sulfuric acid were used—i.e., one is a T i C l 3 solution and the other a solution of an oxidizing agent (e.g., H 2 0 2 , hydroxylamine, or ferf-butyl hydroperoxide), both solutions containing suitable amounts of monomer. The two solutions were mixed rapidly i n the flow system immediately before they entered the flat cell i n the E S R cavity (for details, cf. Réf. 36). Unless otherwise stated, the reacting solutions we used were (a) an acidified (0.022M H 2 S 0 4 ) aqueous T i C l 3 (0.015M) and (b) either an

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


7.

TAKAKURA

A N D RÂNBY

Polymerization

in Solution

127

acidified (0.022M H 2 S 0 4 ) aqueous H 2 0 2 solution (0.2 ~ 0.3M) or an acidified aqueous hydroxylamine hydrochloride ( 0 . 5 M ) . The flow rate was about 4 ml./sec. giving a time lag from mixing to entering into the E S R cell of about 0.02 sec. The E S R measurements were carried out using an X-band spectrometer with 100 kc./sec. field modulation (Japan Electron Optics Laboratory Co., L t d . , model JES-3B) at room temperature ( 2 2 ° ± 2 ° C ) . The magnetic field was calibrated with proton magnetic resonance signals. The g values were measured with the spectrum of l,l-diphenyl-2-picrylhydrazyl i n methanol solution as a reference standard. Results

and

Discussion

Initiator Systems. The initiators used i n this investigation are hydroxyl, amino, and methyl radicals, generated from the rapid reactions of T i ( I I I ) with H2Oo, N H 2 O H , and tert-butyl hydroperoxide, respectively, all i n acidified aqueous solutions. Extensive E S R studies of free radical species from the reaction of T i ( I I I ) with H 2 0 2 have been made by several workers (4, 7, 8, 19, 20, 27, 28, 36). In general, two E S R signals have been reported—i.e., a principal peak at low magnetic field (Peak 1), and a minor peak at high field (Peak 2 ) . However, there is no general agreement on the assignment of these two E S R signals, and conflicting proposals have been presented. W e believe that the H O radicals which attack monomer molecules i n this system show no E S R signal. The H O radicals are highly reactive and, therefore, are too shortlived to reach a steady-state concentration large enough for observation. According to our recent work (34) Peaks 1 and 2 i n this system are unambiguously interpreted as being caused by Η 0 2 · and H O - radicals, respectively, both coordinated with T i ( I V ) ions or a T i ( I V ) - H 2 0 2 complex. Addition reactions of amino radicals, · Ν Η 2 , to a number of vinyl monomers have been studied by Corvaja et al. (5). N o E S R spectrum arising from free N H 2 radicals, however, has yet been observed, prob ably because of the high reactivity and low concentration also i n this case. Methyl radicals are also produced with T i ( I I I ) ions i n aqueous solu tion, probably according to the reaction: Ti(III) + ( C H 3 ) 3 C O O H - > T i ( I V ) + H O " + ( C H 3 ) 3 C O (CH3)3CO-

CH3COCH3 + C H 3

The formation of C H 3 radicals is concluded from the observation of a quartet spectrum with separation of about 23 gauss in this system (7, 8). Recently we found (3) that radical species from reactions in a Fenton's reagent ( F e ( I I ) - E D T A + H 2 0 2 ) can be used as initiator in the flow system, especially for vinyl compounds containing carboxylic


128

ADDITION

A N D CONDENSATION

POLYMERIZATION

PROCESSES


groups—e.g., acrylic and maleic acid. Fenton's reagent was first used for E S R studies by Shiga (29). H e reported that i n the oxidation of alcohols, this reaction system produced free radicals characterized as ω-oxidized products i n which the furthest position from the alcoholic O H was the main point of attack. However, the exact nature of the reacting species in this redox system has not been revealed as yet. To obtain monomer and polymer radicals of concentrations large enough for E S R analysis 10" 6 M) the intiation rate must be much faster than i n conventional polymerizations. I n the initiating system of T i ( I I I ) - H 2 0 2 , the rate of initiation was estimated to be ^ 10" 2 mole/ liter/sec. A c r y l i c and Methacrylic Monomers. Fischer et al. (5, 11, 12, 13, 14, 15) have studied extensively the transient free radicals formed b y addition of Η Ο · , N H 2 , C H 3 , and C H 2 O H radicals to a variety of acrylic and methacrylic monomers. The addition of C H 2 O H radicals to the monomers could be achieved with a large excess of methanol present i n the T i ( I I I ) - H 2 0 2 system ( 5 ) . They have obtained wellresolved spectra assigned to the monomer radicals resulting from the addition of the four initiating radicals mentioned ( R ) to the monomer molecules. A l l these radicals have the structure R — C H 2 — C X i X 2 and not R — C X i X 2 — C H 2 . This result agrees with the general concept that the methylene group of C H 2 = C X i X 2 is normally more reactive toward free radical addition than the substituted carbon atom. F o r some mono mers—e.g., acrylic and methacrylic acid—well-defined E S R spectra attributed to growing polymer radicals were also observed when the monomer concentration was increased. F o r acrylic acid ( A A ) the spec trum of monomer radicals was replaced b y that of the polymer radicals at a monomer concentration of 1.5 Χ 10" 1 mole/liter (14). The coupling constants for the various radicals obtained i n Fischer's work (14, 18) are summarized i n Table I. The variation i n the ^-coupling constant (αΗβ) of R — C H 2 — C X 3 X 2 radicals with different R groups (cf., Table I) has been interpreted satis factorily as being caused by steric hindrance of the R group attached to the ^-carbon atom (15). This is i n agreement with the current theory of H/3-coupling related to conformation described b y Equation 1 (23). a

