Polymerization of dienes on lanthanide containing catalytic systems. Review

Polymer Science U.S.S.R. VOl. 26, 'No. 6, pp. 1251-1268, 1984 Printed in Poland

0032-3950/84 $10.00+ .00 O 1985 Pergamon Press Ltd.

POLYMERIZATION OF DIENES ON LANTHANIDE CONTAINING CATALYTIC SYSTEMS*. REVIEW N. G. MARINA, YU. B. MONAKOV, S. R. RAFIKOV, and KH. K. GADELEVA Institute of Chemistry, Bashkir Branch of the U.S.S.R. Academy of Sciences

(Received 9 March 1983) The results of investigation of the polymerization and copolymecization of the dienes on lanthanide-containing catalysts are reviewed. Attention is focused on c/s-regulating catalytic systems and a classification of them presented. The authors look at the influence of the composition of the lanthanide-containing catalysts and the condition of polymerization on their activity and also the microstructure and molecular mass of the polydienes. The low sensitivity of the stereospecificity of the cis-regulating catalytic systems to wide variation in their composition and the conditions of the process, especially in the case of polymerization of butadiene, is demonstrated. A general characterization of "lanthanide" rubbers and vulcanizates is presented and some of their advantages shown. THE first information on the polymerization of the dienes on lanthanide-containing catalysts appeared two decades ago [1-4] and quite extensive information has now been gathered in this field. However, the existing reviews [5-8] on the subject are very brief. The studies mentioned chiefly underline the possibility of obtaining with lanthanide catalysts highly stereoregular polydienes ~vith improved characteristics but do not systematize the data nor analyse them. The potentialities of these catalysts called for a more detailed examination of the existing information.

GENERAL CHARACTERIZATION OF LANTHANIDE CONTAINING CATALYST

The catalytic systems presented in Table may be divided into individual groups by their composition and stereospeeificity. Group L The initial compound of the lanthanide Ln* is represented by its halide or a complex compound of the halide Ln* Hala.3L, where L is an electron-donor organic ligand. As ligands one uses the esters of o-phosphoric acid [10, 13, 14], sulphoxides [14], alcohols [10] and cyclic ethers [12]. In combination with an organoaluminium compound catalytic systems form giving polybutadiene and polyisoprene with a high content of 1,4-cis units. Group 1I. The initial lanthanide component does not contain a halogen but is the carboxylate Ln* (OCOR)3, alkylphosphate Ln* (OPOR)3 or alcoholate Ln* (OR)3 or * Vysokomol. soyed. A26: No. 6, 1123-1138, 1984. 1251

1252

N . G . MARINA et aL

a chelate compound. The salts of mono- and dicarboxylic acids Cx-C22 are used [19, 21] but primarily carboxylates which possess sufficient solubility in hydrocarbons, i. e. naphthenates [13, 19, 24, 28, 33-35], octanoates [15-20] and stearates [19, 25-27, 36]. The salts of di (2-ethylhexyl) phosphoric acid have also been used [24-26, 36]. In the alcoholates of the lanthanide Ln* (OR)3 the radical R may contain from 1 to 20 carbon atoms [19, 29, 30], preferably not less than C4. The use of the thioalcoholates Ln* (SR)3, the amides Ln (NR'R")3 [29, 30] and a large number of other lanthanide derivatives [19] has also been patented. The chelate compounds of the lanthanide are represented primarily by the fl-diketonates based on acetyl acetone [10, 19, 24, 25], benzoyl acetons [10, 19, 24] and other chelating agents. These triple component systems necessarily contain an halogenating agent as which a wide range of compounds is used: alkylaluminium halides [6, 13, 15-19, 24-30, 35, 36], alkylmagnesium halides [19], alkyltitanium halides [29, 30] and the halides of metals of groups III, IV, V, VI and VIII of the periodic system [19, 20, 24,29, 30], the hydrogen halide acids [15, 18, 19, 29, 30] and halogens in elemental form [15]. The catalytic systems of this group possess the same stereospecificity as the systems based on the halides of the lanthanides. Group III. The two main groups of catalysts in their stereoregulating capacity are joined by a third group of systems in which the lanthanide containing component represents a compound with mixed substituents one of which as a minimum is a halogen. These are two-component systems based on CeXC12 (X-o-hydroxybenzaldehyde) [2] and (CF3COO)2 NdC1.2 Et20 [31]. Groups I-III of c/s-regulating systems contain an organoaluminium component A1R3. As well as the aluminium trialkyls of varied structure, dialkylaluminium hydrides are used [6, 9, 10, 15-17, 19, 24;29, 30]. The results achieved (Table 1) led to the recognition of the lanthanide containing systems as extremely effective catalysts tot obtaining highly stereoregular cis-polydienes. However, recently papers have been published indicating that some organometallic lanthanide derivatives are capable oi polymerizing butadiene and isoprene with tormations of trans-polymers. Such activity is displayed tribenzylneodymium etherate [321 and x-allyl LiLn* (g-C3Hs)4 derivatives in the form of dioxan complexes [6], rise in the polarity of the medium through additions of tetrahydrofuran enabling these lithium lanthanide-containing complexes to carry out 1,2-polymerization ,of butadiene. The formation of polydienes with a mixed microstructure is noted [32] with use of the etherates of phenylcarbyn-neodymium: the polybutadiene contains units of 1,4-eis, 1,4trans and 1,2-structures in comparable amounts, whereas the polyisoprene with complete absence of 1,4-eis units consists of 1,4-trans, 3,4-units and 1"5 % 1,2-traits. The presence of THF in the coordination sphere with use of phenylcarbyn-neodymiumleads (as compared with diethyl ether) to a substantial rise (up to 50 %) in the content of 1,2 and 3,4units in polybutadiene and polyisoprene respectively. Thus, the stereospecificity of lanthanide catalysts may, in principle quite widely vary with the ligand surroundings of the lanthanide atom and the structure of the active compound effecting polymerization. All halogen-containing lanthanide catalytic systems known to date possess high c/s-regulating capacity on polymerization of butadiene and

