Makromol. Chem. 194, 1893-1899 (1993) 1893

Spontaneous polymerization of amphiphilic vinyl monomers, 4 a)

Spontaneous polymerization of methacrylic derivatives of quaternary ammonium bromides with a long alkyl chain

Yutaka Yasuda: Katsuhiko Rindo, Rikio Bushima, Shuzo Aokib)

Wakayama Research Laboratories, Kao Corporation, 1334 Minato, Wakayama 640, Japan

(Received: June 5, 1992; revised manuscript of September 8, 1992)

SUMMARY: Amphiphilic surface active vinyl monomers (alkyl-2-methacryloyloxyethyldimethylammonium

bromides) with a long alkyl chain (C8, C,,, C,,, C,,, and CIS) were investigated with respect to their polymerization behavior in the absence of an initiator. These monomers polymerized spontaneously in water, through a free-radical mechanism, except for the ‘C, monomer’. In the polymerization of the ‘C,, monomer’, the polymer yield depended on polymerization tempera- ture, and the intrinsic viscosity of the resulting polymer varied with the initial concentration of the monomer just like in ordinary polymerization. The ‘C ,8 monomer’, having low reactivity, became highly polymerizable upon mixing with a quaternary surfactant (dodecyl-2-isobutyryl- oxyethyldimethylammonium bromide) in a particular mole ratio, owing to a change in the monomer aggregation state. In benzene, all of the quaternary methacrylates polymerized spontaneously, too. It is obvious that formation of monomer aggregates, micelles or reverse micelles is essential for the spontaneous polymerization. The spontaneous polymerization of other quaternary monomers, having a hexadecyl group and a polymerizable acryloyl, acrylamido, or methacrylamido moiety, did also occur, yet with lower rate than in case of the methacrylate.

Introduction

In the preceding parts of this it has been shown that certain amphiphilic phosphate monomers, sodium alkyl 2-hydroxy-3-methacryloyloxypropyl phosphates (C,-AHMP), polymerize spontaneously through radical mechanism in the absence of initiator, in water above the critical micelle concentration (cmc), and also in benzene and methanol. The formation of monomer aggregates, micelles or reverse micelles is essential for the spontaneous polymerization, and the aggregation state is a key factor for controlling the reactivity in the polymerization, i. e., the activity in the generation of initiating radical species.

Some amphiphilic vinyl monomers containing carboxlate ‘), ethoxylate 5 ) , and quaternary ammonium moieties 6 - 8 ) as hydrophilic group have been investigated with regard to their polymerization behavior, especially their polymerization kinetics. As a rule, if the monomers possess micelle-forming properties, they may undergo spontane- ous polymerization under certain conditions.

a) Parts 1, 2 and 3: cf. refs. b, Present address: Department of Applied Chemistry, Osaka City University, Osaka 558, Japan.

0 1993, Huthig & Wepf Verlag, Basel CCC 0025-1 16X/93/$05.00

1894 Y. Yasuda, K. Rindo, R. Tsushima, S. Aoki

Some quaternary ammonium salts having a vinyl group 6, 7, and poly(ethy1ene oxide) macromonomer with C,, alkyl groups) have been reported to exhibit anomalous polymerization behavior. Alkyl-2-methacryloyloxyethyldimethylammonium bromides with a long alkyl chain (DMAEMA-C,Br) have been investigated in view of their micelle-forming properties and the pronounced effect of monomer aggregation on the kinetic behavior of polymerization in both water and benzene. However, there is no description on the polymerization behavior in the absence of initiator, namely, on the spontaneous polymerization 6* 7).

The present paper deals with the spontaneous polymerization of DMAEMA-C,Br and some analogous compounds.

Experimental part

Cationic amphiphilic vinyl monomers

A series of alkyl-2-methacryloyloxyethyldimethylammonium bromides (DMAEMA-C,Br) were synthesized by the reaction of 2-dimethylaminoethyl methacrylate (DMAEMA) with the corresponding alkyl bromides in acetone. The crude monomers obtained were recrystallized twice from ethyl acetate. All of the purified monomers showed only one spot in thin-layer chromato- graphy (TLC) (silica gel; acetone/aq. NH, = 911 v/v). The structure of these monomers was confirmed by 'H NMR spectroscopy in CDCI,. The results of elemental analysis agreed very closely with the calculated values, indicating that these compounds were highly pure, though the melting points determined by DSC measurement were found to be considerably lower than those reported in the literatures 6 - 8 ) .