m

=

fi

m

(1)

' pa ' cos θ 2

where θ is the angle between the axis of the 2pz orbital of the unpaired electron and the direction of the Ο 3 — Η bond, projected on a plane perpendicular to the direction of the C a — C 3 bond. If the two β-protons are equivalent, the αΗβ values can be given by Equation 2 (14). = βπβ · pa ' 1/4 ( 3 - 2 cos2 φ) = β

Ηβ

·

p

a

· cos2 θ


(2)

7.

TAKAKURA

A N D RÂNBY

Table I.

Polymerization

in

129

Solution

Coupling Constants for Radicals from Acrylic and Methacrylic Acids and Acrylonitrile Coupling Radical

Constants (gauss)

aHa

HO—CH2—CH

&Η€Η*

SLn

Ref.

20.45

27.58

—

—

14

20.17

23.78

—

—

15

21.17

25.03

—

3.40

5

20.23

22.81

—

—

5

—

19.98

23.03

—

14


I COOH CH3—CH,—CH COOH NH2—CH2—CH COOH HOCH,—CH2—CH COOH HO—CH2—CCH3 I Ï O - y C H 2 —CCOHO H V - C HO-VCHo—CH V _ C H 2 — CI H \ COOH Λ COOH CH8—CH2—CCH3

22.62 or 21.34

—

—

14

20.67 —

22.06 15.37

— 21.83

— —

15

—

16.98

24.19

4.96

5

—

14.45

22.27

—

5

—

11.04 13.75

22.45

—

14

HO—CH2—CH

20.10

28.15

—

3.53

14

CH3—CH,—CH

20.10

25.19

—

3.53

15

20.45

23.75

—

3.25

5

20.08

22.89

—

3.44

5

COOH NH2—CH,—CCH,

I COOH HO—CH.,—CCH3

I

COOH H O — ( C H 2 — C C H 3 )„—CH2—CCH,

I

COOH

I

COOH

I CN NH,—CH2—CH

I CN HOCH,—CH,—CH

Platzer; Addition and Condensation Polymerization Processes Advances in C NChemistry; American Chemical Society: Washington, DC, 1969.

130

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

where φ is the free rotation angle between the projection of the C a — R bond and the axis of the 2p orbital representing the average position of the substituent R as illustrated in Figure 1. β β is 58.6 gauss (10), and cos 2 θ is an average for all angles θ attained. z


Η

Figure 1. SteHc conforma tion of the free radical of the type R—CH2—CCa-C&

with the

axis perpendicular the paper plane

to

W i t h increasing bulkiness of the substituent R, i n the order H O , C H 3 , N H 2 , and C H 2 O H , the a /3 values i n most cases tend to decrease. This indicates that the angle of free rotation φ decreases. The smaller α Η β value for polymer radicals of A A can be explained in a similar way as being caused by the much higher bulkiness of the group (i.e., the chain) attached to the β-carbon atom. F r o m the observed spectra, however, it is difficult to distinguish between dimer, trimer, and further growing radical because the variation i n the a /3 values with the degree of poly merization is not significant. H

H

Methacrylic acid ( M A A ) showed an E S R spectrum with well re solved hyperfine structure consisting of 16 lines during redox polymeri zation i n a flow system (13, 14). This spectrum was assigned unambigu ously to the growing polymer radicals. The two ^-protons are apparently not equivalent since they show two different coupling constants (Table I ) . Describing the conformation, the angles θ and θ for β ι - and β protons were 63.1° and 56.4°, respectively, indicating that the R group of long chains in R — C H 2 — C ( C H 3 ) ( C O O H ) is entirely locked i n rela tion to the α-carbon atom. Moreover, this spectrum leads us to the con clusion that the so-called "5 + 5 line" spectrum which has been reported previously for irradiated methacrylic polymers i n the solid state is indeed 1

2


2


7.

