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Free-radical template polymerization
NOTES AND COMMUNICATIONS measurements of T~ and T~p are consistent with the conclusion that T~ is independent of molecular weight for PPOs with OH end-groups. The lowest temperature at which evidence of the glass transition motion is seen in the T~ and T~o results is ,--~I80°K which lies between the values of T~ and T~. The activation energy for the CH.~ reorientational motion derived from the T~ and T~o data is 4.2 kcal mole -~ which is similar to the barrier value derived from the neutron sca,ttering data (3"5 kcal mole -~) and much larger than the value of 1-7 kcal mole -~ previously obtained by fitting *' '* T~ minimum data. It also agrees well with a value derived by interpolation from Stejskal and Gutowsky's calculations 8 of the effect of tunnelling on the CH3 reorientational frequency. There is no evidence from the data in Figure 2, however; of the fall in apparent activation energy which would be expected from the results of these calcula~tions if tunnelling was important at low temperatures.
Thanks are due to Dr P. Brier/or providillg neutron scatterh~g data prior to publication. This work forms part of the research progra~mme of the National Physical Laboratory. T. M. CONNORand A. HARILANO
Molecular Science Division, National Physical Laboratory, Teddington, Middlesex. (Received May 1968) REFERENCES t (a) SLICHTER,C. P. and AILION,D. Phys. Rev. 1964, 135, A 1099 (b) AILION, D. and SLICHTER,C. P. Phys. Rev. 1965, 137, A 235
MCCALL,D. W. and DOUGLASS,D. C. ,4ppl. Phys. Letters, 1965, 7, 12 ~;CONNOR,T. M. and HARTLAND,A. Physics Letters, 1966, 23, 662 4CONNOR,T M., BLEARS,D. J. and ALLEN,G. Trans. Faraday Soc. 1965, 61, 1097 5HARTMANN,S. R. and HAHN,E. L. Phys. Rev. 1962, 128, 2046 CONNOR,T. M. and HARTLAND,A. To be published JONES,G. P. Phys. Rev. 1966, 140, 332 8 STEJSKAL,E. O. and GUTOWSKY, H. S. J. chem. Phys. 1958, 28, 388 9 BRIER, P. Private communication t0 WILLIAMS,G. Trans. Faraday Soc. 1965, 61, 1564 11BAUR,M. E. and STOCKMAYER,W. H. J. chem. Phys. 1965, 43, 4319 1." ~TLEN,G., JONES, M. N., MARKS, D. J. and TAYLOR,W. D. Polymer, Lond. 1964, 5, 547 13FAUCHER,J. A. Polymer Letters, •965, 3, 143 14CONNOR,T. M. and BLEARS,D. J. Polymer, Lond. 1965, 6, 385
Free-radical Template PolymerizatiOn THE polymerization of monomer molecules in organized arrays, for example molecules adsorbed on templates as in biopolymerization, provides a possible method of synthesizing copolymers of predetermined composition and structure. These materials are of potential importance both industrially and academically, yet comparatively little attention has been 595
NOTES AND COMMUNICATIONS paid to template polymerization, no doubt partly because of the difficulty of finding suitable systems with conventional monomers. Ballard and Bamford 1 established the existence of a templute effect (or 'chain effect') in the polymerization of N-carb3xy c~-amino acid anhydrides (NCAs) in the presence of polysarcosine. In th.~se systems adsorption of the NCA molecules on to polysarcosine chains '-3 leads to a rapid polymerization showing unusual kinetic features. Bamford, Block and imanishi 4 showed that when a mixture of two different NCAs is polymerized in this manner, polymers with block-like character are formed if only one species of N C A is adsorbed on to the polysarcosine chain. Some degree of control of copolymer structure by template action is therefore possible in these reactions. Kargin and his colleagues5 hav.e investigated the specific polymerization by an ionic reaction of vinyl pyridinium salts on macromolecular matrices of polystyrene sulphonic acid and K/immerer and Ozaki ~ have described the preparation of stereoregular and molecularly uniform acrylic acid oligomers on a matrix consisting of a polynuclear phenolic derivative. In this latter case the monomer was held to the matrix by covalent bonds so, that the matrix exercised a very precise control over the reaction. As far as we know there are no reports in the literature of free-radical template reaction analogous to the chain-effect polymerization of NCAs. For the past two years we have been studying such a process, viz. the freeradical polymerization of acrylic acid adsorbed on a template of the basic polymer polyethylenimine, and in this communication we present an outline of the kinetic results. Polyethylenimine of degree of polymerization 275 