photocrosslinking of polymethoxymethylstyrene and copolymers containing...

P ho t ocr osslinking of Pol yme t hox yme t h yls t yrene and Copolymers Containing (Z,Z-Disubstituted-1,3-

Dioxolan-4-y1)methoxymethylstyrene

HIROYUKI FUKUDA and YOSHIHIRO NAKASHIMA, Nagoya Municipal Industrial Research Inst i tute , 4-41, Rokuban 3-Chome, Atsuta-Ku, Nagoya

456, Japan

Synopsis

Methoxymethylstyrene (MSt), (2,2-dimethyl-1,3-dioxolan-4-yl)methox~ethylst~ene (MMSt), and (2-ethyl-2-methyl-l,3-dioxolan-4-yl)methoxymethylstyrene (EMSt) were synthesized and homopolymerized and copolymerized. The photochemical behavior of resultant homopolymers and copolymers with methyl methacrylate (MMA) and styrene (St) were investigated. The infrared (IR) and ultraviolet (UV) spectra of poly(MSt) showed that new bands ascribed to methyl benzoate residue increase rapidly with irradiation time in air, but no detectable changes are observed in vacuum. The solubility measurements of poly(MSt) indicate that the main factor in crosslinking is the direct coupling of the benzyl radical generated by UV irradiation, which was confirmed by photopolymerization of MMA by means of benzyl methyl ether. I t was also found that copolymers of MMSt or EMSt with MMA or St are easily crosslinked by UV irradiation. From the results of solubility measurements of these copolymers irradiated both in air and in vacuum, it was concluded that not only the 1,3-dioxolane structure but also the benzyl methyl ether structure takes part in photocrosslinking, as we expected.

INTRODUCTION

It is well known that the methylene group adjacent to the oxygen atom in ether is susceptible to oxidation to form hydroperoxide at elevated temperature or with ultraviolet (UV) light irradiati0n.l This reaction can be used in the oxidative curing of resins for paint or thermosetting. Copolymers containing tetrahy- d r o f u r f ~ r y l ~ , ~ and a lk~xyalkyl~ ,~ acrylates have been synthesized and applied to this field, Although alkyl benzyl ethers are oxidized more easily than dialkyl ethers,l the crosslinking of polymers with pendant alkyl benzyl ether structure have not been reported.

We have investigated the photocrosslinking of polymers bearing the 1,3- dioxolane structure as a photocrosslinkable function.6-8 During this study it was found that polymethoxymethylstyrene is subject to oxidation to give a crosslinked polymer upon UV irradiation. Therefore, if poly[ (2,2-disubstituted-1,3-dioxolan-4-yl)methoxymethylsty~ene] could be prepared, a synergistic effect of the benzyl group and the 1,3-dioxolane structure on photocrosslinking would be anticipated.

In this article we wish to describe the preparation and photocrosslinking of polymethoxymethylstyrene and related polymers containing the 1,3-dioxolane structure. Furthermore, the mechanism of photocrosslinking is discussed in detail.

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 21,1423-1433 (1983) 0 1983 John Wiley & Sons, Inc. CCC 0360-6376/83/051423-l1$02.10

1424 FUKUDA AND NAKASHIMA

EXPERIMENTAL

Materials

4-Hydroxymethyl-2,2-dimethyl-1,3-dioxolane (bp 8O-8l0C/11 mm Hg) and 2-ethyl-4-hydroxymethyl-2-methyl-1,3-dioxolane (bp 68-69"C/2 mm Hg) were prepared from glycerine and the corresponding ketones in the presence of p - toluenesulfonic acid. Methoxymethylstyrene (MSt, bp 83-87"C/9 mm Hg) was synthesized by the reaction of chloromethylstyrene (rn - / p -isomer 60/40) with sodium methoxide. Methyl methacrylate (MMA), styrene (St), N,N-dimeth- ylformamide (DMF), and benzene were purified by accepted procedures. Azobisisobutyronitrile (AIBN) and the other chemicals were of commercial grade.