Polymerization of dienes TABLE 1

1253~

LANTHANIDE-CONTAINING CATALYTIC SYSTEMS FOR POLYMERIZATION OF THE DIENES

Catalytic system

Diene

Butadiene Isop~'ene Piperylene Butadiene Isoprene Piperylene Butadiene Isoprene

Chiefly 1,4-attachment of 79 % 1-4-cis-units Up to 98 % l'4-cis-units Up to 98 % l'4-cis-units Chiefly 1.4-cis.units Up to 98 % 1.4-cis-units Up to 96% l'4-cis-units ~ 80% 1.4-cis-units Up to 99 % 1.4-cis-units Up to 96% 1.4-cis-units

Piperylene Piperylene 2.4-Hexadiene Butadiene

52 % cis-units 80 % l'4-cis-units ~ 100~o trans-units Up to 98 % 1.4-cis-units

Isoprene

Up to 96~/ 1.4-cis-units

Ln*(fl-diketone) a-A(Hal)-AIR3

Butadiene Isoprene

Up to 97 % l'4-cis-units Up to 95% l'4-cis-units

CeXCIz-AIR.3 (CF3COO)2NdCI'2Et 20-AIR3

Butadiene

93 % 1.4-cis-units Chiefly l'4-cis-units Ditto 70-90 Yo l'4-trans-units 80 % 1-2-units 97 % 1-4-trans-units 91% l'4-trans-units 40--45 % 1.4-trans-units 51 ~o 1.4-trans-units

Ln*CI3-AIR3 CcCI3-AIRa (Ln*Cla-R'OH)-AIR3

Ln*HaI3.nL-AIR3

Ln*(OCOR')a-A(Hal)-A1Ra

Ce(OCOR')s-A(Hal)-A1R3 Nd(OCOR')3-A(Hal)-AIR3 Nd(OCOR')3-A(Hal)-AIR3 Ln*(OR') 3-A(Hal)-AIR3

LiLn*(n-C3Hs)4"D LiLn*(z~-C3Hs)4"D-THF (C6HsCH2)aNd'nEt20 C6H,CNd'nEt20 C6HsCNd'nTHF

Butadiene

Mierostructure of polydiene

Isoprene Butadiene

Isoprene Butadiene Isoprene

Literature [3] [2] [9, 10] [9, 10] [11] [10, 12] [10, 12, 13~[.

[14] [15-23] [10, 13, 16-19, 23-26] [181 [27] [28] [6, 19, 29,. 3O] [6, 19, 29, 30] [19] [10, 19, 24, 25] [2] [31] [31] [6] [6] [32] [32l [32] [321

Note. I n * - t h e whole series of lanthanlde; L - a n electron-donor ligand; A.(lrIal) halogenating agent; D - d i o x a n .

isoprene. The use of other variants of Ln* catalysts leads to the formation of the poly-dienes indicated with a very low content or with complete absence of 1,4-cis-units.