TAKAKURA

AND RANBY

Polymerization

i

in

Solution

131

i

Figure 2. ESR spectra of vinyl acetate (VAc) radi cals initiated with the system H202-TiCl3: [H202] = 1.1 X I 0 - J M , m C Z 3 ] = 7 X lO-'M, and [H2SOJ

=

2.2

X

10~*M

A: Monomer radical spectrum at [VAc] — 5.5 X I0~*M. Pi and Pi indicate the position of Peaks 1 and 2, respec tively, which appear from the initiator in the absence of monomer B: Monomer radical and polymer radical spectra over topping at [VAc] = 3.2 X I0"'M. The stick spectrum shows the hyperfine lines assigned to the polymer radicals

caused by polymer radicals. O n the basis of the steric structure and internal rotation of propagating radicals derived from the E S R spectra, Fischer (17) discussed the relative occurrence of isotactic and syndiotactic units in the polymer. The kinetic study of redox polymerization of A A initiated by - C H 3 radicals in aqueous media in the flow system has been made by Fischer (16). The rate constants obtained under the initiation conditions of Ri = 1.83 Χ 10"2 mole/liter/sec. were kp == 0.64 X



132

ADDITION A N D C O N D E N S A T I O N P O L Y M E R I Z A T I O N PROCESSES

10 5 liters/mole/sec., and kt = 1.52 Χ 10 8 liters/mole/sec., both of which were about one order of magnitude higher than for usual solution poly merization. This was interpreted as being caused by the much shorter chain length of the propagating radicals. V i n y l Esters. Various vinyl esters have been studied i n our labora tory (33, 35, 36). The spectra from vinyl acetate radicals ( V A c - ) i n the system of H 2 0 2 + T i C l 3 are shown i n Figure 2. Spectrum A obtained at an ordinary monomer concentration of 5 Χ 10"2 mole/liter can be described as a doublet of triplets of narrow quartets (g = 2.0031). It was assigned to the monomer radical H O — C H 2 — C H ( O C O C H 3 ) . Spec trum Β i n Figure 2 show half of the spectrum obtained at saturated V A c concentration—i.e., about 0.3 mole/liter. Weak signals superimposed on the spectrum of V A c monomer radicals are possibly ascribed to growing polymer radicals. The weak signals Pi and P 2 i n Figure 2A are probably caused by a residue of the initiating system ( H O O and H O - i n complex form) as described previously (34). Using H O - radicals as initiator, E S R spectra for isopropenyl acetate ( I P A c ) , vinyl propionate ( V P r ) , vinyl butyrate ( V B u ) , and vinyl crotonate ( V C r ) were obtained at monomer concentrations below 0.1 mole/ liter. I P A c gave the spectrum of a quartet of triplets. E a c h line split further into a narrow quartet owing to the expected very weak coupling with the three protons i n the acetate group (as shown i n Figure 3 ) . V P r and V B u gave almost the same spectra as V A c except for the narrow triplets arising from the two protons next to the carbonyl i n the ester group. The spectrum from V C r was rather weak. The splitting arising from the ester protons could not be resolved i n this case. Assignment of the observed spectra to vinyl ester monomer radical is verified by using - N H 2 radicals as initiator. W e have observed the spectra from these vinyl esters which showed the expected coupling with the nitrogen atom of the N H 2 group attached to the β-carbon. Coupling with the β-protons was almost the same as that observed for the corresponding HO-adduct radicals. V A c showed an ill-resolved spectrum of low intensity, probably because of the low reactivity of this monomer toward - N H 2 radicals as suggested from its very low Q value (0.026) i n the Q,e scheme of copolymerization (22) (the Q values are considered as a measure of mean reactivity of monomer toward free radical addition). IPc and V P r , however, showed well-resolved spectra. The spectrum of V P r is shown i n Figure 4 where the triplets with the same intensity of components arising from the nitrogen are clearly ob served. The coupling constants obtained for H O - and N H 2 - a d d u c t radicals of various vinyl esters are listed in Table II. For comparison, C H 3 C H O C O C H 3 radicals obtained by Smith et al. (30) from ethyl ace tate by hydrogen abstraction in a flow system are listed also. The spin



7.

TAKAKURA

Polymerization

AND RANBY

in

133

Solution

Figure 3. ESR spectrum of isopropenyl acetate (IPAc) radicals initiated with the system H202-TiCls at UP Ac] = 4.6 X IO"*M, [H202] = 1.5 X 10-*M, [TiCl3] = 7 X I t h ' M , and [ H . S O J = 2.2 X IO"*M

1 I I

I I I

I I I ill

L_LJ

ι_μ

LL

ι 11

|i

J_j

L ± J

ι

11 L_U

L-Ll I J I

L_l—L ill

Lu ι

Figure 4. ESR spectrum from vinyl propionate (VPr) radicals initiated with the system NH2OH-TiCl3 at [VPr] = 5.6 Χ 10~*Μ, [NH2OH] = 2.5 X 10-*M, [TiCls] = 7 X I 0 - ' M , and [ H * S O J = 2.2 X Ι0"*Μ


ADDITION A N D C O N D E N S A T I O N P O L Y M E R I Z A T I O N PROCESSES

134

Table II.