was used and the solvent was a mixture of acetone and water, 2/1 (v/v). The systems were always homogeneous. Figure 1 shows the initial rate of photopolymerization of acrylic acid (M) at 25°C as a function of the total base-molar concentration of polyethenimine IT]0. The initial concentration of acrylic acid was constant at 0"1 ml-1; with this low concentration no significant rate of polymerization is observed in the absence of polyethylenimine under the conditions described. However, the maximum initial rate, which occurs when [T]o/[M],,= 1 approximately, ~[M]0=initial concentration of monomer), is high, corresponding to about 30 per cent polymerization per minute. The shape of the curve in Figure 1 is that expected for a template polymerization. At the lower values of [T]0 the template is effectively saturated with monomer, and in this region the rate increases with increasing IT]0. Eventually, however, as IT]0 increases, a point is reached when there is not sufficient monomer in the system to maintain saturation of the template, and further increase in IT],, then leads to a reduction in rate of polymerization, since the continuity of the adsorbed monomer is interrupted. Other types of free-radical initiation give essentially similar results; we have examined thermal initiation by azo-bis-isobutyronitrile (3 × 10-3 m1-1) at 60°C and photoinitiation by the Mn,2(CO),0-CHC13 system7 ([Mn~ (CO),,,] = [CHC13]=10-3ml-1; h = 4 3 5 8 A) at 25°C. When polyethylenimine was replaced by tetraethylene pen,tamine no template effect was observed at 25°C, the rate of polymerization remaining very low. Before accepting a template mechanism we must ascertain whether 596
NOTES AND COMMUNICATIONS changes in [T], can affect the rate of polymerization by any other mechanism. Both the viscosity and the p H of the reaction mixture are dependent on [T]., and these properties could influence the rate of polymerization of acrylic acid. However, the viscosity is almost proportional to [T].
3° L
c
20
c
o
c
o
,i
i
!
2 lflo/IM] 0
Figure 1--0 Dependence of ini.tial rate of polymerization on base molar concentration of polyethylenimine [T], at 25°C. • Reaction in the presence of tetraethylene pentamine. (IT]0=0). Acrylic acid concentration [M]0=0"l m1-1. Azobis-isobutyronitrile (6×10-3ml -~) as photosensitizer; X in range 3 650-3 663 A over the range studied, so that if the rate increases arise from diffusion control of the termination reaction the rate should increase monotonically with [T]0, which is clearly not so, Katchalsky and Bauer a showed that in the polymerization of methacrylic acid in aqueous media the rate decreases with increasing p H up to a value of six, at which point it is practically zero. In our case (Figure 1) the rate increases as the p H increases up to a value of five. Further, when the p H is controlled by sodium hydroxide in the absence of polyethylenimine no similar increase in the rate of polymerization (which remains effectively zero) is observed. Finally, the contribution of Michael addition of polyethylenimine to acrylic acid to the overall reaction must be assessed. At 25°C the observed rate of reaction in the absence of a free-radical initiator is negligible, showing that Michael addition does not contribute significantly. At 60°C when [T]./[M], ~ 2 the rate of polymerization is low, and the Michael reaction accounts for most of the observed rate. The rate of Michael addition is proportional to [T],,; for [T],/[M] : ~ 1 it makes only a small contribution to the observed rate. Confirmatory evidence for the template nature of the polymerization is provided by the observation that the addition of a non-polymerizable acid may 597
NOTES A N D C O M M U N I C A T I O N S
produce a large decrease in the rate of polymerization. Table I illustrates this effect for addition of ot-hydroxyisobutyric acid HBA. The latter is not a retarder of conventional free-radical polymerization; its behaviour in the present system arises from its competition with the monomer for the template sites. Clearly in the experiments referred to in Table 1 the monomer is largely displaced by the stronger HBA. (The values of Ka for HBA and acrylic acid are 9.2 x 10-~ (18°C) and 5"7 x 10-5 (25°C), respectively.) Table 1.
Effect of a-hydroxyisobutyric acid (HBA) on initial rate of reaction. [T]o/[M]o=0"39; [M]0=0q ml-1; 25°C. Initiation as in Figure 1
[HBA] / [M] 0
0
Initial rate, % conversion per rain
27
1
0'5
2.
