Copolymerization of ethylene with a homologous series of
vinyl esters
Citation for published version (APA):
Meer, van der, R., Gorp, van, E. H. M., & German, A. L. (1977). Copolymerization of ethylene with a homologous series of vinyl esters. Journal of Polymer Science, Polymer Chemistry Edition, 15(6), 1489-1498.
https://doi.org/10.1002/pol.1977.170150620
DOI:
10.1002/pol.1977.170150620 Document status and date: Published: 01/01/1977
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JOURNAL OF POLYMER SCIENCE: Polymer Chemistry Edition VOL. 15, 1489-1498 (1977)
Copolymerization
of Ethylene with a Homologous
Series of Vinyl Esters
R. VAN DER MEER, E. H. M. VAN GORP, and A. L. GERMAN,
Laboratory of Polymer Technology, Eindhoven University of Technology, Eindhoven, The Netherlands
Synopsis
The effect of the alkyl group on the relative reactivity of a homologous series of vinyl esters (2) has been studied with ethylene (1) as reference monomer, tert-butyl alcohol as solvent, a t 62°C and 35 kg/cm2. T h e experimental method was based on frequent measurement of the monomer feed composition throughout the copolymerization reaction by means of quantitative gas-chromatographic analysis. Highly accurate monomer reactivity ratios were estimated in a statistically justified manner by a nonlinear least-squares method applied t o the integrated copolymer equation. T h e reactivity of the vinyl ester monomers towards a n ethylene radical increased with decreasing electron-with- drawing ability of the ester group. All vinyl ester radicals considered turned out t o have the same preference for their own monomer over ethylene (constant r2 = 1.50). Reactivity ratios are discussed in terms of the Q-e scheme and the T a f t relation. I t appeared that chiefly polar factors contribute to the observed relative reactivity, while probably resonance stabilization only plays a minor part. Steric hindrance seems t o impair monomer reactivity, only from vinyl pivalate on. Relative reac- tivities of the vinyl esters are compared with literature values, where other reference monomers have been used.
INTRODUCTION
The correlation between reactivity and monomer structure can be studied conveniently by copolymerizing a homologous series of monomers towards a reference monomer.1,2 In this manner, it has been shown that the relative reactivities of the series of alkyl met ha cry late^,^ alkyl acrylates: methyl a-alkyl a c r y l a t e ~ , ~ and alkyl vinyl ketones6 towards the polystyryl radical can be de- scribed by the Taft relation. For all these series of homologs, relative reactivities appeared to be influenced exclusively by polar factors except for the methyl d-alkyl acrylates where steric factors affected reactivity.
Several investigators studied the copolymerization kinetics of the esters de- rived from vinyl alcohol towards various reference monomers: ~ h l o r o p r e n e , ~ N-vinylcarbazole,8 methyl m e t h a ~ r y l a t e , ~ . ~ styrene,2 vinyl acetate?JO and vinyl chloride. l 1 The reported relative reactivities towards these reference radicals, however, seem to be contradictory. Chloroprene and vinyl chloride radicals revealed increasing relative reactivity for decreasing electron-withdrawing effect of the ester side group, whereas towards N-vinylcarbazole, methyl methacrylate, and styryl radicals the homologous series of the vinyl esters showed a reverse
1489
relative reactivity. Interpretation in terms of the Taft relation showed that relative reactivities were affected primarily by polar factors.2J1
Up to the present, the most obvious reference monomer, i.e., ethylene, was never used, probably because of practical difficulties. However, since German and Heikens12J3 reported a method based on quantitative gas-chromatographic analysis permitting pressures up to 40 kg/cm2, a more accurate determination of monomer reactivity ratios in copolymerizations involving gaseous monomers became possible.
Reactivity ratios are estimated by using the integrated form of the co- polymerization equation14 and a nonlinear least-squares estimation method. Description of the substituent effect in the homologous series of the vinyl esters is achieved by the Taft relation and the Q-e scheme.
