BUTYLLITHIUM-INITIATED POIYMERIZ ATIOX
OF
1983
METHYLMETHACRYLATE
The Butyllithiuim-Initiated Polymerization of Methyl Methacrylate. 111.
Effect of Lithium Alkcoxidesl
by D. M. Wiles and S. Bywater Division of A p p l i s d Chemistry, National Research Council, Ottawa, Canada (Received February 7 , 1964)
Lithium mrthoxidr is produced in the butyllithium-initiated polymerization of methyl methacrylate in toluene solution. The concentration of methoxide increases rapidly in the early stages of the polymerizatio'n, then more slowly, and the final value, which depends on the [monomer]/ [initiator] ratio, represents a considerable proportion of the original initiator eoncentration. Methoxide is also formed when fluorenyllithiurn is used as the initiator. From a series of butyllithium-initiated polymerizations to which lithium methoxide, ethoxide, or n-propoxide was added, it mas learned that although these lithium alkoxides do not by themselves initiate polymerization under the present experimental conditions, they do increase the rate of polymer formation. The molecular weight distribution of poly(niethy1 methacrylate) made in the presence and in the absence of added methoxide is in both cases bimodal, but the molecular weight is higher for the former polpnier. The addition of methoxide to the polyinerization increases the over-all isotacticity of the product slightly. It is postulated that some of the growing ion pairs in a normal organolithium-initiated polymerization are associated or complexed with lithium methoxide produced during polymerization. The coniplexed ion-pair form appears to add monomer more rapidly than the uncomplexed form.
Introduction The low temperature polymerization of methyl methacrylate initiated by butyllithiurn has been found to be more complicated than is the case for a nonpolar monomer. A study of the polymerization kinetics* showed a complex, variable dependence of rate on monomer and initiator concentrations with no simple relationship between molecular weight and the ratio [monomer]/ [initiator]. The distribution of molecular weights has been foundIc to be bimodal for several such ratios. I n addition, the fractions of a polymer sample, . having different molecular weights, have different tacticitle.3. One way of explaining these results invol17es the suggestion of more than one kincl of polymerizing species. l o The presence of two reactive groups in methyl methacrylate is likely to be the cause of some of the complications reported for the anionic polymerization of this monomer. A reaction between the initiator and the est'er group of the monomer could occur lead-
ing to the formation of lithium n i e t h ~ x i d e . ~Presumably the ion pair resulting from the addition of initiator across the vinyl double bond leads to the expected polymer formation. There are, however, two ways by which methoxide ions may be released during propagation as well, i.e., as a result of cyclization of active chain ends4 or from the interaction of polymer chains with the ester function of an inconiiiig monomer mo1ecule.j A preliminary account of the detection of lithium methoxide in organolithium-initiated polymerizations (1) (a) Issued as N.R.C. No. 8025; (b) presented before the Division of Polymer Chemistry a t the 145th National Meeting of the American Chemical Society, New k'ork, N. Y., September, 1963; (c) for part 11, see B. J. Cottam. D. M. Wiles, and S. Bywater, Can. J . Chem., 41, 1905 (1963). (2) D. M . Wiles and S. Bywater, Polymer, 3, 175 (1962) (3) S. Bywater, Pure A p p l . Chem., 4, 319 (1962). (4) W. E.Goode, F. H.Owens, and W. L. Myers, J . Polymer S c i . 47, 75 (1960).
(5) P. Rempp, V. I. Volkov, J. Parrod, and C. Sadron, Bull. soc. chim. France, 919 (1960).
Volume 68, S u m h e r 7
J u l y , 1964
1984
has been published recentlyS6 It is the purpose of this paper to report in more detail the formation and some effects of lithium niethoxide, and the effects of other added alkoxides, in the butyllithiuni-initiated polynierization of methyl methacrylate in toluene solution.
