Stereospecificity of polymers as a function of the conditions of polymerization

STEREOSPECIFICITY OF POLYMERS AS A FUNCTION OF THE CONDITIONS OF POLYMERIZATION* T.M. BIRSHTEIN and O. B. PTITSYN Institute of High Molecular Compounds of the U.S.S.R. Academy of Sciences (Received 27 March 1959) INTRODUCTION

THE tremendous scientific and technical importance of vinyl polymers with a stereoregular structure has in recent years aroused considerable interest in the stereo-chemical structure of macromolecules. Natta's papers [cf. e.g. 1-4] report the preparation of a large number of isotactie vinyl polymers and syndiotactic poly-l,2-butadienes by heterogenous catalysis. Recently [5] stereospecific (apparently syndiotactic [6] polymethyl methacrylate was obtained by Fox et al. who also used free-radical polymerization at low temperature; the stereospecificity of the resulting polymer is inversely proportional to the polymerization temperature [7]. Syndiotactie polymethyl methacrylate can further be synthesized by homogeneous catalytic polymerization at low temperature in a strongly solvating medium [5, 8]; as the solvating power o f the medium diminishes anisotactic polymer is formed [5, 8]. These experimental facts point to the existence of two mechanisms which are responsible for the stereospecificity of the polymerization: the influence of the end of the growing chain which produces a predominantly syndiotactic structure [7], and the influence of the catalyst which, as a rule, yields a chiefly isotactic structure. The former mechanism, the effectiveness of which rapidly increases in inverse proportion to the polymerization temperature, is determinable provided (a)there is no catalyst (free-radical polymerization) or (b)the catalyst has no stereospecific action (homogeneous catalytic polymerization in a strongly solvating medium). As the polymerization temperature rises the influence of this mechanism declines to the point where the polymers obtained in the absence of a stereospecific catalyst are amorphous. For polymethyl methacrylate the "transition" temperature is in the region of 0°C; polyvinylchloride, however, prepared by free-radical polymerization at room temperature retains its crystallizability and its characteristic frequency reveals a syndiotactie structure [9]. * Vysokomol. soedin. 1: No. 6, 846-851, 1959. 288

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CORRELATION BETWEEN THE CONSECUTIVE ADDITIONS OF MONOMER UNITS AND THE FORMATION MECHANISM OF STEREOBLOCK POLYMERS

The stereospecific influence of the end of the growing chain (the likelihood of such an influence existing was first referred to by Huggins [10]) is governed by the differences in the transition state energies corresponding to isotactic and syudiotactie addition. Goode [7] asserts that this difference is associated with the fact that when a monomer is added to the end of the growing chain the CHR-group which terminated the chain before the addition may assume an isotactic or syndiotactie arrangement with respect to the preceding CHI~group. The difference between the transition state energies (i. e. the difference in the activation energies) amounts to the difference in the energies of these two arrangements of the CHR-groups. Schulz [11] has put forward the view that syndiotactie addition is energetically more economical; this was recently borne out b y Fordham's model calculation. The physical cause behind the dependence of the chain energy on the isotactic or syndiotactic arrangement of the CHR-group is the van der Waals interaction between chemically unrelated atoms and groups which is also responsible for retarding the internal rotation in the chain. The authors and Gotlib have formulated a theory to explain the influence of the retardation of internal rotation on the properties of macromolecules in solution which is based on Vol'kenshtein's rotary-isomeric chain model [12] and on the method of determining the rotary isomers of monomer units proposed by one of us together with Sharonov [13]. Comparison of this theory with experimental findings reveals the presence of a definite interaction both between adjacent CR 2 or CHR-groups in polymers of the (-- CH 2 - CE 2-),, and (-- CH 2 - C H E - - )n type, and between the nearest non-adjacent CR 2 or CHR-groups (separated by four members of the principal chain) [14-16]. Thus, for isotactic polystyrene the difference in energy between non-identical and identical spiral configurations of adjacent monomer units governed by the interaction of the nearest, non-adjacent phenyl groups amounts to about 630 cal/mol [14]. These interactions enlarge the chains in solutions but so far as is known cannot be responsible for their rigid spiral configuration in the dissolved state. The opposite view [17] based on an analogy with polymers which form incomparably stronger intermolecular hydrogen bonds we regard as inconclusive. The occurrence of a definite interaction between the non-adjacent CHRgroups must produce a state where the probability of isotactic or syndiotactie monomer addition is governed b y the manner in which the preceding monomer was added. Consequently, notwithstanding the opinion held by Coleman [18], this relationship can also be operative even in the absence of a catalyst. Thus we must be confronted not with two probabilities ~i and a~, but with four; ~ii, ~+~, a~ and ~, (~i~--probability of syndiotactie addition of the n t h

