Cyclopolymerization and Polymers with Chain-Ring Structures

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Cyclization Reaction of N-Substituted Dimethacrylamides in the Crystalline and Glassy States at 77 Κ T. KODAIRA, M . TANIGUCHI, and M . SAKAI Fukui Universiy, Department of Industrial Chemistry, Faculty of Engineering, Fukui 910, Japan

N-Methyldimethacrylamide (MDMA) cyclizes to give a

5-membered cyclic radical at 77 K. This conclusion has been deduced based upon the ESR measurements of MDMA irradiated at 77 K. ESR studies of MDMA deuterated at its four methylene protons of the methacryl groups supported above conclusion. Crystal data of MDMA tell us that the formation of 5-mem bered ring is the most favorable reaction in the crystalline state. It was found that N-benzyl and N-propyl substituents can also form 5-membered ring at 77Kin both their crystalline and glassy states. Molecular structures of these monomers in these states are not available, but these results suggest that ΒDMA and PDMA have favorable conformations for the formation of 5-membered ring. Cyclopolymerization of Ν-substituted dimethacrylamide(RDMA) has been reported by several research groups(1^7). The repeating units which are expected in the polymers prepared from RDMA are 5-membered ring, 6-membered ring, and uncyclic pendant groups. Structural investigations have shown that their main repeating unit is 5-membered ring. They contain small amounts of 6-membered ring but do not contain any detectable pendant double bond04,5) · Propyl(PDMA) and benzyl(BDMA) substituante form glassy and super cooled liquid states when they are quenched rapidly from above their melting points. The polymerization at the supercooled l i q uid state yields polymers with essentially the same structure as those obtained in the liquid state (7)· The contents of 5-membered ring are almost the same for both polymers formed in the liquid and supercooled liquid states while the fraction of 5-membered ring decreases on their polymerization in the solid state. How ever, the polymers formed in the solid state were found to be amorphous and the mechanism of the polymerization could be ex plained fundamentally based upon that of liquid state(7). The de tailed investigation on the cyclopolymerization of BDMA has led to 0097-6156/82/0195-0107$06.00/0 © 1982 American Chemical Society

Butler and Kresta; Cyclopolymerization and Polymers with Chain-Ring Structures ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

108

POLYMERS WITH CHAIN-RING STRUCTURES

the conclusion that the high tendency of RDMA to cyclization was due to the non-polymerizability of i t s monofunctional counterpart (5). N-Alkyl-N-i8obutyrylmethacrylamide(I) just corresponds to the mono functional counterpart of RDMA and i t would not polymerize to give a high polymer (4,5) . I t has been proposed that the lower the polymerizability of the monofunctional counterparts of b i functional monomers, the higher their cyclopolymerizability(5) and the v a l i d i t y of the hypothesis has been proven i n several monomers (8-11). Some of the reported results on the cyclopolymerization of bifunctional monomers are considered to be the additional e v i dences which support above hypothesis, judging from their high tendency to cyclization and low polymerizability of their monofunctional counterparts(12,13,14). During these studies i t was found that the rate-determining step of the cyclopolymerization of RDMA i n the solid and super cooled liquid states i s the intramolecular cyclization reaction (5,7,15). This conclusion has been deduced because only an unCH. 3

CH.

I

CH.

0

I I

CH -C

CH.-C

0

2

CH

2

1

C-CH

1 ι

N-R

I R

H. Ξ

I

I >

2

ι

I

0

RDMA

CH.

