Radioactivation analysis studies of polymerization reactions: (1) Solution polymerization of styrene by radical catalysis

Radioactivation analysis studies of polymerization reactions." (1) Solution polymerization of styrene by radical catalysis T. MAEKAWA,M. MATSUO,H. YOSHIDA*, K. HAYASHI* and S. OKAMURA Radioactivation analysis was used to detect trace amounts of solvent fragments attached to polymer chain ends in a study of the mechanism of chain transfer to solvent in the polymerization of styrene in chloroform, methylene dichloride, ethylene dichloride and bromobenzene. The polymerization was carried out at 60°C with e,e'-azobisisobutylonitrile as initiator. It was confirmed that the fragments of solvent molecule, even with bromobenzene as solvent, became attached to the polymer during the course of the transfer reaction. This is in accordance with the generally accepted mechanisms of the transfer reaction. MANY studies of chain transfer reactions in solution polymerization have been reported. The generally accepted mechanism involves the transference o f the activity of the growing radical to a solvent molecule, thus yielding a solvent radical which may initiate further polymerization1-6. According to this mechanism, fragments of solvent molecule should be incorporated into polymer molecules as end groups. The presence of such solvent fragments has been shown by chemical analysis of the polymer 5, 7, s In the case of monohalobenzenes, however, although reduction in molecular weight caused by the transfer reaction is observed, there is little evidenc~ of halogen in the polymer formed a, 10. As the resulting polymer was analysed using conventional elemental analysis techniques, investigations were restricted to low molecular weight polymer obtained from monomer of low concentration. In this work radioactivation analysis was used to detect fragments of low concentration in high molecular weight polymer which was obtained under the usual conditions of radical polymerization. Polymerization of styrene by radical catalysis was studied in halo-alkyl and halo-benzene solvents. The sensitivity of radioactivation analysis is 10-Sg for chlorine or 10-ag for bromine, which is higher than that of ordinary chemical methods by 3 or 4 orders of magnitude. THEORY According to the generally accepted mechanism of the chain-transfer reac* Faculty of Engineering, Hokkaido University, Sapporo, Japan 342

RADIOACTIVATION ANALYSIS STUDIES OF POLYMERIZATION REACTIONS (1)

tion, fragments of solvent are incorporated into polymer as end groups in the following processes: Mn'+SX

~MnX+S"

(1)

S • + nM -+ SMn • ~ SMnX -+- S •

(2)

where M is a monomer molecule and SX is a solvent molecule. From the above mechanism, the expected number of solvent fragments (number of halogen atoms in the present investigation) combined to a polymer chain is derived in the following way. For a constant concentration of initiator, assuming steady state and a constancy of the rate of initiation and kp/kt~, the number, n, of solvent molecules in one polymer chain is defined as n = kfs[R .]

"=

[S]

[S]/{kt[R .]2 + kys[R .]

I[kt[R']~k~M]+

[S] cs[ M] +

[S] +

,~)

kfm[R "] [M]}

Cm

(3)

where [R .] is the total polymer radical concentration and [M] and [S] are monomer and solvent concentration, k~, kt, klm and kfs are rate constants for propagation, termination, transfer to monomer and to solvent, respectively and Cs = kls/k~, C m = klm/k~. In the equation, the fact that the termination reaction in polymerization of styrene proceeds predominantly by recombination,n, 12 is taken into account. The degree of polymerization in bulk, DPo, is expressed la as 1/DPo kt[R .]/kp[M]0 + Cm (4) =

The relation between n and the mole fraction of monomer in the polymerization system, equation (5), is obtained by applying equation (4) to equation (3). [S] 1

)N)J

(s)

EXPERIMENTAL

Materials Chloroform, methylene dichloride and ethylene dichloride were dried over anhydrous sodium sulphate and distilled twice. Bromobenzene was purified by washing with 10 ~ aqueous sodium hydroxide, cold concentrated sulphuric acid and water, drying over anhydrous sodium sulphate and fractionally distilling. Styrene was washed with 5 ~ aqueous sodium hydroxide and water, dried over anhydrous sodium sulphate and distilled twice just before use.