EFFECT OF COMPOSITION OF LANTHANIDE-CATALYST$ AND THE CONDITIONS OF POLYMERIZATION ON THEIR ACTIVITY AND STEREOSPECIFICITY

Composition o f the initial lanthanide component. The catalysts of groups I and I I pos-sess the same stereospecificity. The role of the halogen-containing component in t h e systems of group I I evidently adds up to the halogenation of the lanthanide througlx

1254

N.G. MARrNAet al.

exchange reactions. Similar processes occur on interaction of the halogenating agent with a carboxylate, alcoholate or an other similar compound of a lanthanide Ln*X3 Ln*X3 + AIR2HaI-, Ln*X2Hal +AIR2X; Ln* X 2Hal + AIR2Hal--, Ln*XHal 2 + AI R2X; Ln*XHaI2 + AIR2Hal--, Ln*Hal3 + A1R2X These exchange reactions in the catalytic systems of group II must result in the formation of active centres based on the trihalide of the lanthanide identical in their structure to the active centres of group I catalysts. However, it may be that in triple-component systems active centres also form based on mono- and dihalogen containing derivatives of the type Ln*X2 Hal and Ln*XHal 2. The results obtained on the catalysts of group I[I confirm the possibility and fiigh probability of this. In addition it should be borne in mind that the systems Ln*Hal3-AIR3 possess extremely low activity [3, 4]. However, if the lanthanide salt is converted to the complex compound Ln*Hal3.3L, then the rate of polymerization of the dienes sharply increases and not only through the better solubility of such compounds in the hydrocarbon medium as compared with the salts of an ionic character LnHal3. A more important factor as postulated by the authors of [9] is that the ligand L by virtue of its electron-donor properties lowers the positive charge on the lanthanide ion and through increase in the degree of covalency of the Ln-Hal bonds promotes the alkylation of the lanthanid¢ halide and the formation of lanthanide-carbon bonds in the reaction Ln*Hai3.3L with AIR3. It is still not clear whether here there is total removal of the ligand L from the coordination sphere of the lanthanide. It is assumed [15, 24, 26] that the active centres of the cis-regulating lanthanide systems represent bimetallic bridge complexes containing the trivalent lanthanide. Consistent with such notions are the results of study of the residues formed on interaction of NdCI3.3L with AI (iso-C4H9)3 in a hydrocarbon solvent. The presence of neodymium and aluminium in the ratio 1 : 1 has been found in these amorphous residues [37]. However, direct experimental data disclosing the structure of the active centres of the lanthanid¢ catalytic systems are still absent. Analysis of the published results indicates that the cis-regulating active centre requires (as a minimum) the presence of one Ln-Hal bond. The two other valencies of the lanthanide atom may also be saturated with halogens but for the realization of activity and high cis-regulating capacity this is not strictly essential as is confirmed by the systems of group III. The established influence of the nature of the substituent X in LnX3 on the activity of catalysts of group II [24-26] may be due to various factors. On the one hand, the ease and completeness of the halogenation of the initial lanthanide compound is determined by the nature of the substituent X and the character of the Ln-X bond i. e. in the final analysis the nature of X must primarily influence the number of active centres. However the presence of the X grouping directly at the active centre with incomplete ilalogenation may also influence its reactivity.