SCHEMES FOR DESCRIPTION OF MONOMER REACTIVITY RATIOS
Several semi-empiric schemes, relating structure parameters of monomers and radicals to monomer reactivity ratios, have been developed. In the Q-e,15 Q-e-e* ,I6 ele~tronegativityl~ and charge-transfer scheme,l* both reactivity ratios
can be described by and derived from the model parameters.
Other schemes, originating from organic chemistry and also used to describe monomer reactivity in copolymerization, contain one or more factors reflecting the effect of substituents in terms of polar, resonance, and steric contribution^.'^
These schemes may be used to decide which factor affects the relative reactivity
of a homologous series of monomers towards a polymer radical. Here the “mo- nomer parameters” only reflect the contribution of the’changing part of the side group, i.e., the alkyl group of the ester side group of vinyl ester or acrylate ho- mologs. The radical influence, which also depends on the reaction conditions, is reflected in the reaction constants. Typical examples of these schemes are the Hammet equation,20 the Yamamoto-Otsu equation,21 and the Taft rela- tion.22
TAFT EQUATION
This equation has been postulated by Taft22 in order to describe the reactivity of the aliphatic ester hydrolysis reactions with varying alkyl groups in terms of polar and steric factors. The effect of the alkyl group on the reactivity has been compared relative to the CH3 group, resulting in two sets of substituent pa-
rameters characteristic of steric and polar effects, successively.
In the same form, this eq. (1) has been applied many times to those co- polymerizations where resonance effects are of minor importance, in aid of the description of the relative reactivity of a homologous series of monomers.
where log
( l h d
= log ( k 1 2 h l l ) represents the reactivity of monomer M2 relative to monomer MI, with regard to a radical chain end with ultimate unit MI; c*,E,= Taft’s polar and steric substituent constant, successively; and p*,6 = the re- spective reaction constants.
COPOLYMERIZATION OF ETHYLENE WITH VINYL ESTERS 1491
EXPERIMENTAL Reagents
Ethylene
Polymerization-grade ethylene (Eth) (DSM) was used without any further purification.
Vinyl Esters
From vinyl formate (VF), vinyl acetate (VAc), vinyl propionate (VP), vinyl butyrate (VB), vinyl isobutyrate (VIB), and vinyl pivalate (VPV) the middle fraction of the distillate was collected and used. The specifications and some physical properties observed are summarized in Table I.
cr,a’-Azobisisobutyronitrile
The initiator AIBN (Fluka) was used without further purification.
tert-Butyl Alcohol
The solvent TBA (Shell) was used after bulk recrystallization and degassing
= 1.3842).
Copolymerization
All Eth (1)-vinyl ester (2) radical copolymerization experiments were per- formed a t 62 f 0.1OC and 35 kg/cm2 with TBA as solvent and AIBN as initiator. The total monomer concentration a t the start of each experiment varied from
TABLE I
Physical Properties of the Vinyl Esters, CH, =CHCOOR
Vinyl R in vinyl Boiling Density a t
ester ester Product source point, “C n D a o 20”C, g/cm3
VF H Koch Light 45.5- 1.3838 0.9612 VAc CH, Shell 72.0- 1.3956 0.9309 VP
c*
H5 Monomer 94.8- 1.4040 0.9105 Lab. Ltd. 47.5 73.0 Polym. 95.0 Lab. Polym. 117.0 Lab. Chemie 104.6 GmbH. VB c3H, Monomer 116.8- 1.4113 0.9014VIB iso-C, H, Wacker 104.2- 1.4042 0.8927
VPV tert-C, H, EGA Chemie 111.8- 1.4055 0.8704
about 1-3 mole/dm3. For each binary combination, the monomer feed ratio was varied between 0.3 and 5. A variable induction period was observed depending on the quantity of initiator (2-6 mmole/dm3) and the efficiency of degassing the reaction mixture. For all combinations involved in the present investigation, the experimental conditions are summarized in Table 11.
