Photocrosslinking of polymers containing cinnamoyl groups: the effects of cinnamoyl contents in copolymers on the reaction in solutions

Eur. Pohm. J. Vol. 25, No. 10, pp. 1033 1037, 1989

0014-3057,89 $3.00 + 0.00 Pergamon Press plc

Printed in Great Britain

P H O T O C R O S S L I N K I N G OF P O L Y M E R S C O N T A I N I N G C I N N A M O Y L GROUPS: THE EFFECTS OF C I N N A M O Y L C O N T E N T S IN C O P O L Y M E R S ON THE R E A C T I O N IN SOLUTIONS YOICHI SHINDO, 1 HIROMASA SATO, 1 TOKUKO SUGIMURA,1 KAZUYUKI HOR1E2 and ITARU MITA 2 ~Department of Chemistry, Faculty of Science, Toho University, Miyama, Funabashi-shi 274, Japan :Research Center for Advanced Science and Technology, University of Tokyo, Komaba, Meguro-ku, Tokyo 153, Japan (Received 5 December 1988)

Abstract--The effects of cinnamoyl contents in copolymers of 2-cinnamoyloxyethyl methacrylate with methyl methacrylate on the competitive intra- and intermolecular photocrosslinking were studied in THF solutions irradiated with a high-pressure Hg lamp at 30. The kinetic parameters for intra- and intermolecular crosslinking in degassed and aerated THF solutions were calculated from the initial concentration dependence of the rate of cyclodimer formation of cinnamoyl groups in copolymers by spectrophotometric measurements or from the data of the change in weight-average molecular weight by light-scattering measurements at various concentrations of copolymers. The quantum yields for intramolecular crosslink formation, q,~tr,, increase linearly with increasing CEMA mol% in a copolymer with a slope of 1 x 10-4 per CEMA mol% in a copolymer. The parameters for intermolecular crosslink formation, K~ ~er, on the contrary, decrease with the extent of reaction, because the increase of intramolecular crosslinks prevents the interpenetration of polymer coils. The second virial coefficient becomes progressively smaller as the cyclodimerization proceeds; at the onset of gelation it reaches almost zero. The magnitudes of < s 2 > ~2 and A),~ are related to the degree of the competition between intra- and intermolecular crosslinking. The contraction of polymer coils proceeds by predominant intramolecular crosslinking in dilute solution, and the contraction ratio, g = < s -~> ~r" < st > ~, becomes 0.3-0.02 at the get point, showing that the volume of the crosslinked polymer coil shrinks to 1/6 1:'350 of the volume of corresponding linear polymer with the same molecular weight.

INTRODUCTION

P h o t o d i m e r i z a t i o n of various c h r o m o p h o r e s has been reported by m a n y a u t h o r s [1-4]. A synthetic investigation of the p h o t o d i m e r i z a t i o n of polymers with cinnamoyl groups in the solid state has been performed by Reiser et al. [5, 6]. We have recently reported some kinetic parameters a n d the change in polymer coil dimension d u r i n g the simultaneous occurrence of i n t r a - a n d intermolecular photocrosslinking in homopolymers containing cinnamoyl groups in dilute solution [7, 8]. The q u a n t u m yields for intermolecular crosslinking formation, inlet q~o , at various polymer concentrations, are very small c o m p a r e d to the q u a n t u m yields for the i n t r a m o l e c u l a r crosslinking formation, ~)/~tra [7]. The c o n t r a c t i o n of polymer coils proceeds by p r e d o m i n a n t i n t r a m o l e c u l a r crosslinking in very dilute solutions. The c o n t r i b u t i o n of i n t r a m o l e c u l a r crosslinking is more m a r k e d in aerated solution, where the reaction proceeds t h r o u g h the excited singlet state, than in degassed solutions where the reaction proceeds mainly t h r o u g h the excited triplet state [7]. In the present work, some copolymers of 2-cinnamoyloxyethyl methacrylate with methyl methacrylate ( M M A ) were prepared to have C E M A contents of 2 . 4 - 2 6 m o l % with a particular number-average degree of polymerization (5600-6400), and fractionated to

o b t a i n a n a r r o w molecular weight distribution. Using ,ae fractionated copolymers, the effects of cinnamoyl contents in copolymers on the reaction rates a n d the changes in coil dimension during the reaction due to the intra- and intermolecular photocrosslinking were evaluated by spectrophotometric, light-scattering and gel p e r m e a t i o n c h r o m a t o g r a p h y measurements. EXPERIMENTAL PROCEDURES

