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European Polymer Journal. 1965. Vol. l, pp. 19-24. Pergamon Press Ltd. Printedin England.
REACTIVITIES OF ESTERS OF METHACRYLIC ACID--II. STUDIES OF RADICAL COPOLYMERIZATIONS J. C. BEVINGTON* a n d B. W. MALPASS~' The Department of Chemistry, The University, Birmingham (Received 6 June 1964) Abstract--Tracer techniques have been used in the determination of monomer reactivity ratios for the radical binary copolymerizations of various esters of methacrylic acid; most of the reactions were performed at 60 °. The results are used for comparison of the reactivities of the monomers towards polymer radicals. There is evidence that depropagation may become significant at 110 ° for the polymerization of phenyl methacrylate. A PREVIOUS paper(~) w a s c o n c e r n e d w i t h t h e r e l a t i v e r e a c t i v i t i e s o f s e v e r a l e s t e r s o f methacrylic acid towards the benzoyloxy radical. This paper deals with the reactivities of the esters towards the polymethyl methacrylate and polybenzyl methacrylate radicals. Monomer reactivity ratios were determined for the radical copolymerizations of certain pairs of methacrylates; tracer techniques were used for the analyses of the copolymers. EXPERIMENTAL Preparations and purifications of the unlabelled monomers have been described already.(l) 3H-methyl methacrylate, 14C-methyl methacrylate and 14C-ethyl methacrylate were prepared by ester exchanges involving the labelled alcohols and the unlabelled monomers; t4C-benzyl methacrylate was prepared by an exchange between 14C-benzyl alcohol and methyl methacrylate; 14C-phenyl methacrylate was made by the reaction of methacrylyl chloride with 14C-phenol. Polymerizations were performed in sealed dilatometers in the complete absence of air and were allowed to proceed to about 5 per cent conversion. For reactions at 60°. azoisobutyronitrile at 0.3 g/l. was used as initiator; the same azonitrile was used as a photosensitizer for reactions at 30 °; polymerizations at 110 ° were initiated by 1-azocyclohexane carbonitrile at 0-035 g/l. Methanol was used as precipitant for all polymers except those containing more than 50 mole ~ of ethyl methacrylate for which n-hexane was used. Polymers were purified by two re-precipitations. Polymers containing ~4C were assayed by gas counting of CO, and those containing tritium by gas counting of C2H2. The procedure for the assay by gas counting of both 14C and tritium in doubly labelled specimens has been described already.t-') Counting rates refer to a given mass of CO2 or of C2H2; specific activities of about 1 t~c/g of C and about 1 ¢az/g of H gave counting rates of 9000 and 400 counts/min respectively. The relative numbers of the monomer units of the two types in a copolymer were calculated from the results of the assays. Suppose that the labelled monomer and the unlabelled monomer have the molecular formulae C, HbOc and CpHqO, respectively and that the copolymer can be represented by the empirical formula (C,HbOc),(CpHqOr) l-n. For labelling with 14C counting rate'~ = (counting rate'I x na forcopolymer! for monomer] n a + ( 1 - - n ) p * The Department of Chemistry, The University, Lancaster. ]' The Research Laboratories, Metal Containers Ltd., Liphook, Hants. 19
20
J . C . BEVINGTON and B. W. MALPASS
For labelling with tritium
countingrate) = [counting rate'1 nb for copolymer t.for monomer/x nb+(l - n ) q The various counting rates are determined experimentally, a, b, p and q are known, and so n can be evaluated. Monomer reactivity ratios were determined by the procedure due to Fineman and Ross. (3) If F = [M~]/[M0] in feed a n d f = [Mal/[Mb] in copolymer then F ( f - - 1)/f.= r~FZ/f - ro.
The conversions were so low that F can be calculated from the initial composition of the feed and f from the average composition of the copolymer. A plot o f F ( f - 1)/fagainst F'-/fis expected to be linear and from the slope and intercept ra and ro can be found.