CH -C»

OCH

0

3

1

ι

/

0

2

R

II

I

oo j

H-C-CH

Κ

3

3

Ϊ

1

I

3

2

ι R

2

3

Τ

1

I

III

R

CH„

I CH « C 2

1 X

3

IV

CH„ H. Ξ

I »

'3

CH.-O 3

1 X

cyclic propagating radical (IV) was observed on the measurements of ESR spectra of irradiated RDMA at polymerization temperature. Radiation polymerization of unsaturated monomers i n the solid state proceeds through the i n i t i a t i o n radical(V) which i s formed by the additiom of hydrogen atom to their double bond(16). Based upon these facts the formation of an i n i t i a t i o n radical(II) i n RDMA by the reaction between a hydrogen atom and one of the double bonds of RDMA i s reasonably assumed. This i n i t i a t i o n radical cy-


9. KODAIRA E T AL.

109

^-Substituted Dimethacrylamides

clizee to give the 5-membered cyclic radical(III) which reacts with other monomer to y i e l d the uncyclic propagating radical(IV). At higher temperature where polymerization proceeds, only the uncyclic propagating radical was detected as mentioned previously. Decrease in temperature reduces molecular motion and the polymerization does not proceed. These considerations suggest that we might be able to observe the i n i t i a t i o n radical(II) and the 5-membered c y c l i c radical(III) at lower temperature. The i n i t i a t i o n radical(II) affords a seven line spectrum i f two methyl groups are rotating freely, and the 5-membered c y c l i c radical gives a 3-line spectrum i f the ro tation abount the C-C bond of >C-CH2* group i s free. A 12-line spectrum has already been assigned to the uncyclic propagating radical(15). Therefore, we can easily distinguish these three rad i c a l s and accordingly, the two reaction steps, i . e . , an intramo lecular cyclization and intermolecular propagation between the radical III and a RDMA molecule, by measuring the ESR spectra of irradiated RDMA. These reactions should be strongly influenced by the phases where they are carried out. The effect of the phases on these re actions can be studied by using RDMA because PDMA and BDMA can form glassy, supercooled l i q u i d and crystalline states. The pur pose of the present investigation i s to identify the i n i t i a t i o n ( I I ) and cyclic (III) r ai cals, and to see the effect of the phases on these intramolecular cyclization and intermolecular propagation reactions. Crystalline structure of MDMA has recently been determined(17). Therefore, these reaction procedures can be studied i n connection with the crystalline structure i n the case of MDMA. Experimental RDMA was prepared according to the procedure described pre vious ly (6^,18). MDMA and PDMA deuterated at their CH -C< protons (MDMA-d^ and PDMA-d^, respectively) were synthesized starting from deuterated methacrylic acid which was obtained by the hydrolysis of deuterated ethylmethacrylate prepared based upon the reported procedure using CD90(19). The degree of deuteration was 100% ac cording to NMR measurements. Single crystals of MIMA and MDMA-d^ were grown by slow evaporation of benzene solution at room temper ature. The crystals obtained from both the monomers had form shown i n Figure 1. The coordinate axis system employed for the ESR measurements i s also given i n Figure 1. Samples for ESR spectra were prepared i n Suprasil tubes. The shape of the bottom of the sample tubes was modified to accomodate the single crystal according to Kurlta(20). However an error of about 10° i n determining the orientation of crystal was i n e v i table because of the d i f f i c u l t y i n settling the crystal. ESR mea surements were made with JEOL JES-FE-1X or Varian E-3 EPR X-band spectrometer. γ-Ray irradiation was carried out by using 60Co source. 2


110

POLYMERS WITH CHAIN-RING

Figure 1.

The crystal form of MDMA and MDMA-à ESR measurements.

k

STRUCTURES

and the axes employed for


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KODAIRA ET AL.