The copolymerization of 2-hydroxyethyl methacrylate (HEMA) and MMA was carried out in dimethylformamide (DMF) at 60 in an evacuated and sealed tube with azobisisobutyronitrile as an initiator. After a time calculated to keep the conversion below 10%, the contents of the tube were poured into methanol with stirring. The precipitated polymer was purified by dissolving in DMF and precipitating in methanol, and then dried at 50 under vacuum. Copoly (HEMA/MMA) was dissolved in the minimum volume of DMF: an equal volume of pyridine was added. Excess cinnamoyl chloride was then added dropwise 1o the polymer solution (molar ratio 1.5 : l ), and kept at room temperature for 24 hr with constant stirring, and then poured into methanol. The precipitated polymer was washed several times with methanol to remove residual cinnamic acid and dried at 5 0 under vacuum. In order to obtain a narrow molecular weight distribution, the reacted polymers were further fractionated by dissolving in a mixed solvent of benzene and methanol and changing stepwise the temperature of the solution from 5 0 to 20. Characteristics of the copolymers are given in Table 1.

1033

YOICHI SHINDO el al.

1034

Polymer

u.v. [C]/mol%

6400 2.4 5100-12 5600-26

2.40 I 1.7 26.0

Table I. Characterization of polymers containing cinnamoyl groups GPC/THF Light-scattering JQ~/~Q~ /~/106 ]"/102A A2/IO 4molmlg -2 2.0 1.8 2.3

1.35 1.09 1.85

About 5 ml of sample solution in a pyrex cell with a polymer concentration of 3.4-0.2 gl t was deaerated by several freeze-pump-thaw cycles at 10 4 torr, and sealed under high vacuum. For an aerated sample, the sample solution was sealed off without evacuation. The sample solutions were irradiated by a 450 W high-pressure Hg lamp (Ushio UM-452) with a glass filter of Toshiba UV-D33S in a thermostat at 30 °. In order to minimize the influence of the heterogeneous nature of the reaction, the solution in the cell was continuously stirred magnetically during irradiation. Actinometry was carried out with an Ushio UIT-100 Type photoresist photometer calibrated by the potassium ferrioxalate/o-phenanthroline system [9]. The incident light intensity was about I o = 1.26 x 10 -8 einstein cm 2 s e c - t The molecular-weight distribution (/Q~/A,~,) were measured with a Toyo Soda HLC-802UR gel permeation chromatograph (GPC) at 40 ° in tetrahydrofuran (THF). The content of cinnamoyl groups in a polymer and the extent of cyclodimer formation of the cinnamoyl groups in solutions were evaluated from the change in optical density at the maximum wavelength (275 nm) as reported previously, in a similar manner to the case of photodimerization of ethyl cinnamate [10]. Weight-average molecular weight, ~Q'~, root-mean-square radius of gyration, < s 2 > 1'2, and the second virial coefficient, A 2, were calculated by a personal computer attached to the light-scattering photometer. Details of the determination have been given [8]. RESULTS AND DISCUSSION

Effect of cinnamoyl contents on the rate o f intra- and intermolecular crosslinking T h e rate o f decrease in c i n n a m o y l g r o u p s c o n c e n t r a t i o n , [C], due to c y c l o d i m e r f o r m a t i o n , is given by e q u a t i o n (1) as in o u r p r e v i o u s kinetic analysis [7], d[C] _ l.b(q)~t~a + q)L]t,r) dt = I.b(Cl)Dintra + KDmter[C])