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T.M. BIRSlITEIN and O. B. PTITSYN

monomer unit to the (n--1)th provided the (n--1)th unit has been added isotactically to the (n--2)th and so forth) related b y the obvious conditions: O~ii+~Xis=1,

O~si+O~ss=1

(1)

and forming a Markoff series. When considered in this w a y the formation of stereoisomeric polymers is formally identical with copolymerization [19]. The presence of four probabilities of addition in copolymerization is governed by the different chemical nature of the two types of monomer unit. The correlations between the successive additions of monomer units can normally be disregarded here [19] as follows from the fact that the steric effects associated with this type of correlation are generally slight compared with the effects arising from the different reactivities of the monomers. However, in the formation of stereoisometric polymers the difference in reaction rates is attributable excll~sively to steric effects. Where there are four probabilities of addition %q we obtain for the proportion of monomer units contained in an isotactic sequence consisting of more than N units the expression: /N=

n (1 - ~ i ) ~ 8 i ~ "-1 (1 - ~,) = ~ ~,2~(14-N~) ,

(2)

n=N+l

where a i = ai ~ii + ~, ~,i = an a priori probability of isotactic addition. If %0 is not dependent on the first term equation (2) becomes fN = ~X~ v + l (l+Na,),

(3)

obtained in paper [7]. Expressions for the proportion of the monomer units contained in sindiotactic sequences can be derived from equation (2) and (3) b y transposing the i and s indices. The theory as to the presence of a correlation between the successive additions of monomer units to the end of the growing chain readily accounts for the existence of crystalline stereoblock polymers, the chains of which consist of alternating isotactic and sindiotactic sequences. Polymers of this type were prepared by the homogeneous catalytic polymerization of methyl methacrylate in a moderately solvating medium at low temperature [7]. It will be clear that the existence of such polymers cannot be explained in terms of the two probabilities a i and ~s (which are independent of time). In point of fact th¢ occurrence of long isotactic and syndiotactic sequences needed to form crystallites of both types necessitates that a i and a s should be close to unity simultaneously, and this conflicts with the obvious condition a i + a s = l . Against this in the case considered above involving four probabilities aii and a** can be as close to unity simultaneously as possible without infringing condition (1). As pointed out earlier the interaction of the adjacent CHR-groups appears to result preferentially in sindiotactic addition, viz. it increases ai8 relative to ~i~, and a,8 relative to ~,i. We shall assume that the interaction of the nearest,

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291

non-adjacent CHR-groups stabilizes an addition analogous to the preceding one, i.e. increases aii and a~s (cf. [17]). When the difference between ~-i~and a~'8 resulting from these interactions becomes smaller, that between ~ au=l ~ grows, and when a~.~and ai~ are similar, ~ )) a~i. Furthermore a syndiotactie polymer is obtained. The catalytic effect which increases ~ii and ~i may lead to the simultaneous fidfilment of aii ) ) ~i8 and ~8~)) a~ which are indispensable conditions for the formarion of a stereoblock polymer (even where the influence of the catalyst is independent of the preceding addition). With increasing stereospecificity of the catalytic influence (owing, say, to a falling-off in the solvating power of the medium) the condition ~ 8 ) ) ~ i will cease to be fulfilled and an isotactic polymer will result. On the other hand if there are just two probabilities ai and ~ such that az ))~i in the absence of catalyst, as the stereospecificity of the catalyst grows we shall first find a~ ~ ~ (atactic polymer) and then ~i )) ~ (isotactic polymer). THE PROPERTIES OF MACROMOLECULES IN SOLUTION AS A METHOD TO STUDY THE STE;~EOCHEMICAL STRUCTURE ("MICROTACTICITY ~) OF THE CHAIN *

In order to study quantitatively the influence of the polymerization conditions (temperature, medium, catalyst used and so forth) on the stereochemical structure of the chains we need to have at our disposal methods with which to determine the stereochemical stru6ture from the properties of the polymers obtained. The X-ray method commonly employed for this purpose has its limitations in that the crystallinity of the polymers rapidly drops to zero with decreasing stereospecificity (it follows from equations (2) and (3) that the proportion of monomer units contained in long sequences is practically zero if ~i and ~ differ noticeably from unity). Any quantitative method to determine the relationship between the stereochemical structure of the macromolecules and the polymerization conditions for atactic polymers, as they are called (with ~i ranging from 0.1 to 0.9) must be based on physical properties that to a large extent are independent of the regularity of the stereochemical chain structure (e.g. the crystallinity) but which are directly governed by the probabilities of isotactic and syndiotactic additions of monomer ("microtacticity" of the chain). The physical properties that have occasionally been used for this purpose such as solubility, degree of swelling, vitrification temperature etc. are unsuitable for the quantitative evaluation of a i because at present there is no theory linking them with the structure of the chain. So far as is known the above applies to the infra-red spectra of stereoisomerie polymers in non-polarized light because considerable difficulty is encountered in working out the relationship between the frequency, and especially the intensity, of the absorption bands of the polymer chain and its stereochemical structure [20].

