July, 1960
BLOCK POLYMERS OF STYRENE AND ISOPRESE
ro. It is logical to conclude either that the probability of orientation of the longest molecular segments is less than that assumed in eq. 1, or that increased coiling with increased chain length shortens the relaxation time, or both. It is evident that the arc plots and the elliptical plots differ t,oo little from one another to make
883
possible a direct distinction between them by means of the data for the alkyl bromides. However, the magnitudes of the relaxation times calculated from the elliptical distribution favor the latter through their consistency with the most reasonable physical picture of the relaxation process i n the alkyl bromides.
BLOCK-POLYMERS OF STYRENE h K D ISOPRENE WITH VAKIABLE DISTRIBUTIOS OF MOSOJIERS ALOSG THE POLYMERIC CHAIS. SYNTHESIS AND PROPERTIES1 BY S. SCHLICK AXD AI. LEVY Chemistry Departnzent, Israel Institute of Technology, Haifa, Israel Recewed J a n u a r y 14, 1960
A number of block-polymers of styrene and isoprene mere prepared by applying the technique of “living” polymers: 1~31. s90M.13400 ; 134oLo. s 9 0 0 0 0 . ~ 3 4 ~ 0;0 I1700. s4sOa. Isroo. ~ 4 s n OI. l i W ; I17000. Siaoa.I340GO. S4mOa. Il76OO ; 11700. sZZ50. I1700. &50C .Il?OO. s2250. I l 7 W ; IliGoo .s22m.I I ~ o ~ ~ . S ~ M ~ ~ ~ . I~ ~I1 3~0 .~~ 2O~ C~ ..~ 1~1 3~0 .~~ 2.2 ~I0 .I~ 2~2 6E0 .O~ ~z 2 ;~ . ~ 1 1 3 0 . ~ 2 2and 5 0 . ~ ~1 11 3~0~~ o o . ~ ~ ~ ~ o o . ~ ~ i ~ o o . ~ z z ~ o o . ~ ~ ~ ~ ~ o . ~ ~ z ~I 1and o.~s 11~~1.~zz~oo denoting the blocks of isoprene and styrene and the subscripts their respective molecular weights. All these polymers have the same composition and form two series of total mol. wt. 15,800 and 158,000, respectively. In addition random copolymers of the same composition and the same molecular weights were prepared. These polymers were characterized and their properties vere investigated. The latter show systematic changes with changes in the block’s size. The dependence of these properties on the block’s size is discussed.
Many methods for preparing block-polymers have been described in the literature, and a comprehensive rexiew of this subject was published in 1956 by Immergut and Mark.2 Most of the known techniques yield a mixture of block-polymers and homo-polymers, and the two components have to be separated before a sample of pure blockpolymer is obtained. l’urthermore, even a purified product is very heterogeneous, being composed of molecules ot variable cmnposition and variable molecular weight. A new method of preparation of block-polymers has been described recently by Szmarc, Levy and M~lkovich.~It as demonstrated that this method yields a pure block-polymer, not contaminated by homo-polymer,” and, moreover, it permits the experimenter to vary at will the distribution of the monomers along the chain of the p ~ l y n i e r . ~A further advantage of the method arises from the fact that under proper experimental conditions a liarrow molecular weight distribution product is formed,j hence block-polymers of uniform architecture can he hynthesized. By applyiiig this technique we succeeded in preparing sample5 of block-polymers of styrene and isoprene ivhich have the same composition and molecular neight and differ only in the distribution of the two monomers along the chain of the iihnlitted by 8. Srhlich in Iiartial fulbllment of the reqiiiremcnt< foi thr A i R degret at the I w n r l Institute of Twhnology ( 3 ) C H Itnmrir it ? n ( l €I \lark, Unkron~n7~kirTnr (’hem 18/19, 322 (1956) 13) AI Szuarc \1 L P I I ind R \Iilkoiich I Ani Chem Sor 78, 2656 11956) I 4) (a) 11 S z a arr \ a f u r 178 1168 (1956), (b) Adbances t n Chem. f ’ h l / a i c s 8 , 147 (1959), Il.lakromolekular Chem., in press (1959). 5 ) R n’nncl, \ Remhdiirii J D (‘oomhes and >I & .n a i f J I m Chem. Soc 79, 2026 11957) ~ ( i )H W MiCorniii ,1 1 P o l u m r i Sca 36,311 f1959)
polymer. This is, to the best of our knowledge, the first time when such material became available and its study permits us to investigate the effect of distribution upon properties of the respective block-polymers. I n this publication we report the details of preparation of such samples, their characterization and :some of their properties which vary with the degree of distribution of the monomers along the chain. Preparation of Block-polymers.-Anionic polymerization carried out in non-protonating solvents and in absence of impurities acting as “killing” agent yields “living” polymers, i.e., polymeric molecules possessing active ends capable of further growth. If the active end of a monomer A initiates polymerization of a monomer B, and vice versa, if the active end of €3 initiates polymerization of A, then block-polymers of the type h
A B . .B A , . A . B
.