Polymerization and purification of polymer Polymerization was carried out in vacuum at 60°C with 5 × 10 -3 mole 1-1 of a,a'-azobisisobutylonitrile (AIBN) as an initiator. The polymerization was limited to less than 5 ~ conversion and the polymer formed was precipitated by the addition of methanol. The polymer obtained was dissolved 343

T. MAEKAWA,

M. MATSUO,

H. YOSHIDA,

K . H A Y A S H I A N D S. O K A M U R A

in benzene and then reprecipitated in methanol so that the remaining chain transfer agent was removed. Five cycles of this dissolving-precipitating procedure were made to purify polymers effectively.

Activation analysis Each polymer sample (20-100 mg) was sealed in polyethylene film. The samples were packed in a polyethylene capsule and irradiated with neutrons. The induced activity of the samples due to chlorine-38 and bromine-82 resulted from nuclear transmutation of C137(n,y)C13a and BrSl(n,y)Br s2 was counted with a multi-channel pulse height analyser. In order to measure absolute value of chlorine or bromine content, standard weighed samples of ammonium chloride or ammonium bromide contained in filter paper were irradiated in the capsule together with the polymer samples, and the induced activity of the latter was compared with that of the former. Before counting the activity, the irradiated samples were cooled for about one hour and one day, respectively, so that the chlorine and bromine content could be measured without interference from short-lived radioactive nuclei, as chlorine38 and bromine-82 decay with half-lives of 37-3 rain and 35-9 h, respectively. Representative spectra of y-rays emitted from polymer samples are shown in Figure 1. The activity of the samples was determined from peak height of 1.64 Mev for chlorine-38 and that of 0.554 Mev for bromine-82. Figure 2 el.- 38

Br- 82 A

•

i

:g 0 lID

'lt// o

5

0

I

I

0.5

1'0

1

1.5 Energy (MeV)

IE t.D O4

I

2.0

2.5

Figure 1 ~,-rayspectra of chlorine-38 and bromine-82 contained in neutron-irradiated polymersamples 344

RADIOACTIVATION ANALYSIS STUDIES OF POLYMERIZATION REACTIONS (1)

.o c~

Figure 2 Calibration lines for quantitative measurement of chlorine and bromine by radioactivation analysis method: (3, chlorine; O, bromine

.>_ "6

I

1

I

I

0-1 0.2 0-3 0"4 A m o u n t of Br (and C[ in s t a n d a r d s a m p l e s (mg)

shows the linear relation between the induced activity and the amount of standard samples. The neutron irradiations were carried out with a JRR-2 reactor at the Japan Atomic Energy Research Institute (irradiation time 5 rain, thermal neutron flux of 5 × 1013 n cm-2s -1) or a KRR-1 reactor at Kyoto University (20 min, 4 × 10lz n cm-2s-1). In order to justify this analytical method, the mixture of polyvinylchloride with polystyrene was irradiated. The results shown in Figure 3 indicate that

-0 U_, Figure 3 Relation between the chlorine "6 content in polystyrene-polyvinylchloride films calculated from the mixture ~I"0 composition and that observed with the o activation analysis method ,.Q

o

O0

1-0 2.0 Ccl[culoted content of C[ {% )

chlorine atoms originally attached to the polymer do not escape from samples during irradiation in the reactor. Pfann et a114 also confirmed this fact for bromine combined with polystyrene and Guinn and Wagner 15 obtained very close agreement between chlorine content values using both the activation analysis method and the conventional elemental analysis method. 345

T. MAEKAWA, M. MATSUO, H. YOSHIDA, K. HAYASHI, AND S. OKAMURA RESULTS A N D DISCUSSION T h e h a l o g e n c o n t e n t a n d the n u m b e r a v e r a g e m o l e c u l a r w e i g h t o f p o l y s t y r e n e o b t a i n e d f r o m p o l y m e r i z a t i o n in c h l o r o f o r m , m e t h y l e n e d i c h l o r i d e , e t h y l e n e d i c h l o r i d e a n d b r o m o b e n z e n e s o l u t i o n a r e s u m m a r i z e d in T a b l e s 1, 2, 3 a n d 4. T h e n u m b e r a v e r a g e m o l e c u l a r w e i g h t , M n was c a l c u l a t e d f r o m t h e i n t r i n s i c v i s c o s i t y u s i n g t h e f o l l o w i n g e q u a t i o n , t6 a s s u m i n g a n o r m a l molecular weight distribution. [~/] :