RESULTS AND DISCUSSION Results of experiments involving methyl a n d ethyl methacrylates at 600 are s h o w n in Table 1 a n d Fig. 1. F o r other pairs of m o n o m e r s also, the F i n e m a n and Ross e q u a t i o n 4
3
2
0
-t 1
I 2
I 3
/ 4
!
5
FZ/f FIG. l. Fineman and Ross plot forcopolymerization o f m e t h y l a n d e t h y l m e t h a c r y l a t e s a t 60°:
O - - reactions in solution ; E) - - reactions in bulk. was obeyed; there were no significant differences between the results for copolymerizations involving b u l k m o n o m e r s a n d those for reactions performed in benzene solution. In those cases where one m o n o m e r was labelled with 14C a n d the other with tritium, the two methods of w o r k i n g led to closely similar results, as illustrated by the data in Table 2. M o n o m e r reactivity ratios for the various pairs of m o n o m e r s are shown in Table 3.
Studies of Radical Copolymerizations
21
Rates of the copolymerizations were recorded. The lines in Fig. 2 show that, for the copolymerizations of various methacrylates with methyl methacrylate, the variation of rate with composition was not large. The values of the product r a.r b are not far from unity (Table 3) indicating that these systems approximate to the so-called ideal copolymerization envisaged by Wall.(4) The monomer units of the two types in the copolymers are evidently not arranged according to a regular pattern. From the monomer reactivity ratios for a set of copolymerizations all of which involve one particular monomer, it is possible to compare the reactivities of monomers towards a reference radical.(5) Results of such comparisons are shown in Table 4. There are significant differences between the reactivities of the various esters of methacrylic acid. Of the monomers copolymerized with methyl methacrylate, only nonyl %
oo
18-
z~
'6 io o~
I
,
60
0.2 Mole
I
0.4
I
0.6
I
0.8
1.0
fraction of MMA in monomer rnixlure
FiG. 2. Rates of.sensitized copolymerizations at 60 ~ for methyl methacrylate with other esters of methacrylic acid. No diluent was present in the following cases: ® - - phenyl methacrylate; (3 - - benzyl methacrylate; • - - ethyl methacrylate. There was 40 per cent of benzene by volume for the copolymerizations involving cyclohexyl methacrylate, indicated by @.
methacrylate has a value of ra in excess of 1 and a value of rb less than 1 ; all the other methacrylates are more reactive than methyl methacrylate towards the polymethyl methacrylate radical, and they are more reactive towards their conjugate radicals also. There are very similar values for the relative reactivities of the monomers towards the polymethyl and polybenzyl methacrylate radicals. It is concluded that highly specific effects do not arise in these copolymerizations involving monomers of very similar structures. It should be possible therefore to predict simply and accurately the monomer reactivity ratios for the copolymerization of any pair of the monomers. The greater reactivities of most of the higher esters of methacrylic acid compared with the methyl ester could be attributed to greater resonance stabilization of the corresponding polymer radicals. This effect could be caused by the greater electron releasing properties of the larger substituent groups; it is, as might be expected, most pronounced for phenyl methacrylate. The treatment due to Bamford et alJ 6) has been applied to the results obtained in
22
J.C. BEVINGTON and B. W. MALPASS