Monomers

(2,2-Dimethyl- 1,3-Dioxolan-4-yl)methoxymethylstyrene (MMSt)

To a solution of 100 mL of DMF containing 16 g (0.12 mol) of 4-hydroxymethyl-2,2-dimethyl-1,3-dioxolane, 4.8 g (0.12 mol) of sodium hydroxide, and a trace amount of cupric chloride, 16.8 g (0.11 mol) of chloromethylstyrene was added dropwise at 0°C. After the addition was completed, the reaction mixture was heated with stirring at 60°C for two days. The mixture was poured into 500 mL of water and then the organic layer was extracted with ether three times. After the etheral solution had been dried over sodium sulfate and concentrated under vacuum, the residue was distilled under vacuum to give MMSt; yield 20.5 g (75%), bp 125-127"C/0.4 mm Hg.

ANAL. Calcd for C15H2003: C, 72.54%; H, 8.13%. Found: C, 72.67% H, 8.21%.

(2-Ethyl-2-Methyl- 1,3-Dioxolan-4-yl)methoxymethylstyrene (EMSt)

The synthetic procedure is similar to the foregoing, starting from 8.8 g (0.06 mol) of 2-ethyl-4-hydroxymethyl-2-methyl-1,3-dioxolane, 2 g (0.08 mol) of sodium hydride, and 10.7 g (0.07 mol) of chloromethylstyrene. The crude product was purified by distillation under reduced pressure to afford EMSt; yield 10.7 g (68%), bp 125-128"C/O.l mm Hg.

ANAL. Calcd. for C16H2203: C, 73.24%; H, 8.47%. Found: C, 73.38%; H, 8.53%.

Polymerization

Homopolymerizations and copolymerizations were carried out in DMF or benzene at 60°C with AIBN as initiator by the sealed-tube method. The polymers were isolated by pouring the polymerization mixture into methanol, and purified by reprecipitation from acetone into methanol and dried in vacuo at room temperature for 24 h.

UV Irradiation

A Toshiba high-pressure mercury lamp, SHL-100 UV (75 W), was employed as a light source in UV irradiation. Films of polymers cast on the glass plates

POLYMETHOXYMETHYLSTYRENE PHOTOCROSSLINKING 1425

were irradiated at a 10 cm distance from the source at 30-35°C. Irradiation in vacuum was carried out in a quartz or Pyrex tube.

Photopolymerization of MMA

Bulk and solution photopolymerizations by means of benzyl methyl ether were carried out by using a Toshiba SHL-100 UV lamp at 30°C. Polymers formed at low conversion (<lo%) were isolated by precipitation with methanol and dried under vacuum at 60°C. The molecular weight of poly(MMA) was determined in benzene viscometrically at 30°C by using the following equationg:

[4 = 8.69 x 10-5~0,.76

RESULTS AND DISCUSSION

Synthesis and Polymerization of Monomers

Methoxymethylstyrene (MSt) was prepared in excellent yield by the reaction of chloromethylstyrene (m-lp-isomer 60/40) with sodium methoxide. The monomers in the type of methoxymethylstyrene having the 1,3-dioxolane structure were synthesized by a similar method; that is, the reaction of chloromethylstyrene with 2,2-disubstituted-4-hydroxymethyl-1,3-dioxolanes in the presence of sodium hydroxide in DMF at 50°C, and (2,2-dimethyl-1,3-dioxolan-4-y1)methoxymethylstyrene (MMSt) was obtained. In the course of synthesis of (2-ethyl-2-methyl-l,3-dioxolan-4-yl)methoxymethylstyrene (EMSt), there was an unexpected difficulty: polymerization took place gradually even in the presence of hydroquinone as an inhibitor. However, EMSt was success- fully prepared by using sodium hydride instead of sodium hydroxide. These monomers, especially MMSt, are comparatively unstable and easily polymer- izable to give crosslinking polymers.

CH,=CH CH,=CH b, ,A CH,=CH HOCHFH-CH,

6 \ - 6 KaOHorNaH in R,/Ch¶ DMF * Q

CHz

0

CH,C-CH, I I

I I

CH, CH,Cl I 0 I CH3

O\ /O /c\

MSt

R, R2 MMSt; R1 = Rz = CH3 EMSt; Ri = CH,CH,, Rz = CH,

It has been reported that radical polymerization of 4-methoxymethylstyrene proceeds with crosslinking by peroxide initiators.lo Therefore, solution polymerization of these monomers was carried out with AIBN as an initiator. Soluble homopolymer of MSt[poly(MSt)] was obtained over a wide range of conversion. On the other hand, poly(MMSt) and poly(EMSt) were homogeneous in the