T._,~E0 . 6 0 ' 8 ~

(.~

-

)Vn

1.1 0.86 0.97

6400 5100 5600

12

%

10

b 6 ~4 ~

2

7

t

I

I

I

t

I

t

w h e r e KT) t~' is a p a r a m e t e r for i n t e r m o l e c u l a r c r o s s l i n k i n g f o r m a t i o n , a n d l,b is the a b s o r b e d dose w h i c h is c o n s t a n t (lab = 1.26 × 10- 5 einstein 1- ~ s e c - 1) i n d e p e n d e n t o f [C] for the p r e s e n t e x p e r i m e n t a l conditions w h e r e all the i n c i d e n t light is a b s o r b e d . 4 ~ tr" is i n d e p e n d e n t o f [C], a n d is f u n c t i o n o f the c o n t e n t a n d the d i s t r i b u t i o n o f c i n n a m o y l g r o u p s in the p o l y m e r chain. T h e z e r o - o r d e r plot o f residual conc e n t r a t i o n of c i n n a m o y l g r o u p s a g a i n s t i r r a d i a t i o n time, t, in a degassed T H F s o l u t i o n was f o u n d to be linear as s h o w n in Fig. 1 as r e p o r t e d previously [7]. T h e value o f - d [ C ] / d t was calculated f r o m the slope of this s t r a i g h t line. - d [ C ] / d t d e p e n d s o n the initial c o n c e n t r a t i o n of c i n n a m o y l g r o u p s , a n d also is affected by c i n n a m o y l c o n t e n t in the c o p o l y m e r as s h o w n in Fig. 2. l.b(/)~ TM a n d l, bKz~ i . . . . are d e t e r m i n e d f r o m the intercepts a n d the slopes o f the s t r a i g h t lines in Fig. 2, respectively. T h e results are listed in T a b l e 2, a n d are p l o t t e d in Fig. 3 as a f u n c t i o n o f the c i n n a m o y l c o n t e n t in c o p o l y m e r s , q)~'~" increases

1

40

/

LA-A--~ _ _ _ _ _

_

80

I

12o

0

40

I

2 4 6 8 [ C ] 0 / 1 0 -3 tool L-1 Fig. 2. Concentration dependence of the rate of cyclodimer formation of cinnamoyl groups in copolymers: (3, 6400-2.4; A, 5100-12; IS], 5600-26. 0

6400-2.4

0.2 F~'ID" ID"~" ID-O~ o_ . 0

(1)

7.4 5. I 5.3

80

120

0

40

80

120

Irrodiotion t i m e / r a i n Fig. 1. Change in concentration of cinnamoyl groups of copolymers due to cyclodimer formation in degassed T H F at 30:. Concentration/g 1-t (moll i): 6400-2.4: (3, 3.4 (8.0 × 10-4); tD, 1.1 (2.6 × 10-4); O, 0.47 (1.1 × 10-4); 01, 0.28 (0.68 × 10-4). 5100-12: /N, 3.4 (2.8 × 10 3); /x, 1.1 (0.95 x 10 3); A, 0.48 (0.40 × 10-a); h , 0.27 (0.23 × 10 3). 5600-26: F-q, 3.4 (6.4 x 10 3); I n 1.1 (2.1 × 10-3); I I 0.47 (0.89 × 10 ~): [ | . 0.25 (0.49 × 10 3).

Photocrosslinking in solution

1035

!!!!+ (o)[b)(c)

Table 2. Q u a n t u m yields for intra- and intermolecular crosslinking of copolymers in degassed T H F irradiated at 30 Copolymer

K'~'+'/I mol

q~t,,,

6400 2.4 5100 12 5600 26

0.40 × 10 ' 1.3 × 10 ' 3.2 × 10 ~

~"

K~t~/I mol

5.6 1.6 0.75

~h

0.55 0.24 0.030 mO

. From bFrom

u.'~. m e a s u r e m e n t s . light-scattering measurements.

x

linearly with increasing cinnamoyl content in copolymers, and oppositely K~ '~ decreases rapidly over the range of about 2 10 C E M A mol%. These results mean that the rate of intramolecular crosslinking becomes progressively greater as the content of cinnamoyl group on the same chain increases. As the reaction proceeds, the extent of the competing intermolecular crosslinking is restricted by the contraction of polymer coils due to intramolecular crosslinking.