B
.
can be prepared readily. The procedure is simplified further if both ends of the polymeric molecule are active, and it was shown that this is the case if the polymerization is initiated by electron-transfer mechanism, e.g., when sodium naphthalene is used as an initiator. Poly-styryl anion initiates polymerization of isoprene and polyisoprene anion initiates polymerization of styrene, hence this pair of monomers suits our purpose. Polymerization was initiated by sodium naphthalene, and the reaction was carried nut in a stirred reactor in uacuo a t 0” by adding wccessively the required amounts of monomers to the initiator. The molecular weight of the obtained polymer is given by ~ M T / I where , MT denotes the total number of moles of added monomers and I the amount of initiator in moles. The molecular wight of each block is giveii by 31, ’ I X ,
884
S.SCHLICK AND M. LEVY
Yol. 64
the molar ratio of S/I being 0.865, but the molecular weight of the 2B series is ten times greater than that of 1B series (molecular weights of 1B = 15,800 and of 2B = 158,000). In addition two samples of “copolymers” were prepared. These samples result from a very slow addition of a mixture of styrene and isoprene (S/I = 0.865) to the initiator, the rate of addition being such that each drop of mixture essentially polymerizes completely before the next is added. The “copolymer” is therefore a material composed of very small and irregular blocks of styrene and isoprene. These samples are denoted by 1C and 2C, the former having molecular weight of the 1B series, the latter that of the 2B series. Each polymerization was terminated by addition of a drop of water to the solution, then the polymer was precipitated in methanol and dried % METHANOL. in vacuo at 60”. Polymers 2B3 and 1C were prel’ig, 1. pared in duplicate. The reproducibility of those samples was satisfactory. Each experiment involved 114 cc. of tetrahydrofuran, and 5 cc. each of styrene and isoprene. The monomers were carefully purified, dried with CaHz and distilled in high vacuum. The yield was quantitative. y Characterization of the Block-po1ymers.-The mol. wt. diktribution and homogeneity of each qample was checked by the turbidimetric technique described by Melville, et ~ 1 The . ~ reliability of the turbidimetric method was checked by precipitating a mixture of polystyrene and polyisoprene (Fig. 1) and by precipitating polystyrene prepared I-ig. 2 . by benzoyl peroxide as initiator (Fig. 2). Turbidimetric titrations were carried out at 30”. The initial concentration of polymer was 5 mg. in 100 cc. toluene. Toluene was used as solvent and methanol as precipitant. The turbidimetric curves are shown in rigs. 3 and 4 for the lorn and high mol. wts., respectively. The curves demonstrate clearly the homogeneity and sharp mol. mt. distribution of samples obtained by this method. It is interesting to note the sharp rise in turbidity in the higher mol. wt. samples up to approximately 90% precipitation and then the gradual bending of the curve. This corresponds to the shape of distribution predicted by Sz1Tai-c and Litt8 from theoretical considerations. Fig. 3 . Attention is drawn to the fact that curves 2B8 and 2Bi are steeper than curves 2ES and 2B9. denoting the numher of moles of the monomer added in the j-th portion, the middle block being The former samples were initiated by styrene an exception since its weight is doubled. The and the latter by isoprene. Apparently the initiation of isoprene is slower than that of styrene samples prepared were and therefore a wider distribution is obtained. IP~CO.