DMAEMA-C,Br: m. p. = 82,O "C (lit. 6): 89,O OC; lit. ,): 84 "C). C16H3,BrN0, (350,3) Calc. C 54,85 H 9,21 N 4,OO

Found C 54,81 H 9,23 N 3,90

DMAEMA-CI2Br: m. p. = 68,2 "C (lit. ,): 88,5 OC; lit. ,): 77 "C). C20H&hN02 (406,4) Calc. C 59,lO H 9,92 N 3,45

Found C 58,55 H 9.97 N 3,43

DMAEMA-C,,Br: m.p. = 75,O"C (lit.@: 96,O"C; lit.,): 105'C). C,,H,BrNO, (434,5) Calc. C 60,81 H 10,21 N 3,22

Found C 60,81 H 10,39 N 3,14

DMAEMA-C16Br: m.p. = 79,O"C (lit.'): 107,OOC; lit.'): 131 "C). C,H4,BrN0, (462,6) Calc. C 62,32 H 10,46 N 3,03

Found C 62,41 H 10,40 N 2,96

DMAEMA-C,,Br: m.p. = 793 "C (lit. 6): 108,O "C; lit. ,): 168 "C). CZ~HSZB~NO, (49096) Calc. C 63,65 H 10,68 N 2,86

Found C 63,47 H 10,81 N 2,73

Other monomers, hexadecyl-2-acryloyloxyethyldimethylammonium bromide (DMAEA-C,,- Br), hexadecyl-3-methacrylamidopropyldimethylammonium bromide (DMAPMA-C16Br) and hexadecyl-3-acrylamidopropyldimethylammonium bromide (DMAPAA-C16Br), were also syn- thesized by the reaction of the corresponding dimethylamino-substituted monomer and hexadecyl bromide, and characterized in similar way as mentioned above.

Spontaneous polymerization of amphiphilic vinyl monomers, 4 1895

DMAEA-C16Br: m.p. = 51,2"C. C,,H,BrN02 (448,5) Calc. C 61,59 H 10,34 N 3,12

Found C 61,49 H 10,16 N 2,86

DMAPMA-CI6Br: m. p. = 36,5 "C. Cz5H,,BrN20 . H 2 0 (493,6) Calc. C 60,83 H 10,82 N 5,68

Found C 60,79 H 10,79 N 5,44

DMAPAA-C,,Br: m. p. = 73,3 "C. Cz4H4,BrN20 (461,6) Calc. C 62,45 H 10,70 N 6,07

Found C 61,90 H 10,44 N 5,90

Cationic surfactant

Dodecyl-2-isobutyryloxyethyldimethylammonium bromide (DMAEB-C,,Br), which may be regarded as the saturated derivative of DMAEMA-C,,Br, i. e., an ordinary cationic surfactant without polymerizable group, was synthesized as follows. Isobutyryl chloride was added dropwise to 2-dimethylaminoethanol (mole ratio 1,l : 1) in chloroform and kept for 1 h at room temperature. After the unreacted acyl chloride had been quenched with methanol, the volatile substances were evaporated. The residue was recrystallized twice from ethyl acetate. 2-Isobutyryloxyethyldimethylamine hydrochloride was obtained in 30% yield. The salt, dissolved in methylene chloride, was neutralized with aq. NaOH and the organic layer was dried over anhydrous Na2S04. 2-Dimethylaminoethyl isobutyrate obtained by solvent evaporation was characterized by 'H NMR spectroscopy (CD30D): 6 = 1,2 (d; 6H), 2,3 (s; 6H), 2,6 (m; 3H), 4,2 (t; 2H). It was then treated with dodecyl bromide in acetone, and the crude product was recrystallized twice from ethyl acetate. The cationic surfactant DMAEB-C,,Br was obtained in 60% yield, m.p. 85,l "C. The purity was checked by TLC: only one spot appeared.

C,,H4,BrN20 (408,5) Calc. C 58,81 H 10,36 N 3,43 Found C 58,82 H 10,29 N 3,33

Polymerization

The polymerization procedure applied was the same as in the previous paper'). The other chemicals used were of extra reagent grade. The conversion was calculated from the weight of the dried polymer obtained by precipitation in acetone. The reduced viscosity of the polymer was measured in chloroform at 25 "C.

Results and discussion

Polymerization of DMAEMA-C,,Br in water

Tab. 1 summarizes the results of the polymerization of micelle-forming quaternary methacrylates above the cmc in the absence of initiator at 60 "C. The monomers with C,,, C,,, CI6 or C,, alkyl chain polymerized spontaneously, in analogy with the case of C,-AHMP2), though the conversion of the 'C18 monomer' was very low and the 'C, monomer' gave only a trace amount of polymer. All the reaction systems became heterogeneous with progress of the polymerization.