The monomer feed composition was determined during the entire course of
the copolymerization reaction by means of quantitative gas-chromatographic
TABLE I1
Experimental Conditions of the Copolymerizations of Ethylene (M, ) with Vinyl Esters (M, ) a
Initial Vinyl monomer ester feed ( M , ) ratio ( q o ) Final monomer feed ratio VB VIB V F 0.4279 0.6637 0.9704 1.5320 2.6768 4.5672 VP 0.2900 0.3165 0.5176 0.6103 0.8199 1.5157 2.4950 3.2547 5.7519 0.4168 0.6643 0.9833 1.6270 2.7748 5.5256 9.2301 0.3192 0.5130 0.5530 0.9061 1.6002 2.4666 4.9605 VPV 0.2570 0.4188 0.7605 1.0595 1.3600 2.8845 4.1635 0.4553 0.7087 1.0334 1.6206 2.9476 5.01 64 0.3577 0.4002 0.5687 0.6946 0.9252 1.6847 2.8518 3.5974 6.1138 0.4778 0.7231 1.0512 1.7268 2.9190 5.6523 9.6627 0.3436 0.5694 0.6016 0.9718 1.6822 2.5479 5.2892 0.3029 0.5045 0.9065 1.2095 1.4682 3.0870 4.5404 Conversion based on M , , % 20.22 20.13 18.44 15.68 23.78 22.19 46.96 50.77 24.78 32.43 30.67 27.50 33.47 26.33 17.03 33.97 22.89 18.65 17.01 14.83 7.03 13.79 19.47 26.06 21.57 17.99 12.91 8.43 15.64 39.05 42.69 40.54 32.27 20.10 17.80 22.04 Initiator concen- tration, mmole/ d m 3 3.7 3.8 3.4 3.4 3.1 3.7 4.7 4.8 5.5 5.3 5.1 4.8 4.8 4.8 5.5 4.2 4.2 4.2 4.2 4.2 4.2 4.2 6 .O 6.1 5.5 6.1 6.1 5.5 6.0 1.6 2.1 3.4 1 . 8 2.1 2.1 2.6 Total initial monomer concen- tration, mole/ d m 3 1.41 1.67 1.80 2.32 2.01 2.55 0.80 0.83 1.19 1 . 2 2 1.42 1.13 1.98 1.54 2.61 1.15 1.30 1.15 1.52 2.91 2.48 1.93 1.57 1.34 0.97 1.28 1.34 1.51 1.31 0.87 0.96 1.13 1.12 1.10 1.46 1.68 ~~
COPOLYMERIZATION OF ETHYLENE WITH VINYL ESTERS 1493
analysis. Samples of reaction mixture were injected directly by a specially constructed sampling valve, permitting pressures up to 40 kg/cm2. The relevant gas-chromatographic conditions were: column temperature, 80 f 0.1OC; sta- tionary phase, 15% by wt of a mixture of diglycerol and quadrol (varying from
5/95 to 30/70% by wt depending on the binary combination involved) on chro-
mosorb P ; 60-80 mesh (Johns Manville).
When the copolymerization reached a degree of conversion of about 25%, the pressure was released and the reaction mixture was collected on some inhibitor (hydroquinone). The precipitation of the copolymer was accomplished by pouring out in a water-methanol (9:l) mixture. Reprecipitation was carried out in acetone-water as solvent-nonsolvent combination. Purified copolymer was dried under vacuum a t a temperature of 40°C.
Estimation of Monomer Reactivity Ratios
By means of the integrated version of the copolymerization equation,14 it is possible to describe an exact relation between the changing molar feed ratio ( q ) , and the degree of conversion (f). For this reason, the integrated form is generally
referr red^^,^^ over the differential form of the copolymerization equation, and also in the present investigation the former has been used for the estimation of the r-values.
Both the monomer feed ratio and the degree of conversion result from the repeated quantitative gas-chromatographic measurements. An improved computational method has been developed, considering experimental errors in both dependent variables q and f , whereas our preceding estimation method considered errors in only one of these variables, e.g., the degree of conversion.