Polymerization of dienes

1255

The use of the alcoholates of the lanthanide Ln* (OR)3 with a higher degree of covalency of the Ln-O bonds than for the carboxylates Ln* (OCOR)3 led to the creation of very active triple-component systems [29, 30] ensuring sufficiently high rates of polymerization at low doses of the lanthanide ( ~ 10-4 mole/l.). It has been shown in the case of neodymium-alcoholate systems [6] that derivatives with a branched hydrocarbon radical, in general, are less active than the alcoholates with linear radicals R; the catalytic activity falls in the series Nd (O-n-C4Hg)3 ~-Nd (O-n-C~ 0H21)3~ N d (O-iso-C3HT)3 >/ t> Nd (O-iso-CsH11)3 In this connexion it should be noted that a similar influence of the structure of the alcohol on catalytic activityhas been found for systems of the type (NdHal3-R'OH)--AIR3 [9]. Thus, with use of C2-C5 alcohols of normal structure the neodymium system possesses identical activity although on further growth of the length of the radical R' the polymer yield tends to fall. It has been shown with reference to four isomeric butanols that on the structure of the radical R' essentially depends the activating effect of the alcohol (n-R'OH > iso-R'OH > tert-R'OH) tertiary butanol not activating the system at all. In all these cases the stereospeeificity of the system does not change. The structure of the X substituent influence not only the activity of the triple-component systems of group I[ but also the region of molar ratios of Hal • Ln* ensuring the maximum activity. Thus, with use of the stearate and di (2-ethylhexyl) praeseodymitma phosphate it has been established [25] that the maximum activity shifts for stearate to smaller ratios of C1 : Pr. A similar picture is also seen on comparing systems based on the carboxylates and alcoholates. In the first case the maximum activity is reached for the ratios C1 : Ln* ~ 2.0-2.5 [15, 17-19, 25], for a higher value of the ratio C1 • Ln* the rate of polymerization sharply drops (Fig. 1). The alcoholate systems display the highest activity for the ratios of CI : Ln*=3 [6]. The use of A1RCI2 instead of A1R2 CI does not change the position of the maximum of catalytic activity as a function of the value CI : Ln* but widens the working region of the molar ratios of CI : Ln* [25]. It may be assumed that the sharp fall in the activity of the systems for doses of dialkyl aluminium halide exceeding the optimal is due to the formation of strong bridge complexes of the type Hal

/ Hal

Hal Ln

\

R

AI

,

/'\ Hal

R

in which the bond L n * - C ensuring polymerization activity is absent. In other words, such a necessary initial component of the catalyst as A1R2Hal may in certain conditions show up as an inhibitor. The extremal dependence of the activity of the systems of group II on the molar ratio of the initial components is a shortcoming since for the realization of high activity it is necessary to hold the doses very precisely. Nature of the lanthanide. All researchers emphasize the close dependence of the acti-

]256

N.G.

MARINA e t al.

vity of lanthanide systems on the chemical individuality of the lanthanide figuring in them. The activity series of the lanthanides have been established on c/s-polymerization of butadiene and isoprene. For the catalysts of group I based on the lanthanide chlorides Nd > P r ) G d > Ce> D y > H o > La > Er> Sm >Tm,-~Yb-~Lu (monomer-butadiene) N d > P r > C e > L a > G d > T b > Dy > Sm~>Ho >Er>~Tm~Yb~-Lu (monomer-isoprene) [10] Nd>Pr>Tb>Gd>Ce>La 700 -

[38]

100

1

~

60

I

I I

\

,\

c~

20 I

0

I

I

30

I

La PP (Pm) Eu Tb HO Tm Lu Ce Nd Sm Gd Dy EP Yb

60 Hal: Ce

FIG. 2

FIG. 1

FlG. 1. Conversion as a function of the ratio Hal : Ce on polymerization of butadiene on catalytic systems containing cerium octanoate. Halogenating agent EtAICIz (1) and EtzAIF (2) [15l. FiG. 2. Conversion as a function of the nature of the lanthanide on polymerization of isoprene on catalytic systems Ln* (Naph)s-Alz (CzHs)3CI3-AI(iso-C4Hg)3 [10].

For the chlorine-containing catalysts of group II based on the carboxylates Nd > P r > Ce > G d > T b > D y > L a > H e > Sm~- Er > T m N Y b ~ L u [10] N d > Pr> Ce > T b > Dy > G d > H e > L a > Sm [5] Nd>Pr>Ce>Gd>La>Sm

[24]

N d > P r > C e > L a > G d > > S m [25, 39] N d > P r > G d [12] These activity series do not concur at all, which is most probably connected with the high sensitivity of the metallo-complex catalysts to impurities and the conditions of the experiment. However, some general patterns may be discerned for all these series and types of catalyst: I. Neodymium-containing catalysts are the most active: this was already noted in the very first publications [40]. 2. The europium-containing systems of groups I and If catalysts practically do not effect polymerization by reason of the transition of this lanthanide to the inactive biva-