2.7 X 1 0 - 4 ( l ' 8 0 M n ) °'66

(12)

Table 1 Observed chlorine contents in polymers obtained by solution polymerization of styrene in chloroform at 60°C with AIBN Polymer

[M]

[S] (mole 1-1)

8"68 8"46 8"27 7' 89 7.23 5'78 4"34 3'04 2'17 1"13

0 0.304 0.594 1"13 2'08 4'16 6-24 8.11 9'37 10-8

Mn x 10 -4

% CI in polymer x 102

C1 atoms in a chain Ncl X 102

6'12 5"85 5"75 5"43 4"05 3'62 2'35 1"55 1'07 0"74

0"876 0'978 0'996 0"947 1'02 0"890 0"910 1'81 2'41 2'64

-1"65 1-94 1-84 1-65 2-41 3-11 4-07 4"62 3"64

Table 2 Observed chlorine contents in polymers obtained by solution polymerization of styrene in methylene dichloride at 60°C with AtBN Polymer

[M]

[S] ~n X 10 -4

(mole 1-1 )

8'68 8'47 8"27 7"89 7"23 6'68 5"78 4"34 3.04 1.13

0 0'383 0'748 1"43 2"62 3"62 5"23 7"85 10-2 13"6

6"60 6' 33 5"97 5' 85 5'05 4"68 3'67 2"34 1"39 0.67 346

~

% El in polymer X 102

CI atoms in a chain Nc1 x 102

0"600 0"606 0"662 O"806 0"768 0"883 1'00 1'32 2"49 4'56

-O'11 1'05 3"40 2"39 3'73 4'14 4'75 7"39 7.47

RADIOACTIVATION ANALYSIS STUDIES OF POLYMERIZATION REACTIONS (1) Table 3 Observed chlorine contents in polymers obtained by solution polymerization of styrene in ethylene dichloride at 60°C with AIBN Polymer

[MI

[S] (mole 1 1)

8.68 8.47 8.27 7.89 7.23 6.68 5.78 4"34 3-03 2.17 I'13

0 0'309 0"602 1-15 2' 11 2"91 4"22 6"33 8.22 9.48 11'0

Mn ×10 4

% C1 in polymer ×10 2

CI atoms in a chain Ncl >'. 102

6.60 6.49 6.12 5"80 5-74 4-09 3.56 2.29 1.66 1'02 0'54

0"633 0'741 0'752 0'763 0.842 0'820 1"01 1"22 1"60 3.60 11.1

-1'97 2.06 2.13 3-45 2.75 3-51 3'72 4.17 7'70 14"4

Table 4 Observed bromine contents in polymers obtained by solution polymerization of styrene in bromobenzene at 60°C with AIBN Polymer

[M]

[S] (mole 1-1)

7'97 7.57 4-55 3-03 2-27

0.850 1.59 4-76 6"35 7-14

Mn × 10 5

Br in polymer x 10a

Br atoms in a chain NBr ~<102

4"34 4.29 2.58 1"43 1"15

0.913 0.887 1.69 3"36 3-14

3"82 3.44 4'43 5'39 3.59

The viscosity was measured at 25°C in benzene solution. It should be noted that polystyrene synthesized u n d e r careful experimental conditions i n bulk polymerization was still c o n t a m i n a t e d with 0.006-0.009 700 of chlorine from u n k n o w n sources. Therefore, to o b t a i n the true content, this value should be subtracted f r o m the chlorine c o n t e n t for each sample. The average n u m b e r of chlorine or b r o m i n e atoms in a polymer chain, Ncl or NBr, calculated from the observed molecular weight a n d the content of the atoms is given in the third c o l u m n of Tables 1, 2. 3 a n d 4. According to e q u a t i o n (5), the theoretically expected value of N c l a n d N•r is calculated f i o m Cs, Cm a n d DPo. F o r chloroform, for example, N c l was calculated f r o m the reported values o f Cs = 5 x 10 -2 (ref. 17), Cm = 5 × 347