This estimation procedure yields statistically more reliable monomer reactivity ratios than did our p r e v i o u ~ ~ ~ , ~ ~ and other existing methods.262s Mathematics and computational procedure will be published separately.
RESULTS AND DISCUSSION
For all six ethylene-vinyl ester combinations considered in the present paper, the overall rates of copolymerization and the number-average degree of polymerization
(p,)
appeared to decrease with increasing ethylene mole fraction in the monomer feed. For an equal ethylene content in the feed, the rate of co- polymerization tended to increase by a factor of about 3 in the order: VIB<
VB1 V P
<
VAc<
VF VPV, whilep ,
varied from 200 to 1500, and increased inthe order: VIB
<
VB V P < VF VPV<
VAc. Probably, the observed dif- ferences may be explained mainly in terms of chain transfer both to monomer and polymer and subsequent slow reinitiation. Differences among the various propagation rates are believed to play a part of only secondary importance. Itis well known that branching of PVAc takes place by prefereqce on the ester side due to the easily abstractable hydrogen atoms on the carbon atom ad- jacent to the carbonyl C-atom. In the series of the vinyl esters considered here,
VF and VPV do not have these acid hydrogen atoms, and as a consequence the overall rate of copolymerization cannot be retarded by this type of chain-transfer reaction with subsequent slow reinitiation. However, all other vinyl esters considered here, VAc, VP, VB, and VIB, contain one or more of the above-
TABLE I11
Calculated Monomer Reactivity Ratios for Ethylene (M, )-Vinyl Ester (M, ) Copolymerizations and Q,, e2 Values for M,
Vinyl ester (M, ) 1 Vinyl formate 2 Vinyl acetatec 3 Vinyl propionate 4 Vinyl butyrate 5 Vinyl isobutyrate 6 Vinyl pivalate 7-1 0.586
*
0.009b 0.740 i 0.007 0.674*
0.007 0.696 f 0.010 0.609 t 0.008 0.645 t 0.006 r2 1.29 i. 0.02b 1.504*
0.008 1.50 t 0.01 1.505 t 0.015 1.49*
0.015 1.49 t 0.01 1 T = r l * r 2 Q z a 0.76 0.028 1.11 0.026d 1.01 0.022 1.05 0.022e 0.91 0.026 0.96 0.024 e2 a -0.73 -0.22d -0.20 -0.20e -0.51 -0.40 a Assuming for ethylene: Q, = 0.015 and e , = -0.20.”,b Estimated standard deviations.
C Recalculated by the improved estimation method, from previously published data.”
d Literature
esupposing r,.r2 = 1.00.
mentioned acid hydrogen atoms, which may lead to a relatively frequent oc- currence of chain-transfer reactions. For the latter monomers, the observed differences in both the overall rate of copolymerization and
pn
may be explained by a different rate of reinitiation: VIB < VP z VB<
VAc, in compliance with the well-known order of reactivity of the pertaining radicals: primary>
sec- ondary > tertiary.Examination of the calculated reactivity ratios of the ethylene (1)-vinyl ester
(2) copolymerizations, summarized in Table 111, reveals gradually decreasing rl-values, and surprisingly almost constant rz-values. However, the Eth-VF combination is deviating in both respects, as can be seen from the mutual posi- tions of the confidence regions (alpha = 95) in Figure 1 for all binary combina-
1.55 I
.5 .6 .7 .8
-
‘1Fig. 1. Confidence regions for alpha = 95 for the copolymerizations of ethylene (Mi) with: ( 1 )
vinyl formate, (2) vinyl acetate, (3) vinyl propionate, ( 4 ) vinyl butyrate, ( 5 ) vinyl isobutyrate, and (6) vinyl pivalate.
COPOLYMERIZATION OF ETHYLENE WITH VINYL ESTERS 1495
tions considered. During the Eth-VF copolymerization precipitation of the copolymer was observed, which may impair23 the kinetic results based on the simple Alfrey-Mayo As a consequknce, this binary combination will be omitted in the following, more detailed discussion.