this work. These authors express the velocity constant (k) for the reaction of a polymer radical with a m o n o m e r thus log k = log
kr+ ~+fl
where k r a n d cr are quantities characteristic of the radical, a n d ~ a n d / 3 are quantities characteristic of the m o n o m e r . Values of:~ and fl have been calculated for the methacrylates examined in this work (Table 5). Bamford a n d Jenkins(7) have given vahies of to the nearest 0-5 unit for various methacrylates, r a n ~ n g from - 0'5 (for the n-propyl and butyl c o m p o u n d s ) to - 2 (for the n o n y l and decyl c o m p o u n d s ) ; their values for/3 all lie between 4.82 and 5-27. There is, therefore, good general agreement between their values a n d those reported here. The copolymerization of methyl and phenyl methacrylates was examined at various temperatures. One of the m o n o m e r reactivity ratios seems to be i n d e p e n d e n t of temperature but the other decreases significantly when the temperature is increased from 60 ~ to 110 °. The heat of polymerization of the phenyl ester is significantly lower TABLE I. COPOLY3,1ERIZATIONS AT 6 0 : OF METHYL METHACRYLATE (MMA) AND 14C-ETHYL METHACRYLATE (a)
Reaction conditions
Mole fraction of MMA in feed
Counts[rain for copolymer
Mole fraction of /VlMA in copolymer
Solution Bulk Bulk Solution Bulk Bulk Solution
0.825 0.771 0-698 0.639 0-542 0.339 0.227
320 416 550 625 781 1058 1210
0.813 0-755 0.671 0.622 0.519 0-325 0.213
(a) Counting rate for 14C-ethylmethacrylate = 1482counts[min; for solution polymerizations, the reaction mixture contained 40~ of benzene by volume. TABLE 2. COPOLYMERIZATIONS A T 6 0 ~ OF 3H-METHYL METHACRYLATE (M.MA) AND 14C-BE,N'ZYL,~,IETHACRYLATE (BMA) ('a)
Reaction conditions Solution Bulk Solution Bulk Solution Bulk Solution Bulk
[MMA]/[BMA] in f e e d 6.46 6.48 2.42 2.43 1.08 1.08 0-40 0-40
Counts[min for copolymer carbon-14 tritium 883 882 1620 1608 2228 2204 2748 2795
2180 2175 1616 1610 1063 1100 525 546
[M MA]/[BMA] in copolymer by carbon-14 tritium 5.86 5'86 2.20 2.22 0.99 1.03 0.39 0-34
6.18 6.18 2.33 2.21 0.97 1.03 0-36 0.38
(a) Counting rate for 3H-methyl methacrylate = 2703 counts/min. Counting rate for 14C-benzylmethacrylate = 3227 counts/rain. For solution polymerizations, the reaction mixture contained 40Yo of benzene by volume.
23
Studies of Radical Copolymerizations TABLE 3. COPOLYMERIZATIONSOF PAIRSor METHACRYLICESTERS
Monomer A (a) MMA 3H-MMA t4C-MMA 3H-MMA 3H-MMA 3H-MMA 3H-MMA 14C-BMA 14C-BMA
Monomer
Temp.
B (a)
(2C)
ra
60 60 60 60 60 30 110 60 60
0'92 1-10 0'86 0'93 0'56 0-53 0-56 0-93 0-65
LaC-EMA NMA CMA 14C-BMA I'~C-PMA I~C-PMA 14C-PMA CMA PMA
Monomer reactivity ratios rb
ra. r b
1'08 0'86 1'15 1'05 1'72 1-67 1'30 1.15 1.42
0'99 0'95 0'99 0'98 0'96 0.89 0.73 1-07 0.92
CMA = cyclohexyl methacrylate BMA = benzyl methacrylate PMA = phenyl methacrylate
(a) MMA = methyl methacrylate EMA = ethyl methacrylate NMA = nonyl methacrylate
TABLE 4. RELATIVE REACT1V1TIES OF MONOMERS TOWARDS REFERENCE RADICALS AT 6 0 °
Relative reactivity towards
Monomer
polymethyl methacrylate radical
polybenzyl methacrylate radical
1-00 (standard) 1-09 0-91 1.16 1-08 1.79
1.00 (standard) --1.14 1.05 1.62
Methyl methacrylate Ethyl methacrylate Nonyl methacrylate Cyclohexyl methacrylate Benzyl methacrylate Phenyl methacrylate
TABLE 5. VALUES OF ~ AND ]3 FOR MONOMERS
Ester of methacrylic acid
Methyl Ethyl Nonyl Cyclohexyl Benzyl Phenyl
~
-
1.50 1-62 1.84 1-68 1-74 1.76
/3
4-90 (standard) 4-97 4-96 5-02 5.00 5.22