TABLE I Polymerization of Monomersa

Feed Mi Mz

MSt MSt MSt MMA MSt St MMSt MMA MMSt St EMSt MMA EMSt St

Solvent

DMF Benzene DMF DMF Benzene Benzene Benzene Benzene

Polymer Yield mlb mzb Mwc M"C

(%) (mol %) (xio-4) (xio-4) M,IM,,

72 100 9.49 74 100 27.9 73 45 55 8.60 81 40 60 7.48 83 37 63 14.5 89 28 72 10.1 79 33 67 13.0 80 29 71 8.57

2.84 3.34 5.10 5.47 2.75 3.13 2.01 3.72 5.00 2.90 3.21 3.16 4.40 2.95 2.67 3.21

a 2 g MI, M2; 1 wt % AIBN; 5 mL solvent; temperature 6 O O C ; time 48 h. Measured by the nuclear magnetic resonance spectrum. Measured by the gel permeation chromatogram.

polymerization tube at less than 50% conversion. Once isolated and dried, these polymers turned out to be insoluble. In order to prepare soluble polymers containing the 1,3-dioxolane structure, copolymerizations of MMSt and EMSt with MMA or St were carried out. The copolymers thus obtained were soluble in acetone, benzene, chloroform, and aprotic dipolar solvents but insoluble in methanol, ether, and hexane. These results are summarized in Table I.

Photochemical Behavior of Polymers

It was found that poly(MSt) is crosslinked readily by UV irradiation. The infrared (IR) and UV spectra of poly(MSt) yere measured both in air and in vacuum. The changes of the spectra of poly(MSt) with UV irradiation in air are shown in Figure 1. New absorption bands at 3450,1730, and 1280 cm-l, which are assignable to hydroxyl, carbonyl in ester, and ether linkage in ester, respectively, were observed on prolonged irradiation. The UV spectra of pol- y(MSt) irradiated in air revealed that new bands in the vicinity of 240 and 280 nm increase rapidly with irradiation time. These spectral changes indicate that poly(MSt) is photo-oxidized by benzylic methylene to produce the polymer containing methyl benzoate residue. Methyl benzoate, in fact, shows A,,, at 230 ( 6 = 1.23 X lo6) and 278 nm ( E = 8.91 X lo2) in the UV spectrum. On the other hand, almost no detectable changes in poly(MSt) were observed from the IR and UV spectra in the case of vacuum irradiation.

Solubility measurements were carried out according to the previously reported method7; the evaluation of the weight loss by immersing the irradiated samples, which were cast on glass plates poly(MSt), in acetone for 24 h at room temperature. The weight loss of poly(MSt) irradiated in air decreased rapidly and poly(MSt) came to be almost completely insoluble after irradiation for about 15 min. An additive effect of benzoin and cobalt naphthenate was also investigated because marked acceleration of crosslinking with these compounds has been observed for the photocrosslinkable polymers.6-8 Contrary to our expec- tation, the effect was found to be suppressive rather than accelerative as shown in Figure 2. UV irradiation of poly(MSt) in vacuum was also performed in a


L LWO 3000 2wO 1600 1200 800

Wave Number Icm- ' )

240 260 280 300 320 Wave Leng th ln rn l

Fig. 1. IR and UV spectral changes of poly(MSt) by UV irradiation in air.

quartz tube. Unexpectedly, it was found that the rate of insolubilization in vacuum is far faster than that in air. These results indicate that benzylic methylene participates in crosslinking of poly(MSt), and that the main factor of crosslinking is not the oxidative curing but the direct coupling of the yielding radical a t benzylic methylene.

I rradiotion t ime (min)

Fig. 2 . Relation between weight loss of poly(MSt) and irradiation time in air: (0) no catalyst, ( A ) 5 wt % benzoin, (A) 5 wt % cobalt naphthenate, (0) no catalyst in vacuum, (+) no catalyst in vacuum by using a Pyrex tube.


1.2

0.8

0.6

ln

0.2

2 4 6 0 1 0 Tim$( h-’)

Fig. 3. Charlesby’s plot of poly(MSt) irradiated in vacuum by using a Pyrex tube.