Polymerization of dienes

1257

lent state on exposure to such a reductant as AIR3 [13, 24]. Reduction to the bivalent state though not so complete also occurs in the case of samarium [13]: the systems based on it are little active. Therefore, such disturbance of the smooth course of the activity curve is also observed in the lanthanide series through samarium and europium (Fig.2). In absence of reductant samarium derivatives display activity at the level of the other lanthanides nearest to it as shown with use of dioxan complexes LiLn* (~ -C3Hs)4 on trans-polymerization of butadiene [6]. 3. The activity of the systems rises from lanthanum to neodymium and then falls. The heavy lanthanides starting from erbium are practically inactive in cis-polymerization of the dienes. Quite marked activity is shown by the systems based on cerium, praeseodymium, neodymium and gadolinium. The position of the catalysts in these series may be influenced by the dependence and concentration of the active centres and their reactivity on the ordinal number of the lanthanide. In the case of the systems LnCI3"3TBP-AI (iso-C4Hg)3 (TBP-tributyl phosphate) ~he authors of [13] established the absence of a correlation between the activity of the catalyst and the frequencies of the characteristic vibrations of the phosphoryl groups of the ligands for various Ln*. The results of laser apectroscopy of combination scatter show that there is also no correlation between the characteristic frequencies (and respectively the energies)of the Ln*-C1 bonds and the activity of the complexes in polymerization. Therefore, the difference in activity is primarily related to the difference in the number off-electrons in the lanthanides [10, 13]. It is assumed [13] that the f-electrons take part to a certain degree in the formation of bonds between the Ln* ion and the diene in the active complex. This assumption is quite feasible since experimental proof of the participation of the f-orbitals in the formation of the bond in organic lanthanide derivatives has already been obtained [41]. The difference in the number off-electrons determines, as assumed by the authors of [10, 13], the value of change in the energy of the electrons on complexation of the trivalent lanthanide ion with a diene or another ligand. Calculations by the method of molecular orbitals gave the following order of change in the energy zlE for the trivalent ions: Ce>Pr>Nd>Pm
[10, 13]

The position of the lanthanides in terms of zlE as a whole corresponds to the reverse order of the series of their catalytic activity. All these findings lead to the conclusion that the reactivity of the active centre of polymerization of the dienes must depend on the individuality of the lanthanide. In fact, it has been established [42] that other things being equal the chemical individuality of the lanthanide influences to a much higher degree the rate constant of the growth reaction kp than the concentration of the active centres on polymerization of butadiene. Therefore, the experimentally observed dependence of the activity of the systems on the nature of the lanthanide is primarily due to the different reactivity of the corresponding active centres. With rise in the ordinal number of the lanthanide the number of active centres tends to fall. The rate constants of the growth reaction (kp = 10-102

1258

N.G.

M A R I N A et al.

1./mole.min) proved to be lower by several orders in numerical value than the rate constants of the complexation of the diene with the LnCIa" 3L compounds (kc = 105-10 ° l./mole'min [43]. This indicates that the limiting stage of polymerization is not the coordination of the diene at the active centre but its penetration over the lanthanoid-carbon bond. The stereospecificity of the catalysts of groups I and II practically does not depend on the nature of the Ln* on polymerization of butadiene and isoprene (Tables 2 and 3). This peculiarity is stressed by all researchers who have tested the series of such catalysts. The 1,2-units in polybutadiene amount to less than 1 ~o. The 1,4-trans units are absent in polyisoprene and the content of the 3,4 units tends to fall to a certain value with rise in the ordinal number of the lanthanoid. All catalysts based on lanthanides most active in polymerization possess the same stereoregulating capacity. The same catalysts give polydienes with roughly identical M with quite high conversion [12,

38, 44]. In this connexion it should be noted that the microstructure of polyacetylene obtained on Ln* catalysts is also roughly identical for all the lanthanides tested (70-80~o c/s-structures) [45] while the other systems of the Ziegler type give mostly the transpolymer. With use of trans-regulating dioxane complexes LiLn* (n -CaI-Is) 4 containing cerium, neodymium, samarium, gadolinium or dysprosium the content of the 1,4-trans units in polybutadiene tends to fall with rise in the ordinal number of the lanthanide, while the activity of these catalysts is practically the same. On addition of aluminium tribomide (A1 : Ln = 0.3) all these catalysts give polybutadiene with 90 ~ 1,4-trans units and similar conversion [6]. Consequently it is quite permissible to use for obtaining highly stereoregular polydienes eatalyts based on a mixture of various lanthanides. Thus, polybutadiene identical in microstructure forms with use of the Nd catalysts and a catalysts based on the mixture N d - P r [46]. On c i s - p o l y m e r i z a t i o n of butadiene successful use was made of a catalytic system in which the lanthanide component was represented by a mixture of