~~~~C.I~~OO This difference in behavior of styrene and isoprene 1Bs = Ii~ro.S4soo.Iatoo.S4aoo.Iiiao was confirmed by independent observations, 1B7 = I1~w.Szz6o.I1~co.S~5oo.Ii~o~.S~zsc.Ii~w using a method described previously.$ 1Bs I I ~ ~ ~ . Q ~ Z ~ . I ~ ~ ~ ~ . S Z Z ~ . I ~ ~ ~ . S Z ZT’iscosity ~ ~ . I ~ ~ ~measurements O . S Z Z ~ ~ I ~were ~ ~ carried out in a 2B3 = I~4ooa.S~oooo.~~~ooo modified Ubbelohde viscometer at 30 i 0.03” 2Bs = Ii~~oo.S~~~~.I~~oco.S~~oo.Ii~ooa using toluene, toluene-methanol, and toluene2B7 = ~ 1 7 L ) 0 0 ~ ~ Z 2 ~ ~ ~ ~ 1 7 0 0 0 ~ ~ 4 M 0 0 ~ ~ 1 i C 0 0 ~ ~ 2 Z ~ C ~ ~ l l G O O isooctane mixtures as solvent?. The solvent flow 2B9 111300 Szz6w.Ii1m. S~~5o(1.Izz~o.Szzjoo.~113w.~zz~.~i~~00 time n as a l n y s over 100 sec. I and S denote the blocks of isoprene and styrene, .1 S. Dunn. B. P. Stead and €I W. 31el\Ille, Trans F a r a d a y respectively, the subscripts being the molecular S a(7) c , SO, 279 (1954). weights of the respective blocks. One notices that (8) 31. Salrarc and AT. Litt, Tais J o n R v a L , 62, 508 (1958). (9) 11.Levy and hl. Szaarc, J A m . Chem. SOC.,82, 521 (ISSO). the compositions of all these polymers are identical,
BLOCKPOLYMERS OF STYRESE ASD ISOPRESE
July, lOG0
Properties of Block-polymers.-Two
SS,’j
properties
of the block-polymers were studied in the courpe of this investigation : their precipitation by addition
of non-solvents and their viscosities in toluene, t oluene-me t hanol and toluene-isooc tane mixtures. It was shown previously3 that a block-polymer of styrene and isoprene is not precipitated from toluene solution by addition of isooctane. This phenomenon now was studied quantitatively using a turbidimetric titration. Khen methanol is used as a precipitant and toluene as a solvent the onset of precipitation seems to be independent of the block’s size. The critical concentration of methanol was found to be 3570 2 for the 1B series and IC (mol. wt. 15,800) and 28y0 f 0.5 for the 2B series and 2C (mol. n-t. 158,000). On the other hand, the size of the block? affected maxkedly the critical concentration of isooctane when this hydrocarbon was used as nonsolvent in the turbidimetric titration. The results are given in Table I. As the size of the blocks decreased, their solubility increased so much that it mas necessary to work with more aiid more concentrated solutions since no precipitation did take place in the dilute solutions. The effect of the block’s size on the solubility of the respective polymer casts doubts on Kilb and Bueche’O calculations of the free energy of mixing of blockpolymers. The viscosity measurements in toluene solution are summarized in Table 11. The results shorn clearly that the intrinsic viscosity increases with decreasing block’s size. This behavior might be accounted for by the incompatibility of polystyrene and polyisoprene-the respective blocks avoid each other and as a result the polymer molecule expands as the number of blocks increases and their size decreases. The same trend is seen in mixed solvents: toluene and methanol and toluene and isooctane (see Tables 11and 111).