111


Results and Discussion ESR Studies of MDMA and MDMA-dA. An isolated radical and rad i c a l pairs are trapped i n MDMA when i t i s subjected to lnonizing radiation at 77 K. The ESR measurements of MDMA at 77 Κ after warming at 195 Κ for 5 min showed that the radical pairs disappear during the heat-treatment but the spectral pattern of the isolated radical does not change though i t s intensity decreases s l i g h t l y (15). The ESR spectrum i l l u s t r a t e d i n Figure 2a i s the one obtain ed on measurements at 77 Κ after the heat-treatment at 195 K. With increasing temperature from 77 Κ to 193 Κ the pattern changes gradually and at 193 Κ i t became the t r i p l e t with an intensity of 1:2:1 and with a coupling constant of 26.5 gauss(Figure 2b). The spectral pattern changed when the ESR spectra were recorded at different orientations of the single crystal which suggests that the hyperfine s p l i t t i n g i s due to α-protons. When temperature was lowered again to 77 R the spectral pattern returned to that of Figure 2a. These phenomena were precisely reproduced when temper ature was increased and decreased repeatedly. The t r i p l e t changed gradually to the 12-line spectrum ascribed to the uncyclic propa gating radical(IV) on further increase i n temperature as shown i n Figures 2c and 2d. The 3-line spectrum i s due to the interaction between an unpaired electron and two protons, and accordingly, i s ascribed to the 5-membered cyclic radical(III). The temperature dependence of the spectral pattern i s interpreted as restricted ro tation about the C-C bond of >C-CH2 group at lower temperature and i t s free rotation at higher temperature. To confirm this as signment ESR spectra of irradiated MDMA-d4 were measured. If the 3-line spectrum i s due to the 5-membered cyclic radical(III), the radical(VI) should be observed i n the case of MDMA-d4, and i t s gives a 5-line spectrum based upon the interaction between an un#

CH.

CH. -CD -C2

•C-CD ·

I

2

VI

-CD

β 'α

--c?

Ι

CH„ 1

C«CD„ σ-

I

VII

paired electron and two deuteriums as shown i n Figure 3c. The coupling constant i s predicted to be 4.1 gauss. The results ob tained by the ESR measurements at the same orientation of crystal as that of MDMA are i l l u s t r a t e d i n Figure 3. The big difference between the ESR spectra of MDMA(Figure 2) and MDMA-d^(Figure 3) indicates that the methylene protons of the methacryl groups are responsible for the spectra i n Figure 2. In accordance with our consideration, the quintet with a s p l i t t i n g constant of 4.0 gauss was detected at higher temperature(Figure 3b). The spectral change between a and b i n Figure 3 indicates the free rotation about the C-C bond of >C-CD2* group at higher temperature and re-


112


STRUCTURES

Figure 2. ESR spectra of MDMA single crystal irradiated at 77 Κ to 5.34 Mrad and heat-treated for 5 min at 195 K. The magnetic field is parallel to the c* axis in the b'c* plane, and recorded at 77 Κ (a), 183 Κ (b), 213 Κ (c), and 243 Κ (d). Spectra b, c, and d were measured after heating to the respective temperature for 20 min. Sensitivity of the instrument was kept constant except for the measurement of spectrum d.


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KODAiRA ET AL.


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10 G

Figure 3. ESR spectra of MDMA-a single crystal measured at the same orienta tion as that of MDMA in Figure 2 after the irradiation at 77 Κ to 5.34 Mrad and subsequent warming to 195 Κ for 5 min, and recorded at 77 Κ (a) and 183 Κ (b). Spectra d and e were observed at 77 Κ after heat-treatment at 213 Κ and 243 K, respectively, for 5 min. Diagrams c and f are predicted patterns for b and e, respec tively. Sensitivity of the instrument was kept constant except for the measurement of spectrum e. k