<0'1
We have developed the following expression for this type of template polymerization. Rv = C exp ( - c~Za/kT) IX] (1) Here R,, is the rate of "polymerization, k, T are the Boltzmann constant and absolute temperature, respectively, IX] is the concentration of adsorbed monomer and Z = IT]/IX], where IT] is the concentration of free template sites. In equation (1)or,/3 are constants, and C is a function expressing the total free-radical concentration on the template molecules which is almost constant under the conditions of our experiments. The exponential term in equation (1) represents the important effects on the rate arising from interruptions in monomer sequences along the templates. The equilibrium constant for the adsorption of acrylic acid on polyethylenimine has been determined, hence IX] may be calculated. InFigure 1 the curve shown is the theoretical one obtained from equation (1) with appropriate values of the parameters; it agrees well with the experimen~tal observations. We hope to extend this work to other free-radical template processes and to explore their suitability for synthetic purposes. C. H. BAMFORDand Z. SHIIKI
Department of Inorganic, Physical and Industrial Chemistry, The University of Liverpool, Donnan Laboratories, Liverpool, 7. (Received May 1968) REFERENCES 1 BALLARD,D. G. H. and BAMFORD,C. H. Proc. Roy. Soc. A, 1954, 223, 495 BAMFORD,C. H. and PRICE, R. C. Trans. Faraday Soc. 1965, 61, 2208 '~BALLARD,D. G. H. Biopolymers, 1964, 2, 463 BAMFORO,C. H., BLOCK, H. and IMANISHI,Y. Biopolymers, 1966, 4, 1067 K^BANOV, V. A., ALIEV, K. V., KARGINA,O. V., PATgIKEEVA,T. I. and KAgGIN, V. A. International Symposium on Macromolecular Chemistry, Prague, 1965. See also: KARG~N, V. A., KABANOV, V. A., ALmV, K. V. and ROZVOLORSKY, E. F. Dokl. A kad. Nauk S.S.S.R., 1965, 161, 1131; KARGIN, V. A., KABANOV,V. A. and KARGIIqA, O. V. International Symposium on Macromolecular Chemistry, Brussels-Louvain (1967) K;(MMERER, H. and OZAKI, SH. Makromol. Chem. 1966, 91, 1 7 BAMFORD,C. H., CROWE, P. A. and WAYNE, R. P. Proc. Roy. Soc. A, 1965, 284, 455 KATCHALSKY,A. and B^UER, G. Trans. Faraday Soc. 1951, 47, 1360
598
Thanks are due to Dr P. Brier/or providillg neutron scatterh~g data prior to publication. This work forms part of the research progra~mme of the National Physical Laboratory. T. M. CONNORand A. HARILANO
Molecular Science Division, National Physical Laboratory, Teddington, Middlesex. (Received May 1968) REFERENCES t (a) SLICHTER,C. P. and AILION,D. Phys. Rev. 1964, 135, A 1099 (b) AILION, D. and SLICHTER,C. P. Phys. Rev. 1965, 137, A 235
MCCALL,D. W. and DOUGLASS,D. C. ,4ppl. Phys. Letters, 1965, 7, 12 ~;CONNOR,T. M. and HARTLAND,A. Physics Letters, 1966, 23, 662 4CONNOR,T M., BLEARS,D. J. and ALLEN,G. Trans. Faraday Soc. 1965, 61, 1097 5HARTMANN,S. R. and HAHN,E. L. Phys. Rev. 1962, 128, 2046 CONNOR,T. M. and HARTLAND,A. To be published JONES,G. P. Phys. Rev. 1966, 140, 332 8 STEJSKAL,E. O. and GUTOWSKY, H. S. J. chem. Phys. 1958, 28, 388 9 BRIER, P. Private communication t0 WILLIAMS,G. Trans. Faraday Soc. 1965, 61, 1564 11BAUR,M. E. and STOCKMAYER,W. H. J. chem. Phys. 1965, 43, 4319 1." ~TLEN,G., JONES, M. N., MARKS, D. J. and TAYLOR,W. D. Polymer, Lond. 1964, 5, 547 13FAUCHER,J. A. Polymer Letters, •965, 3, 143 14CONNOR,T. M. and BLEARS,D. J. Polymer, Lond. 1965, 6, 385
Free-radical Template PolymerizatiOn THE polymerization of monomer molecules in organized arrays, for example molecules adsorbed on templates as in biopolymerization, provides a possible method of synthesizing copolymers of predetermined composition and structure. These materials are of potential importance both industrially and academically, yet comparatively little attention has been 595