Effect oJ cinnamoyl contents on interrnolecular crosslinking density The extent of intermolecular crosslinking was estimated from the change of ~¢~ by light-scattering. The intermolecular crosslinking density, Pro, based on the n u m b e r of monomer units is given by equation (2) [11], 1

Pm

-

-

[-1 M ~ (0)

"~,~(m,O) 1 -

~,,

(2)

where ?~m~.0~and M'~(o~are the weight-average degree of polymerization and the weight-average molecular weight of the copolymer before crosslinking, respectively. The changes of p~ and p~ for initial polymer concentration and cinnamoyl contents in copolymers are shown in Fig. 4, where the subscript, c, designates a quantity based on the number of chromophore units in the copolymer. The plots of p~ against t deviate from linearity to an extent progressively greater as the polymer concentration decreases. This result may be attributed to the effect of the concentration of polymer coils caused by the competing intramolecular reaction. For the experiment of [C]0 = 3.4 g I ~, Fig. 5 shows the plots of the rate of intermolecular crosslinks per polymer chain, p~V,,~,,~, against the extent of photodimerization of cinnamoyl groups, f~x, where the fraction, f~x, of dimerized chromophore units is defined as the conversion, x = (1 -[C]/[C]0 ), multi-

2 u

0

40

8O

120

I r r a d i a t i o n time/rain

Fig. 4. Number of intermolecular crosslinking per CEMA p~ and per monomer unit p~ formed by intermolecular dimerization of cinnamoyl groups of copolymer in degassed THF at 30. Copolymers (concentration/g 1-~) (a) 6400-2.4: O, 3.4: qD, 1.1: O, 0.47. (b) 5100 12: ~ , 3.4; /,, 1.1: A, 0.48. (c) 5600-26: 5 , 3.4: Ill, 1.1; m, 0.47. plied by the cinnamoyl fraction in the copolymer composition. When cinnamoyl contents in a copolymer increase, on the contrary, the initial slope in Fig. 4 decreases. From equation (2), the value of jog for ~,, ~> M~0~ corresponds to the onset of gelation. The star symbol in Fig. 5 represents the point of the theoretical onset of gelation at pg= 1/N~0~. The conversion at which gelation occurs depends on C E M A m o I % in a copolymer. For C E M A = 12mo1% copolymer, for example, the number of cinnamoyl groups which were utilized in inter- and intramolecular crosslinks in 1074 units of cinnamoyl groups per polymer chain at the point of gelation is 91, and the intermolecular crosslink is only 1.1%. As all absorbed dose per volume of solution is .fl~bdt, p+ is expressed in the form

P~=[~o

=ul~h~t+~dt =

l~bKi"~tC]dt.n t J = (I

(3) As photodimerization proceeds mainly due to predominant intramolecular reaction, [C] is approximated by [ c ] = [c]o -- z~ q,~"" t.

(4)

Substituting equation (4) into equation (3) and

1.6

1.2 B u

; o.s IZ 0.4

] 0

5

10

15

20

25

30

moL % CEMA in copotymers

Fig. 3. Dependence of quantum yields for intra- and intermolecular photocrosslinking on polymer composition.

I 0.005

I 0.010

I 0.015

fox

Fig. 5. Plots of p~FV~c,.0~against fox for irradiated copolymer at polymer concentration (3.4gl ~), copolymers: ©, 6400-2.4: /~, 5100 12: [El, 5600 26: ~k, onset of gelation.

YO[CHI SEIINDO et al.

1036 integrating, we obtain the equation illl(:r {

p~=L~bK o t

k

12 --I

Z (~)intra \

1

~b~n

t~

2[C]o J'

For the very initial stage of reaction, the approximation pC = L~bK~t~t holds and hence p~ is independent of [C]0. This is consistent with the results in Fig. 4. For example, LobKi~~ is determined to be about 6.9 x 10 -4 sec -~ from the initial gradient at C E M A = 2.4 tool% copolymer in Fig. 4. By using L~b= 1.26 × 10-5 einstein l-I sec-l, the values of K~ t~ are obtained and summarized in Table 2 together with the kinetic parameters determined by u.v. measurement already mentioned in Table 2. K~" values due to u.v. method are 8 - 2 5 times the values obtained by the change of p~ in Fig. 4. In practice the decreases in concentration of cinnamoyl groups in Fig. 1 are mostly attributed to the intramolecular reaction and only slightly to the intermolecular reaction. However, in the analysis based on the data of u.v. measurements, crosslinks followed by intramolecular reaction between the different chains before irradiation are regarded as intermolecular reaction. By using equation (4), t in equation (3) is replaced by [C], and integrating, we obtain K~'~([C]0 + {C]) (1 P~ =