t h a n t h a t f o r t h e m e t h y l ester, (8) p r o b a b l y b e c a u s e o f g r e a t e r s t e r i c h i n d r a n c e d u r i n g t h e p r o p a g a t i o n r e a c t i o n f o r t h e f o r m e r . It is p r o b a b l e t h a t d e p r o p a g a t i o n f o r t h e case o f phenyl m e t h a c r y l a t e b e c o m e s significant at a t e m p e r a t u r e lower t h a n for methyl m e t h a c r y l a t e ; t h i s effect c o u l d a c c o u n t f o r t h e c h a n g e in o n e o f t h e m o n o m e r r e a c t i v i t y
24
J . C . BEVINGTON and B. W. MALPASS
ratios w i t h t e m p e r a t u r e . O f the f o u r g r o w t h r e a c t i o n s i n v o l v e d in the c o p o l y m e r i z a t i o n o f m e t h y l a n d p h e n y l m e t h a c r y l a t e s , t h a t i n v o l v i n g a p o l y p h e n y l m e t h a c r y l a t e radical a n d a m o l e c u l e o f p h e n y l m e t h a c r y l a t e is p r o b a b l y the o n e m o s t easily affected by the reverse r e a c t i o n ; the effective rate o f this g r o w t h p r o c e s s w o u l d thus a p p e a r to be c o m p a r a t i v e l y low at 1 i0 ° a n d the r e a c t i v i t y ratio, r b, w o u l d be s m a l l e r t h a n m i g h t be e x p e c t e d . Acknowledgements--We thank the University of Birmingham for the award of the A. E. Hills Scholarship to B.W.M.
REFERENCES (1) (2) (3) (4) (5) (6) (7) (8)
J. C. Bevington and B. W. Malpass, J. Polym. Sci. 2, 1893, A, (1964). J. C. Bevington and D. E. Eaves, Makromol. Chem. 36, 145 (1960). M. Fineman and S. D. Ross, J. Polym. Sci. 5, 259 (1950). F. T. Wall, J. Amer. chem. Soc. 66, 2050 (1944). See, for example, J. C. Bevington, Radical Polymerization, p. 79, Academic Press, London (1961). C. H. Bamford, A. D. Jenkins and R. Johnston, Trans. Faraday Soc. 55, 418 (1959). C. H. Bamford and A. D. Jenkins, Trans. Faraday Soc. 59, 530 (1963). L. K. J. Tong and W. O. Kenyon, J. Amer. chem. Soc. 68, 1355 (1946).
R6sumd--La technique des traceurs a 6t6 utilis6e dans la d~termination des rapports de r6activit6 des monom~res pour les copolym/:risations b. radicaux binaires de divers esters d'acide m6thacrylique; la plupart des r6actions ont et6 ex6cut6es ~t 60 °. On a utilis6 les r6sultats en vue de comparer les r6activit6s des monom~res -->- radicaux de polym~re. I1 est maintenant ~tabli qu'il se produit une limitation significative b. 110°, Mrs de la polym6risation du m6thacrylate de ph6nyle. Riassunto--Si sono impiegate tecniche da traccianti radiattivi nella determinazione dei rapporti di reattivit~, monomerica per le copolimerizzazioni binarie radicali di vari esteri dell'acido metacrilico. La maggior parte delle reazioni ~ stata compiuta a 60 °. Si sfruttano i risultati ottenuti a scopo di confronto delle reattivit~ dei monomeri rispetto ai radicali polimerici. Si mette in evidenza come la depropagazione possa farsi rilevante a 110 ° agli effetti della polimerizzazione del metacrilato fenilico. Zusammenfassung--Indikatortechniken sind bei der Bestimmung der Verhiiltnisse von MonomerReaktivit~it fiJr die radikalen binaren Mischpolymerisationen verschiedener Ester der Methakryls~iure angewandt worden; die Mehrzahl der Reaktionen wurde bei 60 ° durchgef'tihrt. Die Ergebnisse werden zum Vergleich der Reaktivit/iten der Monomeren gegentiber polymeren Radikalen benutzt. Es besteht Grund zur Annahme, dass eine Hemmung der Weiterentwicklung fiir die Polymerisierung yon Phenylmethakryls~iureester bei 110 ° bedeutungsvoll wird.