114


STRUCTURES

stricted rotation at lower temperature. On further increase i n temperature the quintet changed gradually to a 12-line spectrum (Figures 3d and 3e). If we assume that the two β-deuteriums of the methylene group of radical VII take the same conformations as those of two β-methylene protons of the uncyclic propagating radical(IV) of MDMA, the 12-line spectrum i s attributable to the uncyclic pro pagating radical(VII) based upon the following consideration. ESR studies of irradiated MDMA showed that the angles between the ir-orb i t a l and the projection of the two C^-H bonds along the C -Cg bond of radical(IV) are 0° and 120°, respectively, and accordingly, each deuterium should give rise to coupling constants of 6.1 and 1.5 gauss. The latter s p l i t t i n g i s considered to be too small to be observed due to line broadening i n present experimental condi tions. Therefore, each component of the quartet produced by the freely rotating methyl group bonded to C should s p l i t s into a t r i p l e t with a coupling constant of 6.1 gauss and with an equal intensity. The 12-line spectrum i l l u s t r a t e d as a stick diagram i n Figure 3f should be observed. I t agrees well with 12-line spec trum obtained from irradiated MDMA-d^ as shown i n Figure 3e. The 12-line spectra i n Figures 2 and 3 are not seen at lower temperature although sensitivity of the instrument was kept con stant during these measurements. These facts suggest that uncyclic propagating radicals were formed during the heat-treatment. The intensities of the 12-line and 3-line spectra of Figure 2 were plotted against temperature from 183 Κ to 263 Κ i n Figure 4. The total amount of radicals and the quantity of the 5-membered c y c l i c radical decrease with increasing temperature, but the 12-line which was not observed at 183 R appears, and i t increases i t s intensity and then decreases. Figure 4 shows that the intermolecular reac tion between the 5-membered cyclic radical (III) and a MDMA molecule occur β at the temperature range from 183 R to 263 K. MDMA molecule can take various conformations. The distance between the two intramolecular double bonds i s rather long i f i t takes an extended structure. Such a conformation i s unfavorable a

a

?

I Me J Me J ?I f

C3

,

H-^ ^C2

C2 ^

I 0

C 3

^H

B-CH · 2

I

f

*ci^ ^cii N

0

IX

I VIII Me Me-Methyl group for cyclization reaction at 77 Κ because of the restricted rotation about chemical bonds. Therefore, i t i s interesting to know the molecular structure of MDMA i n crystalline l a t t i c e . A cryetallographic study of MDMA revealed that the distance between the carbon atoms C2 and C2 (VIII) i s 2.909 Â, and i t i s the shortest one among the distances between possible reaction sites (17). This result ,


KODAiRA ET AL.

180

1

N-Substituted Dimethacrylamides

200

220

240

260

Temp , Κ Figure 4. Variation of radical concentration [R-] trapped in MDMA irradiated at 77 K to 5.34 Mrad, and recorded at each temperature after heat-treatment for 20 min. Key: C , 3-line; ·, 12-line; and O, total concentration.


116

POLYMERS WITH CHAIN-RING STRUCTURES

suggests that the formation of 5-membered ring i s the most favor able reaction i n the solid state of MDMA* It can not be said any thing from the ESR spectra of Figures 2a and 2b on the structure of the part written as Β i n Scheme IX except the fact that i t does not contribute to the hyperfine structure of ESR. However, this crystallographic study supports the assignment of the t r i p l e t to the monomeric 5-membered cyclic radical(III). It appeared that temperature lower than 77 Κ i s required to observe i n i t i a t i o n rad i c a l i n the solid state cyclopolymerization of MDMA. ESR Studies of PDMA, PDMA-d&. and BDMA. To see the effect of the eubstituente on nitrogen of RDMA and the phases where the re actions are carried out, ESR spectra of irradiated PDMA and BDMA were investigated. These monomers are favorable for this purpose because they form glassy, supercooled l i q u i d , and solid states, de pending on the conditions by which they are treated. Irradiated polycrystalline PDMA gives a broad 3-line spectrum with a coupling constant of about 20 gauss as shown i n Figure 5a. With increasing temperature from 77 Κ to 193 Κ the central line of the t r i p l e t sharpens strongly, leaving the side bands almost unaltered(Figure 5b). When temperature was lowered to 77 Κ the spectral pattern returned to that shown i n Figure 5a. These phenomena were essen t i a l l y the same as those observed i n the irradiated single crystal of MDMA. Therefore, the 3-line spectrum i n Figure 5a i s attribut able to the 5-membered cyclic radical(III). ESR spectrum of i r r a diated PDMA-d^ i s shown i n Figure 5c. I t does not show hyperfine structure, but the large difference between Figures 5a and 5c i n dicates that the methylene protons of methacryl groups are closely related to the active species. When temperature was increased to 193 Κ the quintet with a coupling constant of 4.0 gauss was detect ed (Figure 5d). The hyperfine s p l i t t i n g i s considered to be due to the interaction between an unpaired electron and two deuteriums. The spectrum i l l u s t r a t e d i n Figure 5c reappeared when ESR was mea sured again at 77 Κ. These spectral change just corresponds to what has been observed i n irradiated MDMA-d^ and the active species which yields these spectra i s ascribed to the 5-membered cyclic radical(VI). Glassy PDMA irradiated at 77 Κ contains the anion radical of methacryl group which affords 3-line spectrum with a coupling con stant of 11 gauss. This anion radical could be bleached at 77 R by UV-light(21). ESR spectrum obtained after photo-bleach i s shown i n Figure 6a. When temperature was raised, i t s pattern changed to that depicted i n Figure 6b. Central part decreases i t s intensity as temperature i s lowered(Figure 6c). This temperature dependence of spectral pattern i s very similar to that observed i n crystalline MDMA and PDMA. Thus, the radical which yields these ESR spectra i s ascribed to the 5-membered cyclic radical(III). PDMA-d^ i n glassy state yields spectra shown i n Figures 6d-6f. These spectra were recorded after photo-bleach at 77 R. I t i s d i f f i c u l t to identify these spectra, but they clearly show that