NOTES AND COMMUNICATIONS paid to template polymerization, no doubt partly because of the difficulty of finding suitable systems with conventional monomers. Ballard and Bamford 1 established the existence of a templute effect (or 'chain effect') in the polymerization of N-carb3xy c~-amino acid anhydrides (NCAs) in the presence of polysarcosine. In th.~se systems adsorption of the NCA molecules on to polysarcosine chains '-3 leads to a rapid polymerization showing unusual kinetic features. Bamford, Block and imanishi 4 showed that when a mixture of two different NCAs is polymerized in this manner, polymers with block-like character are formed if only one species of N C A is adsorbed on to the polysarcosine chain. Some degree of control of copolymer structure by template action is therefore possible in these reactions. Kargin and his colleagues5 hav.e investigated the specific polymerization by an ionic reaction of vinyl pyridinium salts on macromolecular matrices of polystyrene sulphonic acid and K/immerer and Ozaki ~ have described the preparation of stereoregular and molecularly uniform acrylic acid oligomers on a matrix consisting of a polynuclear phenolic derivative. In this latter case the monomer was held to the matrix by covalent bonds so, that the matrix exercised a very precise control over the reaction. As far as we know there are no reports in the literature of free-radical template reaction analogous to the chain-effect polymerization of NCAs. For the past two years we have been studying such a process, viz. the freeradical polymerization of acrylic acid adsorbed on a template of the basic polymer polyethylenimine, and in this communication we present an outline of the kinetic results. Polyethylenimine of degree of polymerization 275 was used and the solvent was a mixture of acetone and water, 2/1 (v/v). The systems were always homogeneous. Figure 1 shows the initial rate of photopolymerization of acrylic acid (M) at 25°C as a function of the total base-molar concentration of polyethenimine IT]0. The initial concentration of acrylic acid was constant at 0"1 ml-1; with this low concentration no significant rate of polymerization is observed in the absence of polyethylenimine under the conditions described. However, the maximum initial rate, which occurs when [T]o/[M],,= 1 approximately, ~[M]0=initial concentration of monomer), is high, corresponding to about 30 per cent polymerization per minute. The shape of the curve in Figure 1 is that expected for a template polymerization. At the lower values of [T]0 the template is effectively saturated with monomer, and in this region the rate increases with increasing IT]0. Eventually, however, as IT]0 increases, a point is reached when there is not sufficient monomer in the system to maintain saturation of the template, and further increase in IT],, then leads to a reduction in rate of polymerization, since the continuity of the adsorbed monomer is interrupted. Other types of free-radical initiation give essentially similar results; we have examined thermal initiation by azo-bis-isobutyronitrile (3 × 10-3 m1-1) at 60°C and photoinitiation by the Mn,2(CO),0-CHC13 system7 ([Mn~ (CO),,,] = [CHC13]=10-3ml-1; h = 4 3 5 8 A) at 25°C. When polyethylenimine was replaced by tetraethylene pen,tamine no template effect was observed at 25°C, the rate of polymerization remaining very low. Before accepting a template mechanism we must ascertain whether 596
NOTES AND COMMUNICATIONS changes in [T], can affect the rate of polymerization by any other mechanism. Both the viscosity and the p H of the reaction mixture are dependent on [T]., and these properties could influence the rate of polymerization of acrylic acid. However, the viscosity is almost proportional to [T].
3° L
c
20
c
o
c
o
,i
i
!