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As demonstrated by the authors in 1954 [21, 22] .the physical properties of macromolecules in solution (dimensions and dipole moments) must be related to the stereochemical structure of the chains. This conclusion, which was also extended by Gotlib [24] to the optical anisotropy of macromoleeules, was subsequently confirmed by one of us in collaboration with Sharonov [23] allowing for the correlation between the rotations about adjacent segments. A series of papers have compared the dimensions of molecules of isotactic and atactic polystyrene [25-29] and polypropylene [30-33] determined from the intrinsic viscosity [25, 27-33] and from light-scattering [26, 29, 32]: ill both cases the dinmnsions of the isotactic and atactic polymer molecules proved identical. In a number of cases, however, (cf. e.g. [27]), at a given molecular weight, the second virial coefficient of the isotactic polymer is m u c h lower than of the atactic polymer. This led Krigbaum, Carpenter and Newman [29] to assume that at a given molecular weight the "undisturbed" dimensions of isotastic polystyrene molecules (measurable at the &point) must be larger than the dimensions of atactic polystyrene molecules. (According to this theory, in good solvents the increase in the "undisturbed" dimensions is offset by the reduced influence of the space effects). Unfortunately all attempts so far to measure the properties of isotactic macrvmolecules at the 0-point have been unsuccesful [29]. The difficulties involved in measuring the dimensions of isotactic polymers in &solvents make it particularly desirable to obtain experimental proof of the theories [22-24] which relate the dipole moments and the optical anisotropy of the macromolecules to their stereochemical structure because these properties are practically insensitive to the nature of the solvent. Such proof was recently adduced by Tsvetkov and Magarik [34] who showed that the optical anisotropy of an isotactie polystyrene macromolecule in solution is greater than that of the normal atactic polymer by a factor of about 1.5. Moreover they were the first to confirm experimentally the relationship of macromolecules in solution to their stereochemical structure. Accordingly the properties of macromolecules in solution can be used for the quantitative d~termination of chain microtacticity. The findings reported in paper [34] attest that the stereochemical structure of polystyrene is by no means isotactic (cf. [22]). Pro-requisites for a more accurate determination of the microtacticity are a knowledge of the optical anisotropy of sindiotaetic polystyrene and the development of a theory to account for the physical properties of the atactic molecules in solution. At the same time the occurrence of a retarding effect in respect of the internal rotation m a y lead to a non-linear relation between the properties of the isotactic and syndiotactic polymers (a theory dealing with the analogous non-linear relation for copolymers is formulated in paper [35]). Information on the micr0tacticity of chains can also be gained b y studying their infra-red dichroism [20] and (in particular cases) by studying the kinetics of certain reactions within the chains [36].

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One of the problems whose solution requires a knowledge of the ~)ficrotacticity of the chain is the e x p e r i m e n t a l s t u d y of the relationship between the p r o b a b i l i t y of stereospecific addition of the m o n o m e r unit to the end of the growing chain ~nd the degree of Crystallinity of the p o l y m e r (cf. equations (2) and (3)). Block c o p o l y m e r s c o m p o s e d of isots~ctic a n d atactic sequences and h a v i n g various degree~ of crystallinity are suitable material for solvin~ this problem [37, "~]. CONCLUSIONS

(1) The interaction between nearest, n o n - a d j a c e n t R - g r o u p s in (--CH~ - C H R - ) , ~ t y p e p o l y m e r s m u s t resl~lt in a correlation between the successive additions of m o n o m e r units. (2) The existence of such a correlation a c c o u n t s for the p h e n o m e n o n of stereoblock macromolecules consisting o f alternate isotactic and sindiotactic sequences a n d f o r m e d b y h o m o g e n e o u s polymerization in a m o d e r a t e l y solvating m e d i u m where the stereospecific influence of both the c a t a l y s t and the end of the growing chain is substantial. (3) The properties of p o l y m e r s in solution can serve as a m e t h o d to examiile the m i c r o t a c t i c i t y of macromolecules. T r a , slated by C,. CAMERON REFERENCES

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