Coupling Constants and Spin Densities Coupling


Substrate

Radicals

Initiator

VAc IPAc

HOHOH2N-

H O — C H 2 — C H ( OCOCH3 ) H O — C H 2 — C ( C H 3 ) (OCOCH3) H 2 N — C H 2 — C (CH3) (OCOCH3)6

20.3 ± 0.1 — —

VPr

HOH2N-

HO—CH2—CH(OCOCH2CH3) H2N—CH2—CH(OCOCH2CH3)6

20.2 ± 0 . 1 20.5 ± 0.1

VBu VCr EAce

HOHOHO-

HO—CH2—CH(OCOCH2C2H5) HO—CH2—CH(OCOCH=CHCH3) CH3—CH(OCOCH3)

20.1 ± 0 . 1 20.4 ± 0.3 18.8 ± 0.7

β Designations a and β refer to the position relative to the carbon which bears the unpaired electron. , # 6 In acid media they are protonated—i.e., H 3 N — C H 2 — C H ( OCOR' ). c Obtained from the Û H 3 values. The average value of Δ(ΟΟΟΟΗ3) obtained from C H

densities for these radicals were determined from the a - C H 3 coupling constants of I P A c monomer radicals using Fischers data (14, 18). W h e n methyl radicals were used as initiator, no detectable amount of V A c radicals was obtained under any experimental conditions. The reactivity of V A c is apparently very low compared with the results for acrylic monomers (14). These findings are, however, i n agreement with the data from methyl affinity studies by Szwarc (32), who reported the reactivity of V A c monomer towards methyl radicals to be about l / 4 0 t h that of acrylonitrile ( A N ) and methyl methacrylate. As seen from Table II, all the vinyl ester monomer radicals show almost the same, very small ^-coupling constants independent of the ester group. Comparing these values with those for C H 3 C H O C O C H 3 radicals, one notices that the α Η β value is much less when one of the hydrogens of methyl group attached to the tervalent carbon is replaced with a substituent such as H O or N H 2 group. There is no significant variation i n the α Η β values for V A c monomer radicals with temperature— i.e., 12.0 gauss ( 8 ° C ) , 12.2 gauss ( 2 0 ° C ) , and 12.8 gauss (~5Q°C). These results indicate that the vinyl ester monomer radicals have the / ? - C H 2 group locked with respect to the α-carbon group i n such a way that the two hydrogens remain equivalent. The free rotation angle φ calculated from the a /3 values using Equation 2 are as follows: 0 ° for H O - V A c - radical, 14° for H O - I P A c - radical, 0 ° and 15° for H O - and N H 2 - V P r - radicals, respectively, and 4 5 ° for C H 3 C H O C O C H 3 . This means that θ is nearly 6 0 ° for both methylene hydrogens and that the R group ( R = H O or N H 2 ) of R — C H 2 — C H ( O C O C H 3 ) is above or below the radical plane. This phenomenon cannot be understood as being caused by enhanced bulkiness of the substituent R, attached to H


7.

TAKAKURA

A N D RANBY

Polymerization

135

in Solution

for Various Vinyl Ester Monomer Radicals Constants0 a


Spin Density pa

ZJJOCOR'

s

CH

H

22.5 ± 0.1 23.0 ± 0.1

12.2 ± 0.1 12.5 ± 0.1 13.1 ± 0.1

1.30 ± 0.04 0.40 ± 0.03 unresolved

— — 8.2 ±

—

12.3 ± 0.1 14.2 ± 0.1

—

12.1 ± 0.2 12.3 ± 0.3

24.0 ± 0.9

—

0.1

0.836 0.768 c 0.785 c

1.61 ± 0.05 1.62 ± 0.05

—

8.3 ± 0.1

0.836 d 0.855 d

1.34 ± 0.04 unresolved 1.44 ± 0.8

— — —

0.836 d 0.836 d 0.819

the p„ values was 0.104, which agrees well with the value 0.109 calculated from the data for ethyl acetate. d A(OCOR') is assumed to be 0.104, irrespective of R group. e Ethyl acetate, from Ref. 30.