2 , ~ '~

[C]%

-~

(5)

(6)

- [c]0/

- \4

~tD

o

ID

~ ~ ' ~7'?~4~"~'=I]~*L-*--H-I*J-UJ 10 6

10 7

10 8

10 9

fiw Fig,

7. P l o t s

of

A 2 against

~tw f o r

irradiated

copolymers

at polymer concentration (l.l gl-~). Copolymers: ID, 6400-2.4; Ix, 5100-12; Ifl, 5600-26; :,1¢,onset of gelation. decrease in chain interpenetration and, consequently, during the reaction, K~ ter becomes small increasingly as [C]0 or C E M A mol% in a copolymer increases. From the light-scattering measurements, we could obtain the relationship of second virial coefficient, A 2, with .~tw as shown in Fig. 7. The plots of A2 against /Q,~ display downward curvature. The change in A 2 is marked in the initial stage, and eventually it reaches almost zero at the gel point. Because of the effect of the contraction of polymer coils caused by predominant intramolecular crosslinking, the rate of the decrease in A 2 against AIw becomes greater with increasing C E M A mol% in a copolymer. Change in polymer coil dimension during the reaction

With x, equation (5) takes the form

The values of root-mean-square radius of gyration, < s 2 > ~2, are plotted on a double logarithmic scale [C]--~= 2q)~ 'r" (2 - x ) x . (7) against Mw in Fig. 8. The plotted points give three different lines according to the initial polymer conAccording to equation (7), the theoretical values centrations. This indicates that the magnitudes of follow an identical straight line regardless of [C]o. In practice, however, the higher [C]0, the greater becomes its tendency to deviate from the theoretical straight line in the initial stage of reaction as shown in Fig. 6. The deviation becomes very marked with increasing C E M A tool% in a polymer. These results suggest that the contraction of polymer coils caused by the intramolecular reaction is responsible for the [ I I lillllt I I IIIIIII ginter O

Pc

4°I(o)

o,~ 4°I(b)

40 I ~

3o

-I

V

[

I

I IIIIII1

[

I

t IIIllll

I

I I IIII11

I

I IIIIIII

o 20 tj i_l

1()

z:,-o-o---cri 0

I 0.2

i

I 0.4

i

I 0.6

I

I 0.8

(2-x)x Fig. 6. Plots of p¢/[C]0 against (2 - x)x for photodimerization of a copolymer (6400-2.4) in degassed THF at 30 . Concentration/g I-i: O, 3.4; ID, l.l; O, 0.47. Linear relationship according to equation (7) is expressed by a dotted line.

10 6

10 7

10 0

fiw Fig. 8. Molecular weight dependence of t 2 of irradiated copolymers. Copolymers (concentration/g l-I): (a) 6400-2.4: C), 3.4; IDA.I; O, 0.47; (b) 5100-12: A, 3.4; Ix, 1.1; A, 0.48; (c) 5600-26: V], 3.4; I n l.l; I , 0.47.

Photocrosslinking in solution < s ~ - > ~2 and M~ are related to the competition between intra- and intermolecular crosslinking. When the concentration increases, the dimension of polymer coils increases gradually owing to the increase in molecular weight caused by the intermolecular reaction overcoming the contraction caused by the intramolecular reaction. However, the values of < s - ' > ~ 2 f o r [ C ] 0 = 0 , 4 7 g l ~ of C E M A = 2 6 m o 1 % copolymer are almost unchanged despite the large change of M~. This could be attributed to the accidental cancelling of the expansion due to the molecular weight increase with the contraction due to intramolecular reaction. In order to evaluate the net contraction of polymer chains due to intramolecular crosslinking, the g value was introduced as the ratio of the mean-square radius of gyration of the intramolecularly crosslinked polymer, < s 2 > o r , to that of corresponding linear polymer with the same weight-average molecular weight, < s : > ~, [8].

g-

,/~.