TABLE 2. MICROSTRUCTURE OF POLYBUTADIENE OBTAINED ON THE SYSTEM

(Ln*CIa-C2HsOH)-A! (C:~Hs)3. [10]

Lanthanide

La

Ce Pr Nd Sm Gd

Content of units in polybutadiene, y. 1,41,4cis trans 1,2 97"2 97"2 97.2 97"3 98-0 97"3

2"1 2'1 2"2 2"2 1-6 2"2

0"7 0.7 0"6 0.5 0-4 0-5

Lanthanide

Content of units in polybutadiene, % 1,4-

c/s Tb Dy Ho Er Tm Yb

97"9 97"5 96"7

93"0 90.6 97-1

1,4- 1 I

1,2

1.6 J

0-5 0"6 0-5 0-9 0.7 0"3

trans

1"9 2"8 6"1 8-7 2"6

1259

Polymerization of dienes TABLE 3. MICROSTRUCTUREOF POLYISOPRENEOBTAINEDON LANTHANIDE-CONTAININGCATALYTICSYSTEMS[10]

Content of units in polyisoprene, 1,4-cis ] 3,4 1,4-cis I 3,4 Ln* naphthenate-AIz(C2Hs)~ CI3(LnCI3 -- C2H~OH)AI(iso-C, Hg)3 AI(iso'C,H9) 3

Lanthanide

94"1 94"8 94'9 94'7 96"7 96'8 97"2 97"2 97"4 97"4 97"6 98"0

La

Ce Pr Nd Sm Gd Tb Dy Ho Er Tm Yb TABLE 4.

5.9 5.2 5.1 5.1 3.3 3"2 2.8 2-8 2.6 2.6 2-4 2.0

94-2 93.6 93-9 95-0 94"6 96"0 95-2 95.2 95.5 94.7

5-8 6-7 6-1 5.0 5-4 4-0 4.8 4.8

4.5 5-3

EFFECT OF THE NATURE OF THE HALOGEN ON THE

STEREOSPECIFICITY OF THE SERIES OF CATALYTIC SYSTEMS CONTAINING HALOGEN DERIVATIVES OF d AND f ELEMENTS

Halogen F C1 Er I

Content of 1,4 cis-units in polybutadiene,% Ni [15] u [47] Co D5] Nd [lOl Ti [15l 35 75 87 93

93 98 91 50

98 85 80 10

95"7 96'2 96"8 96"7

99.O 98.5 98-5

neodymium, lanthanum and praeseodymium salts in the ratio 72 : 20 : 8 [20]. All this is very important in view of the lower cost of lanthanide mixes. Nature of halogen. Depending on the nature of the halogen the catalytic systems o f group II line up in the following activity series: B r C I > I > F [5, 15, 24, 26, 39]. f f the dialkylaluminium halides are used as halogenating agent, the region of ratios o f Hal : Ln* ensuring the maximum activity practically does not depend on the nature o f the halogen (except for fluorine) and amounts to 2.0-2.5 [15, 26]. Fluorine-containing systems of group II polymerize the dienes for a ratio F : Ln* of not less than 10 and the maximum activity corresponds (Fig. 1) to the ratios ~ 40 [15, 16, 19]. This is connected with the fact that the dialkylaluminium fluorides in solutfon are almost completely associated to tetramers and therefore fluorination of the lanthanide requires higher doses of this component [15]. The influence of the nature of the halogen on the activity of systems of group I1 is evidently connected to a certain extent with the completeness and conditions of halogenation of the lanthanide and hence with the number of active centres formed. There-

1260

N . G . MARn~A e t

al.