the β-carbon. It seems more likely that some type of intramolecular interaction leading to a locked conformation is operative i n this case. A t the moment two probable explanations are conceivable. The first is a formation of intramolecular hydrogen bond between the hydrogen of the β-hydroxyl group or β-amino group and the carbonyl oxygen of the ester group, giving a seven-membered ring structure. F r o m a molecular model, this conformation seems to be feasible although the whole struc ture is not planar. The other interpretation is a titanium chelate of two vinyl ester monomer radicals i n which titanium ions may be coordinated between the two polar side groups. The formation of H O adduct radicals of V A c is mainly related to the concentration of initiator, monomer, and sulfuric acid. The intensity of the signal is affected strongly by the H 2 0 2 / T i C l 3 molar ratio. The maximum intensity was obtained with a ratio close to r = 15. W i t h decreasing ratio below 10, either by increasing the T i C l 3 concentration or by decreasing the H 2 0 2 concentration, the intensity of the E S R signal was decreased considerably. According to simple kinetics, however, the highest intensity is expected at a molar ratio of unity (2). W h e n the H2O2/T1CI3 ratio was kept constant at the optimal value r = 16 with increasing initiator concentration, the intensity at first increased, passed through a maximum, and then decreased. After adding a small amount of ferric chloride, which is known to be a radical scavenger i n the reaction system, the signal intensity decreased markedly down to the E S R noise level. These results suggest that species like T i ( I V ) ions, derived from the redox reaction, may take part i n the termination—e.g., through a process of electron transfer, as previously proposed by Bamford et ai. ( 1 ).


136

ADDITION AND CONDENSATION

POLYMERIZATION

PROCESSES


Such scavenging effects of metal halides seem to be enhanced for reactive radicals having negative e values, such as the V A c radicals. As seen from Spectrum Β i n Figure 2 weak signals caused by V A c polymer radicals were observed at saturated monomer concentration. However, no more distinct spectrum than this one has been obtained so

[VAc] mole/1.

Figure 5. Intensity of monomer (M-) and poly mer radical (P-) spectra at increasing VAc concentration and other conditions as given in Figure 2

30-,

,

[Acrylic acid], mole/l.

Figure 6. Radical concentrations from ESR signal intensities of monomer and polymer radicals of acrylic acid (AA) at increasing concentrations. Reac tion conditions same as in Figure 5



7.

TAKAKURA AND RÂNBY

Polymerization

in Solution

137

Figure 7. ESR spectrum for a polymerizing system of vinyl acetate (VAc) and acrylonitrile (AN) in aqueous solution. The predominant spectral component is assigned to the radical HOCH2CH(OCOCH3)CH2CH(CN). Concentrations are 5.5 X 10~2M of VAc and 1.9 10~2M of AN—i.e., a molar ratio of VAc/AN = 75/25. The stick spectra at the top show the expected relative intensities of the hyperfine lines for VAc and AN monomer radicals

far, owing to the limited solubility of the monomer. The coupling constants obtained from this spectrum are α Η α = 20.3 gauss, α Η β = 17.5 gauss, and a H 0 C 0 C H 3 = 1.3 gauss. The formation of V A c monomer and polymer radicals at increasing monomer concentrations is shown i n Figure 5. In most cases, no polymer-like substance could be detected i n the reaction streams immediately after mixing the two solutions. The very low concentration of V A c polymer radicals is i n marked contrast to the results for acrylic monomers (14) for which intense, well-resolved spectra arising from polymer radicals are easily obtained even at mono mer concentrations below 5 Χ 10"2 mole/liter. For comparison, data obtained for A A under identical conditions i n our work are given i n Figure 6. These results suggest that the highly reactive V A c radicals could terminate preferentially with species such as H O - radicals, V A c monomer radicals, or titanium ions i n the flow system rather than undergo propagation. This is probably caused by the low reactivity of V A c as a



138

ADDITION A N D

CONDENSATION

POLYMERIZATION

PROCESSES

monomer. As reported elsewhere (33), however, introduction of small amounts of a more reactive second monomer such as A N into the V A c system has given a well resolved spectrum attributed to the initial copolymer radicals resulting from addition of comonomer to the V A c monomer radicals—e.g., H O — V A c — A N ·. Copolymerization of VAc with Various Comonomers. The monomer mixtures used i n our experiments were V A c as monomer M i and A N , A A , M A A , acrylamide ( A A m ) , maleic acid ( M A ) , and fumaric acid ( F A ) as monomer M2 (33). A l l the E S R spectra obtained from the binary monomer systems containing vinyl ester and small amounts of M 2 Table III.