d

Figure 5.

ESR spectra of polycrystalline PDMA and PDMA^ irradiated at 77 Κ to 5.34 Mrad. Recorded at 77 Κ (a,c) and 193 Κ (b,d).


118


STRUCTURES

a

Figure 6. ESR spectra of glassy PDMA and PDMA-â^ bleached by UV-light after the irradiation at 77 Κ to 5.34 Mrad, and recorded at 77 Κ (a,d) and 163 Κ (b,e). Spectra c and f were observed at 103 Κ after the measurement at 163 K.


9. KODAiRA E T AL.


119

the methylene protons of methacryl groups are responsible for the spectra i n Figures 6a and 6b and support their assignment to 5membered cyclic radical(III). BDMA irradiated at 77 Κ i n the s o l i d state contains some un known species, judging from the spectrum shown i n Figure 7a. How ever, i t disappears on Increasing the temperature of the system and spectrum shown i n Figure 7b was observed at 173 K. This spec trum changes to the pattern given i n Figure 7c on measurement at 103 K. This spectral change between Figures 7b and 7c i s revers i b l e , depending on temperature. ESR spectra of glassy BDMA are depicted i n Figure 8. They are taken after photo-bleach at 77 R because an anion radical of methacryl group was detected. ESR spectra of BDMA i n crystalline and glassy state and their tempera ture dependence just correspond to those observed i n crystalline and glassy PDMA, respectively. These facts permit us to consider that 5-membered cyclic radical(III) i s formed i n both glassy and crystalline states of BDMA irradiated at 77 K. Conclusion A l l these results show that cyclization reaction occurs at 77 R i n the three substituents studied. Crystal data of MDMA t e l l us that the formation of 5-membered ring i s the most favorable reac tion i n the crystalline state of MDMA(17). Based upon this result PDMA and BDMA are considered to have favorable conformation for the cyclization to form 5-membered cyclic radical(III) i n both the crystalline and glassy states. To detect the i n i t i a t i o n radical (II) irradiation and ESR measurements have to be carried out at lower temperature than 77 R.


120


STRUCTURES

Figure 8. ESR spectra of glassy BDMA bleached by VV-light after the irradiation at 77 Κ to 5.34 Mrad, and recorded at 77 Κ (a) and 193 Κ (b). Spectra c was ob served at 103 Κ after the measurement at 193 K.


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Acknovlegements The Grant-in-Aid for individual research(365322) from the Ministry of Education, Japan, is acknowledged. A part of this work was done under the Visiting Researchers Program of Ryoto University Research Reactor Institute. 9

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February 1, 1982.


Cyclopolymerization and Polymers with Chain-Ring Structures

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