T. MAEKAWA, M. MATSUO, H. YOSHIDA, K. HAYASHI AND S. OKAMURA m

10 -5 (refs. 18-20), DPo = 5.9 x 102 a n d Ncl -----3n* (see Figure 4). The o b s e r v e d values o f Nc~ were also p l o t t e d in Figure 4. D e v i a t i o n s o f the o b s e r v e d values are insignificant. A l t h o u g h s u b t r a c t i o n o f a ' b a c k g r o u n d ' value o f the c h l o r i n e c o n t e n t f o u n d in b u l k p o l y m e r i z a t i o n might cause an u n c e r t a i n t y in the values o f N c l listed in Tables 1-3, this is affected less at s m a l l e r m o l e fractions o f m o n o m e r . It is o b v i o u s that the observed values

0"2 a

# 0.1

0.0 ~,

b

0.1 0 0/W'~

°2F :~ 0 0

, o,

io

,°,o

,o

o "

0

0 ,

q~

° 1

I

~

0

;

O, CI3

0.5 Mole fraction of monomer, m

1"0

Figure 4 Number of chlorine atoms, No, attached to a polymer chain plotted as a function of mole fraction of styrene: (a) Polymerization in chloroform at 60°C with AIBN. The line is the theoretical curve calculated from equation (5), with C~ = 5 x 10-5 (refs 17, 22), Cm = 5 x 10-5 and Net = 3n; O, experimental points (b) Polymerization in methylene dichloride, (:'8 = 1-8 x 10-5 (refs 17, 23) and Ncl = 2n (c) Polymerization in ethylene dichloride. C8 = 4 x 10-5 (refs 22-24) and Ncl = 2n

a r e in a c c o r d a n c e with those expected f r o m the generally accepted m e c h a n ism as illustrated in r e a c t i o n s 1 a n d 2. Similar results were o b t a i n e d for the p o l y m e r i z a t i o n s in m e t h y l e n e d i c h l o r i d e a n d ethylene dichloride. F o r b r o m o b e n z e n e , NBr is c a l c u l a t e d f r o m Cs = 1.3 x 10-5; a c o m p a r i s o n w i t h e x p e r i m e n t a l values is shown in Figure 5. I n this case discretion m u s t be used in c h o o s i n g a Cs value. B r o m o b e n z e n e m a y behave m u c h like c h l o r o benzene in transfer. The transfer constants o f c h l o r o b e n z e n e at 155°C a n d 60°C are 3-1 x 10 -4 (ref. 9) a n d 1.33 × 10 -5 (ref. 22), respectively, a n d the transfer c o n s t a n t o f b r o m o b e n z e n e at 155°C is 3 x 10 -4 (ref. 9). Since the transfer c o n s t a n t o f b r o m o b e n z e n e at 155°C is 0.97 t h a t o f chlorobenzene, the transfer c o n s t a n t o f b r o m o b e n z e n e at 60°C should be 1.3 x 10 -5. K a p u r et al 2t have r e p o r t e d Cs = 1.775 x 10 -4 for b r o m o b e n z e n e at 60°C. H o w e v e r , their d a t a seem to be systematically higher t h a n other d a t a by * Obviously Net = 2n for methylene dichloride and ethylene dichloride 348

RADIOACTIVATION ANALYSIS STUDIES OF POLYMERIZATION REACTIONS

(1)

a b o u t one o r d e r o f m a g n i t u d e . In our d a t a at 60°C, (2'8 lies in the range 2-7 × 10 -5, m o s t p r o b a b l y 2 - 4 × 10 a. The observed Ngr values are in g o o d a g r e e m e n t with those e s t i m a t e d f r o m the n o r m a l transfer m e c h a n i s m using C8 = 1.3 × 10 -5. But considering the reliability o f the chosen C8 value, p a r t i a l occurrence is not excluded o f a transfer mechanism suggested by

Z

~0'3 o

B

,-0-2

m

~60'1 6 z

00

!