Coupling Constants for Free Radicals Monomer Substrate

Radical

HOCH2CH(CN) HOCH2CHCHoCH ( C N )

AN" VAc + A N

OCOCH3 HOCH 2 CCH 3 CH 2 CH ( CN )

IPAc + A N

I

OCOCH3

HOCH2CH(COOH) HOCH2CHCH2CH ( COOH )

AAa AAa

COOH HOCH2CHCH2CH ( COOH )

VAc + A A

I

OCOCH3

HOCH ( COOH ) CH ( COOH ) HOCH2CHCH ( COOH ) CH COOH

OCOCH3

M A or F A a VAc + M A or VAc + F A

HOCH2CCH3(COOH)

MAAa

HOCH 2 CCH 3 CH 2 CCH 3

MAAe

COOH

COOH

HOCH2CHCH2CCH3 (COOH)

VAc + M A A

OCOCH3

HOCH2CH(CONH2)

AAm

HOCH2CHCH2CH ( CONH2 )

VAc + AAm

OCOCH3



7.

TAKAKURA AND RANBY

Polymerization

in Solution

139

showed predominant, new signals assigned to the radical species H O ( vinyl ester ) - Μ 2 · . These spectra are distinguishable from the corre sponding monomer radical spectra of H O - M 2 - by the difference i n the cim values. The initial copolymer radicals are characterized by their low dm values. N o evidence of the reverse type of copolymer radicals—i.e., H O - M s — V A c · was observed. A typical spectrum obtained for the system V A c - A N is shown i n Figure 7. The coupling constants for the initial copolymer radicals are given in Table III together with those of the corresponding monomer radicals. The monomer radicals included give coupling constants, which are i n Obtained from Single and Binary Monomer Systems Coupling

Constants, gauss

a**

Others

20.1 ± 0.1 20.3 ± 0.2

28.2 ± 0.1 20.6 ± 0.2

a N = 3.45 ± 0.07 a N c = 3.42 ± 0.07

20.0 ± 0.2

19.3 ± 0.2

flN

20.6 ± 0.1 21.1 ± 0.2

27.8 ± 0.2 22.6 ± 0.2

20.4 ± 0.2

21.2 ± 0.2

20.80 ± 0.05 20.75 ± 0.05

12.7 ± 0.1 11.1 ± 0.1

19.9 ± 0.1

—

13.8 ± 0.2 11.0 ± 0.2

—

15.5 ± 0.2 9.5 ± 0.2

c

cx

=

3.33

±

0.07

a 3 = 23.0 ± 0.1 CH H

a H C H e = 22.4 ± 0.2

a 3 = 22.4 ± 0.2 CH H

coNH2=L8

±

flH

20.1 ± 0.3

26.6 ± 0.3

aNcoNH2

20.0 ± 0.3

21.0 ± 0.3

flHCONH2=L8 flNcoxH2

=

.0

±

2.0

±

2

o.3

0.3

± 0.3 =

0.3

"The results are in agreement with Fischer's data (14).



140

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

good agreement with data reported by Fischer (14). The smaller α Η β values indicate that the rotation of the R C H 2 group around the C — C bond is restricted to a greater extent i n the copolymer radicals than i n the corresponding monomer radicals. This is probably caused by steric hindrance of the bulky V A c units R, attached to the β-carbon atom. I n comparison, the H O groups on the corresponding monomer radicals are small. The relative concentrations of the various radical species during copolymerization have been estimated from the intensities of the E S R spectral components. Typical results obtained from the V A c - M A and V A c - F A system are shown i n Figure 8, giving the relative radical con centration as a function of comonomer concentration. The addition of small amounts of M2 (molar ratio M 2 / M i $C 0.05) i n each case caused a sharp decrease i n the concentration of V A c monomer radicals, while the

[Maleic acid] or [fumaric I

0

l

l

0.1

Mole fraction of MA

0.2

acid], mole/l. ι

0.3

(or F A ) in VAc - MA (or VAc - FA) system.

Figure 8. Concentration of different radicals measured from ESR spectra during copolymerization of vinyl acetate (VAc) with maleic acid (MA) ana of VAc with fumaric acid (FA) at different molar concentrations of MA in the VAc-MA system and FA in the VAc-FA system, respectively. (VAc) and (MA-) refer to monomer radicals and (VAc-MA-) and (VAc-MA-) to the copolymer radicals observed. [VAcJ = 5.5 X 10~2M; [ M A ] or [ F A ] : variable



7.