Decreasing rl-values indicate increased monomer reactivity of the vinyl esters towards an ethylene radical, as the ester side group has a lower electron-with- drawing character. In contradistinction to the varying r l-values, the constancy of the r2-values surprisingly suggests that each vinyl ester radical exhibits an equal preference for adding to its corresponding monomer over ethylene. These findings appear to fit in with common polymerization behavior. A general rule in polymerization states that monomers of high reactivity yield polymer radical adducts of low reactivity, while the reverse is also true.33 Moreover, it has also been p r ~ v e d ~ ~ s ~ that the reactivity of the polymer radicals is the basic factor, determining the rate of radical propagations. Therefore, vinyl ester homopro- pagation reactions (1222) may also proceed increasingly slower when decreasing radical reactivity overrules the relatively small increase of the vinyl ester mo- nomer reactivity. This may lead to a practically constant value of r2 = 1.50, as a result of compensation by secondary monomeric effects. However, the detailed physical background of such a specifically steering influence of the polymer radical on relative ’monomer reactivity remains unexplained.
As can be seen from Table 111, all products of monomer reactivity ratios are close to unity, indicating only small departures from ideal copolymerization behavior. For two binary combinations, VAc and VB, the product of reactivity ratios turns out to be significantly greater than unity. In such cases, application of the Q-e schemeI5 is infeasible. This semi-empiric model only permits prod- ucts of monomer reactivity ratios smaller than or equal to unity. For this reason, the scheme has often been c r i t i ~ i z e d . ~ ~ , ~ ~ However, for the sake of comparison,
it is assumed here that for the Eth-VB copolymerization the product of reactivity ratios is equal to unity. The calculated Q-terms for all vinyl esters appear to be almost identical, which leads to the conclusion that resonance stabilization is a contribution of minor importance to the observed differences in reactivity among the vinyl esters. The calculated e-terms of the vinyl esters become more negative, as the ester side group becomes less electron-withdrawing. Physically, this means that the “double-bond charge” (e) on the monomer is increasing. A recent theoretical calculation of Roth and F l e i ~ c h e r ~ ~ suggests that it would be more meaningful to consider the product e1.e2 to be proportional to the inter- action of the dipole moments of the monomer and the radical in the transition state. The latter interpretation would by no means conflict with the present findings on the effect of substituents on reactivity.
Constant r2-values lead to a relation between Q2 and e2 for the vinyl esters considered. The physical background of the observed linear dependency of log Q2 and e2 for the present homologs remains unexplained. Also, Kawabata et al.38 recognized a linear correlation between log Q and e for various types of monomers.
By means of the Taft relationF2 it becomes possible to decide whether the relative reactivity of a homologous series is affected by steric factors, polar factors,
or both. A graphical representation of log (llrl) versus the polar substituent constant ts* for the various binary combinations is shown in Figure 2. A linear relationship is observed for this plot only if polar factors exclusively affect re-
Fig. 2. Relation between log ( l / r l ) and -u* for the copolymerizations of ethylene ( M I ) with a homologous series of the vinyl esters: (2) vinyl acetate, (3) vinyl propionate, ( 4 ) vinyl butyrate, ( 5 ) vinyl isobutyrate, and (6) vinyl pivalate.