fore, systems of group I based on the trihalides of the lanthanides are a more convenient object for evaluating the influence of the halogen on the reactivity of the active centres and as a whole the activity of these catalysts. With use of the system (NdHaI3C2HsOH)-AI (C2Hs) a it has been established [10] that the yield of polyisoprene is 84, 42 and 5 ~o for the chlorine, bromine and iodine catalysts respectively in identical conditions. The dependence of the activity on the nature of the halogen is strongly marked .and the most active is the chlorine-containing systems. However in this case, too, the system based on neodymium trifluoride proved extremely little active (polymer traces). The microstructure of the polybutadiene does not depend on the nature of the halogen as part of the lanthanide catalyst [5, 10, 15]. As may be seen from comparison of the data (Table 4) such insensitivity of the stereoregulating capacity to the nature of the halogen is a distinguishing feature of catalytic systems based on the f-elements. On polymerization of isoprene [10, 24, 26, 39] there is a certain rise (from 5 to 10%) in the content of 3,4-units in the chain with use of Cl-, Br- and I-containing catalytic systems [I0, 24, 26, 39] with absence in all cases of 1,4-trans units in the polymer. Structure of organoaluminium component. Already at the earliest stages of study of the polymerization of the dienes on Ln* systems it was noted [40] that variation in the alkyl radicals in AIRa has an appreciable effect on the activity of the catalyst. For the ~ratalytic systems of group II the following activity series have been established for the organoaluminium component: AI(iso-C4Hg)zH>AI(iso-C4Hg)a>AI(C2Hs)3 >Al(CHa)3 [10, 24] Al(iso-C4H9)3/> AI(iso-C,Hg)2H>>AI(C2Hs)a >>AI(CH3)3 [6]

Al(iso-C,H9)3> AI(CH2CH2- ~/--~)3 [42, 48] For the system Ln*CIa-R'OH-A1Ra a somewhat different sequence was established

i9] AI(C2 Hs)3 > Al(iso-C4Hg)3 > Al(iso-C4Hg)2H >~AI(CHa)3 It should be noted that the experiments were run by different investigators and not in identical conditions which probably also accounts for a certain discrepancy in the findings. The least active are the Ln*-systems consisting of trimethylaminium. The low activity of the systems including tiethylaluminium [6, 10, 24] is apparently connected with its dimer state at temperatures of polymerization close to room. Fairly close activity, to judge from the findings [6, 10, 24], is shown by triisobutylaluminium and diisobutylaluminium hydride. With rise in the length of the alkyl radicals R the alkylating capacity of AIRa falls accompanied by drop in the polymerization rate through fall in the number of active centres with their reactivity remaining unchanged i.e. the rate constants of the growth reaction and stereospecificity [42, 48]. With variation in the organoaluminium component in the Ln*-systems their stereoregulating capacity does not change [9, 10, 24, 38, 42, 48]. From these data it may be assumed that each of the above listed series, in fact, reflects the level of formation of uniform active cent res.

Polymerization of dienes

1261

The activity of the systems is also influenced by the ratio ~1 : Ln*. The rate of polymerization sharply rises with increase in this ratio in the range ~4-20, with further increase in the ratio the activity of the catalyst changes little [6, 9, 15, 25, 38, 42]. The dependence of the polymerization rate and the number of aetive centres on the concentration of A1R3 (Fig. 3) allows one to relate the changes in rate with variation in the ratio AI : Ln* for a given lanthanide system to the different concentration of identical active centres since as shown for neodymium-containing systems [42] in the whole range of ratios A1 : Nd = 7-80 studied, the value kp remains unchanged. Another experimental fact is also well consistent with these findings. The activity of catalyst based on neodymium naphthenate is related by a direct dependence to absorption at 582 nm; the stronger the absorption for the catalyst the higher is its activity [33]. It has also been established that with rise in the ratio A1 : Nd the intensity of absorption and catalytic activity increase, evidently due to rise in the concentration of the uniform particles absorbing in this region.

Hv "10-s

.lO,

w" 10z7mole/l., rain i

C.d

# ~

0

I

I

I

20

0,0

60

Ft~. 3

t

l'i

80 AI:Nd

/sI 12 I

-

/

o/I /

8

2

lio,:. 20

60

I00

Conversion, % Fxo. 4

F~G. 3. Rate of polymerization (I) and number of active centres (2) as a function of the molar ratio AI : Nd on polymerization of butadiene on the neodymium-containing catalytic system [42]. FI~. 4. Molecular mass of polyisoprene as a function of conversion on polymerization on Ln*-eatalytic system for the ratios: A1 : Ln* 17-5 (1), 30 (2) and 50 (3) [10]. The organoaluminium compound as a component of the catalytic system exerts an appreeiable effect on the molecular-mass characteristics of the polydienes. Rise in the concentration of A1R3 leads to fall in M [10, 12, 15, 26, 38, 42, 44], which indicates that AIR3 is a transfer of the growing chain. With use of triisobutylaluminium it has been established that the relative constant of transfer to this component k ~ : kp depends little on the nature of the lanthanide (the cerium systems are a certain exception) and amounts to 0.05 (25°C) on polymerization of butadiene [42] and 0.03 (20°C) on polymerization of piperylene [44]. Study [35] gives a value close in order of the ratio koAI : kp = 1 : 90. Increase in the length of the radicals in AIR3 promotes rise in M of the polydienes [6, 9, 10, 15, 24, 42] which is due, according to [42], to fall in the value of the relative constant of transfer of the chain to AIR3. As compared with the alttminium