TAKAKURA AND RANBY

Polymenzation

in

Solution

141

Concentration of comonomer, [M ], mole/1. 2

Figure 9. Plots of concentration ratio [VAcM * ' ] / [ V A c * ] v s - concentration of comonomer M2 for the systems (I) VAc-FA, (II) VAc-AN, (III) VAc-AA, and (IV) VAc-MA FA = fumaric acid AN = acrylonitrile

AA = acrylic acid MA = maleic acid

[VAc] = 5.5 X 10'M (constant)

concentration of the corresponding copolymer radicals V A c - M 2 - i n creased. This indicates a rapid reaction of V A c monomer radicals with M 2 resulting in an almost instantaneous transformation to V A c - M 2 radicals. M A and F A gave the same E S R spectrum as expected. F A (trans isomer) reacted much faster than M A (cis isomer) with V A c monomer radicals. Plots of the radical concentration ratio [ V A c - M 2 - ] / [ V A c ] vs. concentration of M 2 are shown in Figure 9. The data give straight lines with a characteristic slope, which is a measure of the relative rate of conversion of H O - V A c - to H O - V A c - M 2 - for the different M 2 comonomers. The results obtained are in good agreement with the reported reactivity ratios (22) for these V A c copolymerization systems as shown in Table IV. The conversion rates (i.e., slope coefficients i n Figure 9) can be correlated with the reciprocals of fi values on a qualitative basis. The enhanced reactivity of F A compared with A A and A N is described as a higher value of both Q and e for F A . The difference i n reactivity between F A and M A is most reasonably interpreted in terms of steric effects. It is interesting that the reactivity values for V A c monomer radicals derived from the initial copolymerization reaction step are i n harmony with the data for the reported copolymerization kinetics, although the reaction


142

ADDITION

Table IV.

CONDENSATION

Conversion Rate, liters/mole

(Mt- + A f f - »

PROCESSES

Q,e Values for M

2

ri

VAc

Maleic acid

0.8 Χ 10 2

0.12

VAc VAc VAc

Acrylic acid Acrylonitrile Fumaric acid

1.2 Χ 10 2 1.3 Χ 10 2 2.5 Χ 10 2

0.10 0.061 0.023 a

VAc VAc

Methacrylic acid Acrylamide

IPAc Acrylonitrile Acrylic acid Acrylic acid α

POLYMERIZATION

Conversion Rates of M i * to Μ ι - Μ 2 · Radicals and ri Values from ESR Data

System


AND

— —

I Αι

Q

8.3 0.09 10.0 1.15 16.4 0.60 43.5 0.76

0.01 100.0 2.34 0.016 e 62.5 1.18

0.6 Χ 10 2

0.032α

0.3 Χ 10 2

—

e 1.27 (dimethyl maleate) 0.77 1.20 1.49 (dimethyl fumarate) 0.65 1.30

31.2 0.60

1.20

1.15

0.77

Calculated from Q,e values.

conditions in the flow system are different from the usual polymerization conditions. Miscellaneous Monomers. Butadiene has been studied using H O radicals as initiator in our laboratory (36). The observed spectrum is probably caused by the radical H O — C H 2 C H C H = C H 2 ±± H O — C H 2 C H = C H — C H 2 , formed by hydroxyl addition to the monomer. The major product of the reaction of hydroxyl radicals generated from the system of ferrous ions and H 2 0 2 with butadiene is known to be l,8-dihydroxy-2,6octadiene (21). This product would result from the dimerization of H O C H 2 — C H = C H — C H 2 . Griffiths et al. (21) have also studied buta diene, and they obtained almost the same spectrum. In addition, they recorded the spectrum obtained by the addition of N H 2 to butadiene. The spectrum, however, was highly complex, presumably because isomeric radicals were formed, and an unequivocal assignment was not possible. Griffiths et al. also studied ethylene, vinyl chloride, and vinyl fluoride. The spectra obtained were assigned to the monomer radicals formed by hydroxyl radical addition to the terminal methylene group of the mono mers. However, no spectrum attributed to propagating radicals has been observed, probably because of the very limited solubility—and resulting low concentration—of these monomers i n aqueous media. In addition to the above-mentioned monomers, various acrylic and methacrylic esters, methacrylonitrile, itaconic acid, crotonic acid, methyl vinyl ketone, allyl alcohol, allyl amine etc., have been studied by Fischer



7.

TAKAKURA AND RÂNBY

Polymenzation

in

143

Solution

et al. (5, 14). A l l the monomers show well-resolved spectra arising from monomer radicals. Recently, Smith et al. (31) studied allyl alcohol i n detail using the T i ( I I I ) + H 2 0 2 system and obtained weak signals arising from a growing polymer radical. The coupling constants for some of the monomer radicals are summarized in Table V . The small α Η β values recorded for HO-adduct radicals of vinyl fluoride and vinyl chloride may be interpreted as steric effects arising from certain intermolecular inter action in an analogous way as described for the H O adduct radicals of vinyl esters. Table V. Coupling Constants for HO-Adduct Radicals of Butadiene, Vinyl Fluoride, and Vinyl Chloride Coupling Radicals

HO—CH2—CH—CH=CHo HO—CH2—CH=CH—CH2 HO—CH2—CH

aHa

(doublet)

Constants, gauss

SLh&

(triplet)

SL

X

Ref.