The 'C, monomer' even in the presence of a radical initiator, 2,2'-azobis(2-amidino- propane) dihydrochloride, showed lower polymerizability or slower rate of polymeriza-

1896 Y. Yasuda, K. Rindo, R. Tsushima, S . Aoki

Tab. 1. Polymerization of DMAEMA-C,Br in water a)

DMAEMA-C,Br cmc b, [Initiator] C) Temp. Time Conv. [?I mmol/L mmol/L in O C in in % dL.g - '

CS 134 None 4

c12 $93 None 4 None 8

c14 1,28 None c16 0,56 None ClS 0,13 None

60 48 60 5 45 5 45 5 60 24 60 24 60 24 60 24 60 24

Trace - 6 Trace - 83 41 4,40 86 3,lO 59 - 63

-

-

- I -

a) [DMAEMA-C,Br] = 400 mol/L. b, Critical micelle concentration, cf. refs. 6*7).

') 2,2'-Azobis(2-amidinopropane) dihydrochloride.

tion as compared with the 'C12 monomer' (see Tab. I), similarly to the results reported by Nagai et al. '). This seems to be the most probable reason for the lack of reactivity of the 'C8 monomer' in the spontaneous polymerization, though the generation of initiating radical species might be rendered more difficult too.

In order to further investigate the spontaneous polymerization of quaternary methacrylates, the ' c 1 6 monomer', with the highest polymerizability among these monomers, was chosen. As shown in Tab. 2, no polymer was obtained in the presence of p-benzoquinone, diphenylpicrylhydrazyl or air, indicating that the spontaneous polymerization proceeded through a free-radical mechanism, similarly to the case of the amphiphilic phosphate monomers. The effect of penetrating light was ignored because of the result obtained with the brown-colored glass tube. The polymer yield increased enormously with an increase in polymerization temperature, as observed in

Tab 2. tor, temperature and monomer concentration a)

Polymerization of DMAEMA-CI6Br without initiator in water: effect of radical inhibi-

Radical inhibitor [Monomer] Temp. Conv. [?I mmol/L in % dL.g - ' in "C

None 400 400 400 200 100

None 400 p-Benzoquinone (1 OOO ppm) 400 Diphenylpicrylhydrazyl (1 OOO ppm) 400 Air 400

60 45 35 60 60 60 60 60 60

63 20 3 81 75 70 b, 0 0 0

8,20 -

6,lO 5,50

a) Polymerization time: 24 h. b, Performed in a brown-colored glass tube to eliminate the effect of penetrating light.

Spontaneous polymerization of amphiphilic vinyl monomers, 4 1897

the case of the phosphate having a C,, alkyl group2). The intrinsic viscosity of the resulted polymer depended on the initial concentration of the monomer, just like in ordinary radical polymerization.

Effect of cationic surfactant on the polymerization of DMAEMA-CI8Br

Tab. 3 shows the effect of addition of the cationic surfactant DMAEB-C,,Br, i. e., the saturated analogue of the ‘C,2 monomer’. The reactivity of the ‘Cl, monomer’, possessing low reactivity in the spontaneous polymerization, was increased to give the polymer spontaneously, by mixing with 37 mol-Yo of this surfactant. On the other hand, addition of only 25 mol-Vo o f this surfactant did not lead to the polymerization of the ‘CI8 monomer’, though the presence of a radical initiator induced easily the formation of the polymer. It appears that mixing with an adequate amount of the cationic surfactant DMAEB-C,,Br triggers the generation of initiating radical species, similarly to the effect of addition of sodium dodecyl glyceryl phosphate (DGCP) to the phosphate monomer with a C,, alkyl group3). Thus, mixing with the surfactant seems to convert the original aggregation state of the monomer to a less restricted one which is better suited for the generation of initiating radical species. This has been recently confirmed by the increase of the ‘H spin-lattice relaxation time of protons of a phosphate monomer upon addition of DGCP ’).

Tab. 3. CI2Br) in water without initiator a)

Polymerization of DMAEMA-C18Br in the presence of a cationic surfactant (DMAEB-

~

Mole ratio DMAEMA-C18Br/DMAEB-C,,Br

Conv. in Vo

1 oo/o 75/25

63/37 50/50

0 0

92 b, 21

5

a) [DMAEMA-C18Br] + [DMAEB-C12Br] = 400 mmol/L, temp.: 6 0 T , time: 20 h. b, 2,2’-Azobis(2-amidinopropane) dihydrochloride (8 mmol/L) was added.