D. nI.
WI~,ES AKD
S.BYWATER
7
Experimental All polymerizations were carried out in a glass vacuum apparatus using the techniques described previously,2a t - 30" unless otherwise specified. I n those experiments designed to detect and measure the quantity of lithium methoxide, polymerizations were stopped a t various stages by adding small quantities of acetic acid. It was assumed that niethoxide ions would be converted quantitatively to methanol. Samples of the reaction mixtures were analyzed gas chromatographically and measurements of the methanol concentrations were made. I n two experiments fluorenyllithium, instead of butyllithium, was used as the initiator, still with toluene as solvent. I n another set of reactions, sinal1 quantities of the appropriate alcohols mere added, prior to monomer addition, to solutions of butyllithium in toluene. I n this way polymerizations would occur in the presence of butyllithium and comparable amounts of either methoxide, ethoxide, or n-propoxide ion pairs.' The amount of product and the molecular weight and tacticity of polymer produced in these experiments were measured and compared with the corresponding values obtained in the kinetic experinientsJ2ie., in the absence of added alkoxide, for the same polymerization time. A polymei~ization involving larger quantities of reagents was carried out in the presence of a quantity of lithium methoxide equal to that of butyllithium initiator (7.6 niM). The product was fractionated and the fractions were characterized by the methods outlined in part 11.1~
Results and Discussion The concentrations of lithium niethoxide found in polymerization systems a t - 30' a t various stages of reaction, for different initial concentrations of monomer and butyllithiuin initiator, are shown in Fig. 1. It was found that for polymerizations at 10' the concentration of 'methoxide had approxiniately the same value early in the reaction as a t -30°, but the former increased more rapidly and to a higher value with increasing polymerization time. The trend a t both temperatures seems to be a rapid production of niethoxide early in the reaction followed by a gradual increase as the polyiiiei ization proceeds. The higher terminal values for niethoxide concentration at the The Journal of Physical Chemistry
I I I 100 200 300 PO LY M E R I2 AT IO N T I M E ( m in .I
4
Figure 1. Dependence of methoxide ion concentration on polymerization time; temperature = -30". [BuLi] = 7.6 mM; f, [ X M A ] = 0.25 M ; 0, [MMA] = 0.125 ill. [BuLi] = 3.8 mM; ill, [MMA] = 0.25 144; A, [MMA] = 0.125 M.
higher temperature niay be connected with a greater prevalence for chain terniination above Oo. In all cases, including the fluorenyllithium-initiated polynierizations, the final methoxide concentration represents a surprisingly high proportion of the initiator added originally. The reaction of butyllithiuni with monomer ester groups3 could account for the initial lithium methoxide concentrations shown in Fig. 1. The subsequent gradual increase in concentration could be a result of cyclization4 and/or termination5 of some of the growing polymer chains. The same considerations seein to apply when initiation is by fluorenyllithiuin. It is reasonable to assume that the niethoxide formed during polynierization associates with growing polymer ion pairs. Thus, in addition to the unconiplexed ionpair chain ends, there niay be active polymer chains present in the form of polymer ion pairs coniplexed with one or more lithium methoxide ion pairs. Although there is no experimental evidence as to the nature of the coniplexed form, it niight be expected to add nionoiner a t a different rate and with a different stereospecificity than does the uncoinplexed ion-pair form. The effects of the added lithium alkoxides on the rate and stereospecificity of butyllithiuni-initiated polyni(6) D. M .Wiles and S. Bywater, Chem. Ind. (London), 1209 (1963) (7) Since lithium methoxide is only slightly soluble in toluene a t - 3 0 ° , even in the presence of monomer, it was thought that the effect of adding the more soluble lithium ethoxide or propoxide, although similar, would be more easily detected.
BUTYLLITHIUM-INITIATED POLYMERIZATIOX OF METHYL METHACRYLATE
1985
Table I : The Effect of Added Alkoxide Ione
[ROLi], moles/l.
x
RO -
MeOMeOMe0 MeOMeOEtOEt0Et0 Pro-
103
10 20 10 10 20 7.6 7.6 7.6 3.8 3.8 7.6
ProPro-
[BuLi], moles/l.
x
102
3.8 3.8 3.8 7.6 7.6 3.8 7.6 7.6 3.8 3.8 7.6
[ M M A 1,
Conversion,
mole/l.