14.9

13.8,12.6

—

36

14.4,4.0

13.4,12.4

—

21

19.1

11.2

17.7

14.4

57.6 (X = F)

21

F HO—CH2—CH

3.0 (X = CI)

21

CI Conclusion

By applying a flow method using rapid redox initiation, E S R studies of vinyl polymerization in solutions have been made. This approach has provided new information on the transient free radicals involved, par ticularly at the initial stage of polymerization. Although the information obtained is significantly helpful for studying ordinary polymerization, the situation in the flow system is quite different from the normal solution polymerization. In this investigation, considerably high concentration of initiator is required, and turbulent flow on mixing may affect the reaction condition appreciably. As a consequence the effect of diffusion should be taken into account, especially when we determine rate constants and monomer reactivity ratios for the reactions occurring i n the flow system. Acknowledgment

This research program is supported by M A L M F O N D E N — S w e d i s h Foundation for Scientific and Industrial Development.


144

ADDITION AND CONDENSATION POLYMERIZATION PROCESSES


Literature

Cited

(1) Bamford, C. H., Jenkins, A. D., Johnston, R., J. Polymer Sci. 29, 355 (1958). (2) Baxendale, J. H., Evans, M.G.,Park, G. S., Trans. Faraday Soc. 42, 155 (1946). (3) Ciubotariu, C., Rånby, Β., unpublished data. (4) Chiang, Y. S., Craddock, J., Mickewich, D. M., Turkevich, J., J. Phys. Chem. 70, 3509 (1966). (5) Corvaja, C., Fischer, H., Giacometti, G., Z. physik. Chem., N.F. 45, 1 (1965). (6) Dewing, J., Longster, G. F., Myatt, J., Todd, P. F., Chem. Commun. 1965, 391. (7) Dixon, W. T., Norman, R. O.C.,Nature 196, 891 (1962). (8) Dixon, W. T., Norman, R. O.C.,J. Chem. Soc. 1963, 3119. (9) Dixon, W. T., Norman, R. O.C.,Buley, A. L., J. Chem. Soc. 1963, 3625. (10) Fessenden, R. W., Schuler, R.H.,J. Chem. Phys. 39, 2147 (1963). (11) Fischer,H.,Z. Naturforsch. 18a, 1142 (1963). (12) Ibid., 19a, 267 (1964). (13) Fischer,H.,J.Polymer Sci. B2, 529 (1964). (14) Fischer,H.,Z. Naturforsch. 19a, 866 (1964). (15) Fischer,H.,Giacometti,G.,J.Polymer Sci. Pt. C, 2763 (1967). (16) Fischer,H.,Makromol. Chem. 98, 179 (1966). (17) Fischer,H.,Kolloid-Z. Z. Polymere 206, 131 (1965). (18) Fischer,H.,Z. Naturforsch. 20a, 428 (1965). (19) Fischer,H.,private communication. (20) Fischer, H., Ber. Bunsenges. Phys. Chem. 71, 685 (1967).

(21) Griffiths, W. E., Longster, G. F., Myatt, J., Todd, P. F., J. Chem. Soc. Β 1967, 530. (22) Ham, G. E., Ed., "Copolymerization," Interscience, New York, 1964. (23) Heller,C.,McConnell, H. M., J. Chem. Phys. 32, 1535 (1960). (24) Kharasch, M. S., Arimoto, F. S., Nudenberg, W., J. Org. Chem. 16, 1556 (1951). (25) Livingston,R.,Zeldes,H.,J.Chem. Phys. 44, 1245 (1966). (26) Livingston,R.,Zeldes,H.,J. Am. Chem. Soc. 88, 4333 (1966). (27) Piette, L.H.,Bulow,G.,Loeffler, K., Am. Chem. Soc., Div. Petrol. Chem. Preprints, April 1964. (28) Sicilio,F.,Florin, R. E., Wall, L. Α., J. Phys. Chem. 70, 47 (1966). (29) Shiga, T., J. Phys. Chem. 69, 3805 (1965). (30) Smith, P., Pearson, J. T., Wood, P. B., Smith, T. C., J. Chem. Phys. 43, 1535 (1965). (31) Smith, P., Wood, P. B., Can. J. Chem. 45, 649 (1967). (32) Szwarc, M., J. Polymer Sci. 16, 367 (1955). (33) Takakura, K., Rånby, Β., J. Polymer Sci. B5, 83 (1967); IUPAC Symp. Macromol. Chem., Brussels-Louvain, 1967.

(34) Takakura, K., Rånby, Β., J. Phys. Chem. 72, 164 (1968). (35) Takakura, K., Rånby, Β., J. Polymer Sci., in press. (36) Yoshida,H.,Rånby, Β., J. Polymer Sci. C16, 1333 (1967). RECEIVED March 27, 1968.


ESR Studies of Vinyl Polymerization in Solution

Recommend Documents