(8)

The values of < s -~>., are those of < s 2> for irradiated copolymers given in Fig. 8. The values of < s 2 > ~were calculated from values of the molecular weight by using the relationship < s 2 > j ~ M ~z'3)~t+") where a, the viscosity coefficient, was assumed to be 0.6. The contraction ratio, g, plotted against h4',. decreases with the progress of reaction, more rapidly for the lower initial polymer concentration, and finally approaches 0.35 0.25 for copolymer with C E M A = 12 m o l % as shown in Fig. 9. Fig. 10 shows the change of g against f~x at [ C ] 0 = 0 . 5 g l ~ for evaluating the effect of the amount of C E M A m o l % in a copolymer on the contraction due to intramolecular crosslinking. The value of g decreases more rapidly in the initial stage of reaction as the amount of C E M A tool% in a copolymer decreases, and then becomes 0.02 at the gel point. The results suggest that the volume of the crosslinked polymer coil shrinks to 1/350 of the volume of corresponding linear polymer with the same molecular weight. The large motion of a polymer coil becomes limited in the initial stage of crosslink formation, and consequently, the contraction of polymer coils becomes small, since the crosslinks are produced with cinnamoyl groups in the vicinity of the excited group in the same chain. In conclusion, the effect of cinnamoyl contents in copolymers on each reaction during the competitive 1.o o,8 0.6 g 0.4

O.~ i

0

10 6

i t Liilll

I 10 "t

I i lllill 10 8

Fig. 9. Plots of g against ,Q~ for irradiated copolymer (510(}-121 concentration/gl ~: A, 3,4: IX, 1.1: A , 0.48.

1037

1.0i-0.8 0.6 g 0.4

0,~

0

~ll~

1

0.05

0.10 0.1,5 0.20 0.25 f¢ X Fig. 10. Plots of g against fox for irradiated copolymers at polymer concentration (0.47-0.48 gl-t). Copolymers: 0 , 6400 2.4; A , 5100-12; I , 5600-26; ~ , onset of gelation.

intra- and intermolecular crosslinking reactions were investigated, q~tr, obtained from the initial concentration dependence of the rate of cyclodimer formation of cinnamoyl groups increases linearly with increasing C E M A m o l % in copolymers. On the other hand, since the contraction of polymer coils occurs because of predominant intramolecular crosslinking and prevents the interpenetration of polymers, K~ t°r becomes smaller as the amount of C E M A in a copolymer increases. The deviation of p~ from the theoretical straight line in the initial stage of intermolecular crosslinking becomes progressively greater as the reaction proceeds. The second virial coefficient becomes increasingly smaller as the photocyclodimerization proceeds, and at the gel point it reaches almost zero. In particular, the increase of C E M A content in a copolymer is effective for the decrease in A 2. The magnitude of < s 2 > 12 and ~S¢w are related to the degree of the competition between intra- and intermolecular crosslinking. The contraction ratio, g, of polymer coils becomes 0.3-0.02 at the gel point due to predominant intramolecular crosslinking, showing that the volume of the crosslinked polymer coil shrinks to 1/6-1/350 of the volume of the corresponding linear polymer with the same molecular weight. REFERENCES 1. Y. Suda, Y. Inaki and K. Takemoto. J. Polym. Sci.; Polym. Chem. Edn 22, 623 (1984). 2. J. S. Hargreaves and S. E. Webber. Macromoleeules 17, 235 (1984). 3. H. Ushiki, K. Hirayanagi, Y. Shindo, K. Horie and 1. Mita. Polym. J. 17, 671 (1985). 4. J. Uchida, E. Takahashi, T. ]izawa and T. Nishikubo. J. chem. Soc. Japan 65 (19;35). 5. A. Reiser and P. L. Egerton. Macromolecules 12, 670 (1979). 6. P. L. Egerton, E. Pitts and A. Reiser. Maeromolecules 14, 95 (1981). 7. Y. Shindo, T. Sugimura, K. Horie and I. Mita. J. chem. Soc. Japan 184 (1984). 8. Y. Shindo, T. Sugimura, K. Horie and I. Mira. Eur. Po(vm. J. 22, 859 (1986). 9. S. L. Murov. Handbook of Photochemistry, p. 119. Dekker, New York (1973). 10. Y. Shindo, K. Horie and 1. Mita. Chem. Lett. 639 (1983). 11. C. David and D. Baeyens-Volant. Eur. Po(vm. J. 14, 29 (1978).