I

o°-,7--

I

i

oo

0"5

I

1"0

Mole fraction of monomer, m Figure 5 Number of bromide atoms, NBr, attached to a polymer chain plotted as a function of mole fraction of styrene. Polymerization in bromobenzene at 60°C with AIBN. The line is the theoretical curve calculated from equation (5) with C,~- 1.3 ~: 10 5, C,, -- 5 × 10 5 and NBr = n; ( , experimental points

M a y @ in which the solvent molecules are not a t t a c h e d to p o l y m e r during the chain transfer to reaction solvent. It is c o n c l u d e d that activation analysis is useful for the analysis o f trace a m o u n t s o f chemical c o m p o n e n t s in p o l y m e r s a n d it is therefore applicable to the kinetic study o f solution p o l y m e r i z a t i o n . C o m p a r e d with conventional chemical analysis which can detect only high concentrations o f solvent fragments a t t a c h e d to very low m o l e c u l a r weight polymers, this m e t h o d is m o r e suitable for analysing high m o l e c u l a r weight p o l y m e r s o b t a i n e d u n d e r the usual conditions o f p o l y m e r i z a t i o n . Department o f Polymer Chemistry, Kyoto University, Kyoto, Japan

(Received 7 November 1969) (Revised 23 March 1970)

REFERENCES 1 2 3 4 5 6 7 8 9

Smith, V~'.V. J. Amer. Chem. Sac. 1946, 68, 2059 Mayo, F. R. Discuss. Faraday. Sac. 1947, 2, 328 Mayo, F. R. J. Amer. Chem. Sac. 1948, 70, 2373, 3689 Gregg, R. A., Alderman, D. M. and Mayo, F. R. J. Amer. Chem. Sac. 1948, 70, 3740 Walling, C. J. Amer. Chem. Sac. 1948, 70, 2561 Mayo, F. R. and Lewis, F. M.J. Amer. Chem. Sac. 1954, 76, 457 Breitenbach, J. W. and Maschin, A. Z. physik. Chem. 1940, A187, 175 Bamford, C. H. and Dewar, M. J. S. Proe. Roy. Sac. 1948, A192, 329 Mayo, F. R. J. Amer. Chem. Sac. 1953, 75, 6133 349

T. MAEKAWA, M. MATSUO, H. YOSHIDA, K. HAYASHI AND S. OKAMURA 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Breitenbach, J. W. and Schindler, A. Monatshelftefiir Chemie, 1957, 88, 810 Bevington, J. L. et al, J. Polym. Sci. 1954, 12, 449; 1954, 14, 463 Kolthoff, I. M. et al, J. Polym. Sci. 1955, 15, 459 Mayo, F. R. J. Amer. Chem. Soc. 1943, 65, 2324 Pfann, I-I. F., Salley, D. J., and Mark, H.J. Amer. Chem. Soc. 1944, 66, 983 Guinn, V. P. and Wagner, C. D. Anal. Chem. 1960, 32, 317 Pepper, D. C. J. Polym. Sci. 1952, 7, 347 Gregg, R. A. and Mayo, F. R..i. Amer. Chem. Soc. 1953, 75, 3530 Baysal, B. and Tobolsky, A. V. J. Polym. Sci. 1952, 7, 529 Smith, W. V. J. Amer. Chem. Soc. 1949, 71, 4077 Mayo, F. R., Gregg, R. A. and Matheson, M. S.J. Amer. Chem. Soc. 1951,73, 1691 Kapur, S. L.J. Polym. Sci. 1953, 11,399; 1954, 14, 489 Misra, G. S. and Chadha, R. N. Makromol. Chem. 1947, 23, 134 Our data Bamford, C. H. and Dewar, M. J. S. Discuss. Faraday Soc. 1947, 2, 314

350