7%
0.25 0.25 0.125 0.25 0.125 0.25 0.25 0.125 0.25 0.25 0.125
62 38 97 48 80 71 74 88 31 85 80
x
n.?
Isotactic triads,
10-5
70
.. .. 4.6
.. 1.5 3.2 2.6 1.6 1.5 4.0 0.75
, .
, .
..
.. 85 .. , .
92 78 84 91
Kinetic expt. of same polymerization time; ------no added alkoxide-----ConIsotactic version, 8, triads, 70 x 10-5 %
60 32 60 38 52 60 38 52 18 60 52
3.0 1.2 3.0
65 ..
65
..
..
0.98 3.0
74 65
..
..
0.98 0.54 3.0 0.98
74 58 65 74
-.
erizations of methyl methacrylate are indicated in Table I Comparison with typical kinetic experinients2 of the same polymerization times performed in the absence of added alkoxides shoffs that, generally speaking, the systeiiis to which alkoxide has been added polymerize faster to form more highly isotactic products with highLer molecular weights. Because adding methoxide increases the rate of polymerization it may not, in fact, be enhancing the stereoregulation at all or be changing the number of active species. This follows since, in all polynierizations without added methoxide, as the conversion of monomer to product increases the isotacticity inLcreases1c*2 and so, of course, does the molecular weight. I n the case of added lithium ethoxide or lithium propoxide, however, the isotacticities of high conversion polymers are at higher values than would be reached a t 10070 conversion in the absence of added lithium salts. I n other words, all the added lithium alkoxides have increased the polymerization rate but only the ethoxide or propoxide salts appear to have increased the stereoregularity of the polymer appreciably. The different effeci of added methoxide, at the lower butyllithium concentration, for the l,wo monomer concentrations presumably reflects the different proportions of initiator whiich have led to methoxide formation during a normal polymerization (cf. Fig. l ) , i.e., a t [ I I M A ] = 0.25 M the concentration of lithium methoxide may be sufficiently high so that adding an additional quantity has little effect. The different effects of the different added alkoxides may be due in part to increased solubility in the series AleO-, EtO-, PrO-; the possibility of inducing some heterogeneous reaction by adding lithium methoxide, for example, cannot be disregarded. Finally, it must be emphasized
that the experimental difficulties in the measurements are such that the data in Table I can be considered as semiquantitative only. Figure 2 shows a comparison between polymerization rates in the presence and in the absence of added lithium methoxide. Figure 3 illustrates the comparison between rates of polymerization with and without added lithium propoxide. The two-stage monomer consumption rate, which normally is characteristic of these monomer-initiator ratios, is absent with added propoxide and perhaps less well defined with added methoxide. Since the first stage, the period of slower growth, yields polymer of lower isotacticity than does the second stage,2 removal of the first stage by the addition of lithium propoxide might be expected to increase the isotacticity of the polymer. Adding lithium methoxide which does not completely remove the period of lower rate would be expected to have a smaller effect on the isotacticity of the polymer. It can be seen in Table I that this is indeed the case. Figures 4 and 5 shorn a comparison between molecular weight distributions, a t complete conversion, of tm7o polymers prepared under identical conditions except for the lithium methoxide added before the preparation of one of them. Addition of these ions has resulted in an appreciable increase in the viscosity-average molecular weight of the whole polymer (from 1.6 to 2 . 5 X lo6). The bimodal molecular weight distribution is broader for the polymer made in the presence of added lithium methoxide (curve 11; aq./iqn = 67) than it is for the other polymer (curve I ; i1Tn/an = 32). The broadening of the distribution reflects the fact that in the presence of added methoxide the high polymer grows to a higher molecular weight, whereas the amount of product soluble in Volume 6 8 , Xumber 7
J u l y , 1964
1986
D. M. WILESAND S.BYWATER
0.9
+.-
I C - -
r
-
0-1
'
0 8 -"
0.7
wi
""p
0.2
I
0.1 8-
J I I I I I ( I I I I
0
40
80
I20
160
200
0
I
l
l
1
2
3
5'
6I
7
8
" J *9
'
iiiVx IO-'
POLYMERIZATION TIME (min.)