Polymerization of DMAEMA-C,@ in organic solvents

All of the quaternary methacrylates polymerized spontaneously at 60 “C in benzene (Tab. 4). This solvent is known to induce formation of monomer aggregates in the form of reverse micelles. The polymerizations proceeded homogeneously to give transparent gels. The polymerization reactivity in benzene is higher than that in micellar solution, and the order of the polymer yield varies with the alkyl chain length as follows: C,, > C,, > C,, > C, > C18. It is apparent that in benzene the monomer aggregation acts also as a trigger of the initiation of polymerization.

In methanol, which is known to give an isotropic solution, the polymerization of DMAEMA-C 16Br occurred spontaneously in the absence of initiator only at high

1898 Y. Yasuda, K. Rindo, R. Tsushima, S. Aoki

Tab. 4. Polymerization of DMAEMA-C,Br in benzene without initiator a)

DMAEMA-C,Br Conv. in 070

c8

'14

c16

c16

c12

36 14 87 59 29

a) [DMAEMA-C,Br] = 400 mmol/L, temp.: 60 "C, time: 18 h.

monomer concentration (2 mol/L) to reach 29% conversion at 60 "C after 24 h. At a lower monomer concentration (400 mmol/L), no polymer was obtained without initiator. These results show that some mutual monomer interaction seems to take place at high concentration in methanol and to participate in the initiation of polymeriza- tion, similarly to the case of the phosphate monomer I).

Polymerization of other amphiphilie quaternary monomers

The spontaneous polymerization of quaternary monomers other than DMAEMA- C,Br was investigated to confirm the effect of the polymerizable group. Tab. 5 summarizes the results of the polymerization of quaternary monomers composed of a lipophilic hexadecyl chain and a polymerizable moiety (acryloyl, acrylamido or methacrylamido group), at a concentration of 400 mmol/L, which is sufficient to form micelles or reverse micelles. These monomers also polymerized spontaneously in the

Tab. 5. water and in benzenea)

Polymerization of various quaternary ammonium vinyl monomers without initiator in

Monomer b, Solvent Temp. Time Conv. in "C in h in Vo

DMAEMA-C,,Br Water

DMAEA-C,,Br Water

DMAPMA-C16Br Water

Benzene

Benzene

Benzene Water Benzene

DMAPAA-C,,Br Water Benzene

60 60

60 60

60 60 60 80 80

60 60

24 18 24 18 24

5 18 5 5

24 18

63 59

11 20 Trace 86 ') 0 4

'Race

16 2

a) [Monomer] = 400 mmol/L. b, See Experimental part for the abbreviations. ') 2,2'-Azobis(2-amidinopropane) dihydrochloride (8 mmol/L) was added.

Spontaneous polymerization of arnphiphilic vinyl monomers, 4 1899

absence of initiator in water and in benzene. The polymerization reactivities of these monomers were inferior to that of the methacrylate, and the order was as follows: DMAEMA-C,,Br > DMAEA-C,,Br = DMAPAA-C,,-Br > DMAPMA-C,,Br. These results lead to the conclusion that the spontaneous polymerization of amphiphilic quaternary monomers is not a specific phenomenon of DMAEMA-C,Br but also occurs with quaternary monomers having other polymerizable groups.

Polymerization of methyl methacrylate by DMAEB-Ct2Br

Some quaternary salts, such as benzyldimethylphenylammonium bromide, have been reported to initiate the polymerization of methyl methacrylate (MMA) in methanol through the formation of initiating alkyl radicals9). This may be correlated with the initiation mechanism in the present spontaneous polymerization of quaternary monomers, at least in methanol. Thus, the polymerization of MMA was attempted by using an amphiphilic quaternary salt without polymerizable group, DMAEB-C,,Br. However, no polymer was obtained under the condition described in the literature ') where poly(MMA) was obtained in almost quantitative yield with benzyldimethylphenylammonium bromide (4 mmol of quaternary salt, 2,5 mL of MMA, 1 mL of methanol, at 60 "C, for 7 h). From this result it seems that the initiation mechanism is different in both cases.

Conclusion

The spontaneous polymerization of amphiphilic quaternary monomers occurs in the aggregate state through radical mechanism in analogy with the phosphate monomers. The modification of aggregation state of the monomer by the addition of a cationic surfactant changes the activity in the spontaneous polymerization.

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