When a Pyrex tube was used, vacuum irradiation of poly(MSt) brought about crosslinking gradually. The data were analyzed by Charlesby’s equation,ll which is applicable to the simultaneous chain scission and crosslinking of polymers with random initial molecular distribution. For photoprocess, the equation is

S + S1I2 = alp + l/flFnJot

PolylEMSt- S i 1

PoIyIEMSt- MMA)

3000 2000 1600 1200 800

Wave Number lcrr-’1

Fig. 4. IR spectral changes of poly(EMSt-St) and poly(EMST-MMA) by UV irradiation in air: (-1 before irradiation, (- - -) after irradiation for 2 h.


Irradiation timelrninl

polylEM St- MMAI

i c"

20 LO 60

polylEMSt-St I 'o

Irradiation time lminl

Fig. 5. Relation between weight loss of copolymers and irradiation time in air: (0) no catalyst, (A) 5 wt % benzoin, (A) 5 wt 5% cobalt naphthenate, (0) no catalyst in vacuum.

where S is the weight fraction of the soluble part of the polymer after irradiation time t , a and /3 are constants which are proportional to the rate constants of the chain scission and crosslinking, respectively, 10 is the incident intensity of UV light, and P,, is the initial number-average polymerization degree. Upon plotting S + S1/2 against llt, a straight line was obtained as shown in Figure 3. The intercept alp = 0.23 suggests that the rate of crosslinking is faster by about 4.3 times compared with that of chain scission. This value is considerably smaller than that obtained from polystyrene, 0.42,12 and similar to that observed for poly(p-methylstyrene), 0.26.12

20 LO 60

Irradiation tirnelminl

Fig. 6. Relation between weight loss and irradiation time in air: (0) Poly(EMSt-MMA), (A) poly(MSt-MMA), (0) poly[ (2-ethyl-2-methyl-l,3-dioxolan-4-yl)methyl acrylate].


TABLE I1 Photopolymerization of MMA by means of BME at 3OoC

[BME] X lo2 R, x 105 [11 (mol/L) (mol/L s ) (dL/g) M, x 10-5

0.93 2.89 2.60 7.76 1.97 3.28 2.41 7.02 4.34 4.04 2.08 5.75 9.11 5.11 1.80 4.79

12.24 5.67 1.65 4.27

Photocrosslinking of copolymers containing MMSt or EMSt was also investigated. The typical IR spectra of copolymers before and after UV irradiation pre displayed in Figure 4. The characteristic absorptions were observed at 3450 and 1730 cm-' due to hydroxyl and carbonyl groups, respectively, which reflects the oxidation of benzyl methylene as mentioned above. The weight loss of copolymers irradiated in air as well as in vacuum was measured. The weight loss of copolymers irradiated in air decreased at a relatively high speed compared 'with that in vacuum as shown in Figure 5. As for the additive effect, benzoin and cobalt naphthenate accelerate the crosslinking of poly(MMSt-St) and pol- y(EMSt-St) remarkably; however, neither promotion nor suppression of crosslinking takes place in poly(MMSt-MMA) or poly(EMSt-MMA). The slow crosslinking of copolymers containing styrene may be ascribed to the st,eric hindrance of the phenyl group in styrene. In order to confirm the synergism of the benzyl methylene group and the 1,3-dioxolane ring on photocrosslinking, the results of solubility measurements of poly(EMSt-MMA) (33:67) and pol- y(MSt-MMA) (45:55), together with poly(2-ethyl-2-methyl-1,3-dioxolan-4- y1)methyl acrylate7 are redrawn in Figure 6. It is obvious that poly(EMSt-MMA) is more crosslinkable than the others in spite of its low content of the functional monomer unit, as we expected.

Crosslinking Mechanism

As described in the preceding section, it can be assumed that the main factor in photocrosslinking of poly(MSt) is the direct coupling of the yielding radical a t benzylic methylene. In order to verify this hypothesis, photopolymerization of MMA by means of benzyl methyl ether (BME) as a photoinitiator was carried out. The results of bulk polymerization in concentrations of BME varying from 9.30 X to 1.22 X lo-' mol/L at 30°C are presented in Table 11. The polymerization rates R, were calculated from the slopes of the conversion-time plots. The relations of R, vs. [BME]1/2 and l/Pn vs. [BME]1/2 are given in Figure 7. It is clear that this polymerization system obeys the rule of square root of the concentration of BME. The monomer exponent was also determined in the diluent systems by use of ethyl acetate to be 1.02 as shown in Figure 8. The chain transfer constant Ci to BME was also estimated to be 1.10 X which is almost the same as the chain transfer constant to dibenzyl ether13 (1.04 X in photopolymerization of MMA with AIBN.