Figure 4. Comparison of integral weight distributions of polymer prepared in the presence and absence of added lithium methoxide: [BuLi] = 7.6 mM; [MMA] = 0.125 M ; I, no added methoxide; 11, added [MeO-] = 7.6 mM.
Figure 2 . Effect of added lithium methoxide on polymerization rate: [BuLi] = 3.8 mM; [MMA] = 0.125 ill; temperature -30".
80
4"
-
t
z
I
I
0
40
80 120 160 POLYMERIZATION TIME (min.)
200
Figure 3. Effect of added lithium propoxide on polymerization rate: [BuLi] = 3.8 mM; [MMA] = 0.25 M ; temperature -30".
pet>roleum ether (#,, = 8.50 in both cases) remains almost unchanged. This latter product accounts for 2.45 aiid 2.8 m M initiator in the abseiice and in the presence, respectively, of added methoxide indicating a small iiicrease in the number of short (aiid presumably largely inactive) polymer chains. In coiitrast, the amount of material of higher molecular Th'e Journal of Physical Chemistry
0
I
I
I
2
4
6
i;ii, x IO+
Figure 5. Comparison of differential weight distributions of polymer prepared in the presence and absence of added lithium methrhxide: [BuLi] = 7.6 mall; [MMA] = 0.125 M ; I, no addefi methoxide; 11, added [MeO-] = 7.6 mM.
weight has decreased from 0.22 to 0.1.55 mM on the addition of lithium methoxide, and this checks roughly with the increased molecular weight of this petroleum
BUTYLLITHIUM-INITIATED POLYMERIZATIOK OF METHYLMETHACRYLATE
ether-insoluble polymer. The increased rate must therefore be due to an increased reactivity of these chains, and not to an ;Increased number of active chains. In both cases, i e . , with and without added lithium methoxide, the initiator balance (calculated from the number of polymer molecules found) is poor as is often observed in these systems. A much better balance can be obtained by making the assumption that the apparent deficiency is caused by attack of butyllithium on the ester group of the monomer, and that this can be measured by the initial rapid formation of methoxide. In this particular case, an apparent loss of about 4.2 to 4.4 mM initiator would result. The detailed situation may, however, be complicated by some multiple attack of butyllithium on monomer, or by the loss of very low molecular weight chains in the recovery of these products by evaporation of the petroleum ether used as high polymer precipitant. As only a minute amount of the added initiator appears as reactive chains (reactive enough to grow into high molecular weight products), whose reactivity depends on the presence of lithium methoxide formed in side reactions, no simple relationship between initiator concentration and either polymerization rate lor polymer molecular weight can
1987
be expected and in fact was not observed in earlier studies. The effect of lithium hydroxide on the stereospecificity of the butyllithium-initiated polymerization of styrene iii toluene solvent has been shown8 to be considerable. By analogy one might expect that lithium methoxide affects the stereospecificity of methyl methacrylate polymerization and that adding more methoxide would enhance the effect. Nevertheless, the isotacticit,y of sample I1 (86y0 isotactic triads, with lithium methoxide added) is only slightly higher than that of sample I (82% isotactic triads). I n the absence of a larger difference it is not certain, even from measuring tacticities of fractions of these polymers, whether a polymer ion pair complexed with lithium methoxide adds moiiomer with a higher or lower probability for isotactic placement than does the uncomplexed ion-pair form.
Acknowledgnzents. The authors are grateful to Dr. S. Browiistein for the proton magnetic resonance measurements, to Dr. B. J. Cottam for some viscosity measurements, and to Rir. P. E. Black for the gas chromatographic analyses. ~
~~~~~~
(8) D. J. Worsfold and S. Bywater, Makromol. Chem., 65, 245 (1963).
Volume 68, Number 7
J u l y , 1864