Fig. 4. TABLEI11 G./100 CC. O F BLOCK-POLY\fERS I N TOLUENE-ISOOCTdZE hfIXTUREb
I A T R I S S I C VISCOSITIES I S
Type of blockpolymer
1B7 1Bs 1c
2B3 2B3 2B; 2By 2C
0
20
-% of Isooctane 40
60
GO
70
... ... ... 0.220 0.200 0.185 ,232 ,215 .194 0.188 0.170 ... ,220 ,210 190 .. . ,250 .233 ,220 .. . .a55 ,233 ,200 ... .lo0 . . . ,185 . . . ,165 0.150 ,810
.878
.so0 .980 1 Ui5
,690 ,810 ,840 ,908
.
,
,515 ,670 ,710 ,805 ,980
...
...
...
,320 ,583
...
.., .,.
... ... ...
,580 ,725 0.530
,..
Rossi, et al.,” described a method bf determining [77]0 at constant temperature by varying the com-
position of the solvent. Plotting log [7] versiis log (M) at each composition one determines the exponent a of the [VI = Killa relation as a function of the solvent ccmposition, and by ertrapolation (or interpolation) the composition for which a = 0.5 could be found. Solvent of this particultu. composition is assumed to be the 0 solvent. TABLE I It remains to be shown that for a series of blockConcn. of Type of blocks R isooctane polymer, my./lOO c c . polymers characterized by the same distribution 2% 55 2 5 of monomers along the chain but of varying 2B5 57 10 molecular weights, the log of intrinsic viscosity is 2B7 62 15 linear with log of mol. wt. However, if such a reBY 68 25 lation is assumed then our data permit to find a KO pptn. even at the highest concn. of for each type of block-polymers and for each com2c isooctane position of the mixed solvent. Figure 5 illustrates this method. Here a plot of a as a function of % TABLE I1 of methanol in the solvent is given for the copolyIKTRINSIC VISCOSITIES I N ~ . / 1 0 0cc. OF BLOCK-POLYMERS IN mer, and determination of the composition of the TOLUESE-METHANOL MIXTURES 0 solvent from this graph is self-evident. In this T y p e of % methanol may the data listed in Table IV have been obtained. 20 24 0 10 block-polymer These show a definite trend in the composition 0.195 ... of thedata 0.220 0 210 IBs “0 solvent” with the decreasing block’s . I 80 ,232 .212 1Bs size, and they are therefore interesting whatever is .250 .220 ,200 ... their meaning. Since the polymer seems to expand 1Bs ,255 ,224 ,190 ... IC .I90 ,185 . I55 0.148 as the block’s size decreases it is natural to expect that the proportion of the precipitant in the “fl 2% .810 .695 .530 ... solvent” should increase accordingly. 2Br 878 ,735 ,545 ... Table IT’ lists also the intrinsic viscosities of the 2B7 .890 .780 .545 ... respective polymers in their solvents.” AlBY .980 ,864 ,650 . . though considerable experimental uncertainties 2c 1 075 .970 680 0.550 are involved in the interpolation (or extrapolation) c
(10) R. W. Kilb and .4. AI. Bueche. J . Polymer Sa.. 28, 285 (1958).
(11) C . Rossi,
U. Rinnrhi a n d T . AIagnavo. zbid.,
30, 175 (19.58).
ssti
JEFFC. D A ~ I JR.. H . ASD KEXNETH S.PITZEI~
7-01, 64
case, although the similarities of the respective [TIS add weight to the assumption that one deals here indeed with B solvents, the difficulties of application of a 0 solvent concept to solutionr of blockpolymers shoidd he clearly recognized. TARLE IV 5%
5
---
Type of Methanol [vlgIsodctane blockin " R 3101. u-t. 3101. wt. in "e w l y m e r solvent" 158,000 15,800 solvent" S p
13; Hi
I
-
133
0
10
20
27 30
(*
12.5 17.0 17.0 24.0 27.0
O.fi5 .OO .63 .