If the C-0 bond cleavage of poly(MSt) takes place through photochemical


6 -

5 -

4 -

3 -

2 -

m 0 a (L

’ t i l - 0 1 2 3 4

[ BME 1’2 i 10

Fig. 7. Plots of R, vs. [BME]’/* and l/pn vs. [BME]l/z for photopolymerization of MMA in bulk with BME at 30°C.

excitation, the IR absorption band at 1100 cm-l ascribed to the ether linkage should decrease markedly. However, no detectable changes were observed in the case of vacuum irradiation and the crosslinking rate of poly(MSt) in vacuum is far more rapid compared with that in air as described in the previous section. In addition, it is obvious from Figure 1 that the hydroperoxide formed by the photo-oxidation decomposes to an ester group by elimination of water. This fact is coincident with the results of the decomposition of the poly(alkoxyalky1 acrylates) reported by Rehberg and Fancette4 and Costanza and V ~ n a . ~

Taking these facts into account, we propose the following mechanism for the photochemical reaction of poly(MSt):

hv - 0 2

On the other hand, polymers having the 1,3-dioxolane ring have been reported to photocrosslink with the coupling of radicals generated by the scission of the 1,3-dioxolane ring.6v7J4 Taking account of the above facts, we suggest that photocrosslinking of copolymers containing MMSt or EMSt proceeds according to the following scheme:

1432

I I 0

I I 0

CHZ-CH-CH, CHZ-CH-CHZ ‘ A o\c/o 0Lc/ I 1

RC ‘Rz R( \Rz

FUKUDA AND NAKASHIMA

hv

CHZ I

Q 0 I

CH,-CH-CH I I

R C ‘Rz O\&O

Q o r Q - Path A \

I Path D I

Cross - linking

Scheme 1


I

0.7

a [L

0.6 - Y)

I

0.51

0.8 oas 0.9 0.9s

log [ M M A]

Fig. 8. Plot of logR, vs. log[BME] for photopolymerization of MMA at 30°C in ethyl acetate; [BME] = 1.22 X mol/L.

Paths A, B, and C seem to participate in photocrosslinking of copolymers containing MMA because there is almost no difference in the results of solubility measurements of irradiation in air and in vacuum. On the other hand, crosslinking of copolymers containing styrene is more rapid in air than in vacuum, which indicates that path B is almost excluded, and that not only the coupling of radicals generated by the scission of the 1,3-dioxolane ring (path A and C) but also oxidative crosslinking (path D) takes part in photocrosslinking in air.

References

1. Y. Ogata, Ed., Yukikagobutsu no Sanka to Kangen, Nankodo, Tokyo, 1963. 2. K. Noma and R. Yosomiya, Shikizai Kyokaishi, 37,51 (1964). 3. Y. Nakashima and M. Kawai, Shikizai Kyokaishi, 45,21 (1972). 4. C. E. Rehberg and W. A. Fancette, J. Org. Chem., 14,1094 (1949). 5. J. R. Costanza and J. A. Vona, J. Polym. Sci. A-1,4,2659 (1966). 6. Y. Nakashima, Shikizai Kyokaishi, 49,341 (1976). 7. Y. Nakashima and H. Fukuda, J. Polym. Sci. Polym. Chem. Ed., 17,245 (1979). 8. Y. Nakashima and H. Fukuda, Kobunshi Ronbunshu, 39,515 (1982). 9. T. G. Fox, J. B. Kisinger, H. F. Mason, and E. M. Shuele, Polymer, 3,71 (1962).

10. R. 0. Symcox and J. D. Cotman, Jr., J. Polym. Sci. A-1,5,1165 (1967). 11. A. Charlesby, Atomic Radiation of Polymers, Pergamon, Oxford, 1960. 12. N. A. Weir, J. Appl. Polym. Sci., 17,401 (1973). 13. K. Tsuda, S. Kobayashi, and T. Otsu, Bull. Chem. Soc. Jpn., 38.1517 (1968). 14. G. F. D’Alelio and R. J. Caiola, J. Polym. Sci. A-1,5,287 (1967).

Received September 13,1982 Accepted December 13,1982

photocrosslinking of polymethoxymethylstyrene and copolymers containing...

Documents