2 lflo/IM] 0
Figure 1--0 Dependence of ini.tial rate of polymerization on base molar concentration of polyethylenimine [T], at 25°C. • Reaction in the presence of tetraethylene pentamine. (IT]0=0). Acrylic acid concentration [M]0=0"l m1-1. Azobis-isobutyronitrile (6×10-3ml -~) as photosensitizer; X in range 3 650-3 663 A over the range studied, so that if the rate increases arise from diffusion control of the termination reaction the rate should increase monotonically with [T]0, which is clearly not so, Katchalsky and Bauer a showed that in the polymerization of methacrylic acid in aqueous media the rate decreases with increasing p H up to a value of six, at which point it is practically zero. In our case (Figure 1) the rate increases as the p H increases up to a value of five. Further, when the p H is controlled by sodium hydroxide in the absence of polyethylenimine no similar increase in the rate of polymerization (which remains effectively zero) is observed. Finally, the contribution of Michael addition of polyethylenimine to acrylic acid to the overall reaction must be assessed. At 25°C the observed rate of reaction in the absence of a free-radical initiator is negligible, showing that Michael addition does not contribute significantly. At 60°C when [T]./[M], ~ 2 the rate of polymerization is low, and the Michael reaction accounts for most of the observed rate. The rate of Michael addition is proportional to [T],,; for [T],/[M] : ~ 1 it makes only a small contribution to the observed rate. Confirmatory evidence for the template nature of the polymerization is provided by the observation that the addition of a non-polymerizable acid may 597
NOTES A N D C O M M U N I C A T I O N S
produce a large decrease in the rate of polymerization. Table I illustrates this effect for addition of ot-hydroxyisobutyric acid HBA. The latter is not a retarder of conventional free-radical polymerization; its behaviour in the present system arises from its competition with the monomer for the template sites. Clearly in the experiments referred to in Table 1 the monomer is largely displaced by the stronger HBA. (The values of Ka for HBA and acrylic acid are 9.2 x 10-~ (18°C) and 5"7 x 10-5 (25°C), respectively.) Table 1.
Effect of a-hydroxyisobutyric acid (HBA) on initial rate of reaction. [T]o/[M]o=0"39; [M]0=0q ml-1; 25°C. Initiation as in Figure 1
[HBA] / [M] 0
0
Initial rate, % conversion per rain
27
1
0'5
2.
<0'1
We have developed the following expression for this type of template polymerization. Rv = C exp ( - c~Za/kT) IX] (1) Here R,, is the rate of "polymerization, k, T are the Boltzmann constant and absolute temperature, respectively, IX] is the concentration of adsorbed monomer and Z = IT]/IX], where IT] is the concentration of free template sites. In equation (1)or,/3 are constants, and C is a function expressing the total free-radical concentration on the template molecules which is almost constant under the conditions of our experiments. The exponential term in equation (1) represents the important effects on the rate arising from interruptions in monomer sequences along the templates. The equilibrium constant for the adsorption of acrylic acid on polyethylenimine has been determined, hence IX] may be calculated. InFigure 1 the curve shown is the theoretical one obtained from equation (1) with appropriate values of the parameters; it agrees well with the experimen~tal observations. We hope to extend this work to other free-radical template processes and to explore their suitability for synthetic purposes. C. H. BAMFORDand Z. SHIIKI
Department of Inorganic, Physical and Industrial Chemistry, The University of Liverpool, Donnan Laboratories, Liverpool, 7. (Received May 1968) REFERENCES 1 BALLARD,D. G. H. and BAMFORD,C. H. Proc. Roy. Soc. A, 1954, 223, 495 BAMFORD,C. H. and PRICE, R. C. Trans. Faraday Soc. 1965, 61, 2208 '~BALLARD,D. G. H. Biopolymers, 1964, 2, 463 BAMFORO,C. H., BLOCK, H. and IMANISHI,Y. Biopolymers, 1966, 4, 1067 K^BANOV, V. A., ALIEV, K. V., KARGINA,O. V., PATgIKEEVA,T. I. and KAgGIN, V. A. International Symposium on Macromolecular Chemistry, Prague, 1965. See also: KARG~N, V. A., KABANOV, V. A., ALmV, K. V. and ROZVOLORSKY, E. F. Dokl. A kad. Nauk S.S.S.R., 1965, 161, 1131; KARGIN, V. A., KABANOV,V. A. and KARGIIqA, O. V. International Symposium on Macromolecular Chemistry, Brussels-Louvain (1967) K;(MMERER, H. and OZAKI, SH. Makromol. Chem. 1966, 91, 1 7 BAMFORD,C. H., CROWE, P. A. and WAYNE, R. P. Proc. Roy. Soc. A, 1965, 284, 455 KATCHALSKY,A. and B^UER, G. Trans. Faraday Soc. 1951, 47, 1360
598