activity. Steric factors, if present, will decrease the relative reactivity expected on account of the u* constant. Moreover, these steric effects will be the slightest,
if not completely absent, in case of the smaller side groups, i.e., for VAc and VP. From Figure 2 it then seems obvious to suppose that for VIB and VB, steric hindrance does not play a noticeable part in relative reactivity towards the ethylene radical chain end (p* = -0.42). On the other hand, a significantly lower reactivity than would be expected considering its polar substituent constant only, is observed for VPV. Evidently, the bulky tert-butyl group decreases monomer reactivity, indicating that steric hindrance does play a part in the formation of the transition state starting from VPV, whereas for vinyl esters with smaller side groups addition to a polyethylene radical is not impeded. A steric effect, how- ever, is rather unexpected, since up to this moment only in 1,l-disubstituted vinyl. monomers a steric influence on relative reactivity has been ~ b s e r v e d . ~ On the other hand, the results of Nozakura et al.39 provide further supporting evidence to our conclusion. They reported an enhanced preference for syndiotactic over
TABLE IV
Copolymerization of Various Reference Monomers (M, ) with a Homologous Series of Vinyl Esters
p* from Ref-
Reference monomer (M, ) Q , C e, C Taft equation erence
Chloroprene 7.26 -0.02 +0.58a 7 Methyl methacrylate 0.74 0.40 -0.25 2, 9 Vinyl chloride 0.044 0.20 +0.094 11 Ethylene 0.015 -0.20 -0.42 (b) Vinyl acetate 0.026 -0.22 -0.57 40 N-Vinylcarbazole 0.41 -1.40 -0.85a 8
a Calculated from values given in t h e respective references.
b From this investigation.
COPOLYMERIZATION OF ETHYLENE WITH VINYL ESTERS 1497 isotactic addition reactions in vinyl ester homopolymerization starting from VIB, which is also indicative of steric effects.
In the present case, steric hindrance can be explained only in terms of a pen- ultimate effect or shielding of the a-electrons by the ester side group. An ex- tension of the vinyl ester copolymerization towards other reference monomers seems to be very useful here, as it may yield additional information on the origin of steric hindrance.
The sign of the polar reaction constant p * , from the Taft equation, defines the reactivity order insofar as no other contributions affect the observed reactivity. As for the present study, the sign of p* agrees with other copolymerization in- vestigations on vinyl ester monomers, where methyl m e t h a ~ r y l a t e , ~ . ~ N-vinyl- carbazole: and vinyl acetate40 have been used as reference monomer, as can be seen from Table IV. On the contrary, towards chloroprene7 and vinyl chloride radicalsll the sign of p* was reversed, while towards styrene2 and vinyl acetate (cf. Bevington and Johnsong) no accurate decision about the sign of p* could be made, probably due to less reliable monomer reactivity ratios. As already sug- gested by Cameron et a1.,2 the sign of p* probably is defined by the different electrophilic character of the free radicals. However, a trial on our part to cor- relate p* with a parameter related to the nature of the radical, in particular the polar term (e) of the Q-e scheme, failed. Possibly, the strongly varying Q-values of the reference monomers (see Table IV), and the different reaction conditions of the above-mentioned copolymerizations are responsible for this failure.
Copolymerizations of a homologous series of vinyl esters towards other ref- erence monomers under more comparable reaction conditions may lead to better correlations and consequently to a deeper insight into the physical background of radical polymerizations. A t present such experiments are in progress in our laboratory.
CONCLUSIONS
Relative overall rates of copolymerization of the present ethylene-vinyl ester binary combinations appear to be defined chiefly by chain transfer and the rate of the subsequent reinitiation reactions.
By means of a statistically reliable method for the determination of monomer reactivity ratios, a detailed examination of the relative reactivities of the vinyl ester monomers towards the polyethylene radical becomes possible.
The relative reactivity of the vinyl ester homologs towards the ethylene radical increases as the electron-withdrawing effect of the ester side group decreases, indicating that the relative reactivity is mainly affected by polar terms. Reso- nance stabilization plays only a minor part in relative reactivity, while for VPV
steric hindrance impairs reactivity towards the ethylene radical.
The unexpectedly constant r2-values, observed for the present series of co- polymerization reactions, as well as the various reported literature values for Taft’s polar reaction constant ( p * ) , indicate a dominating influence of the dif- ferent electrophilic nature of the varieus reference radicals on copolymerization behavior.
The authors gratefully acknowledge the contribution of M.A.S. Mondal to the kinetic experiments.
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Received June 26,1976 Revised July 30, 1976