1262

N.G. MARINAet al.

trialkyls much stronger chain transfer agents are the dialkylaluminium hydrides [6, 9, 10, 15, 24]. The characteristic viscosity of polyisoprene, for example, is ~ 2 times lower with use of diisobutylaluminium hydride than in the case of triisobutylaluminium [10, 24]. Moreover, A1R2H promotes the widening of the MD of the polydienes and its conversion to bimodal [10]. The relative constant of chain transfer to A1R3 for the Ln* systems is lower by about one order than the corresponding transfer constant in the case of polymerization of butadiene in the cis-regulating system TiI2C12-A1Ra for the same AIRa [48]. This primarily ensures the preparation of high molecular weight polydienes on Ln*-catalyst. At low concentrations of A1R3 in presence of these catalysts "live" polymerization is observed, the M of polydiene increasing in proportion to conversion (Fig. 4), and the maximum of he MD curve shifting to higher M with barely no change in the width of the MD [10], i.e. termination and transfer of the chain are almost completely absent in certain conditions. It is assumed [6] that the lanthanide catalysts ensure the more distinct character of "live" polymerization than the usual Ziegler-Natta catalysts. Cerium systems somewhat differ from the other lanthanide systems in respect of the influence of the organoaluminium component on the characteristics of the polydienes, in particular, polybutadiene. Thus, there is some fall in the content of 1,4-cis units in polybutadiene with rise in the A1 : Ce ratio and the fraction of 1,4-trans units rises [15]. With increase in the AI : Ce ratio above 20 kp falls [42]. The relative constant of chain transfer to triisobutylaluminium for the cerium catalytic system is somewhat higher than for the other Ln* systems [42, 44] as result of which the cerium systems lead to the formation of lower molecular weight polydienes. Conditions of polymerization. On polymerization of the dienes on Ln*-catalysts various solvents are used: individual aliphatic, cycloaliphatic and aromatic hydrocarbons and also mixtures of hydrocarbons, for example, hydrogenated benzine. The influence of the nature of the solvent on the stereospecificity of the catalyst on polymerization of butadiene and isoprene is not noted. However on polymerization in aliphati¢ solvents, as compared with aromatic, higher molecular weight polydienes form [12, 15, 25, 44]. Moreover, the aliphatic solvents ensure higher polymerization rates [12, 15, 42]. The difference in the activity of the catalysts is due to the influence of the solvent on the growth rate constant which is ,~ 3 times higher on polymerization in heptane than in toluene [42, 44]. Toluene probably enters the coordination sphere of the lanthanid¢ at the active centre and thereby influences its reactivity and also stability. The rate of polymerization of butadiene in toluene is ,-~30 ~o higher at 70°C than at 50°C while on polymerization in heptane in the same conditions the rates are practically identical [15] which indicates the appreciable deactivation of the active centres in heptane at raised temperatures of polymerization. A similar picture was also noted in the reaction in hydrogenated benzine [35]. Characteristic of polymerization in toluene [42, 44] is the somewhat higher level of formation of the lanthanide active centres than in heptane which is connected with the stabilization of these centres in aromatic solvents.

Polymerization of dienes

1263

It should be noted that although the method of polymerizing dienes on Ln* catalysts over a very wide temperature range (from - 6 0 to +150°C) is patented [19, 30] it is preferable to run the process at temperatures of 20-80°C At low temperatures polymerization is heavily delayed not only through fall in kp but slow initiation already at 0°C whereas at 20 and 50°C initiation occurs instantly [35]. At raised temperature thermal deactivation of the catalysts ensues [15, 35] especially in a medium of aliphatic solvents. In choosing the temperature conditions it is also necessary to take into account the following factors. Polymerization of the dienes on Ln* catalysts even at raised temperatures is not accompanied by their secondary oligomerization and the polymer does not contain highly pungent and extractable impurities [15]. The stereoregularity of the chain of polybutadiene and polyisoprene is little sensitive to the temperature of polymerization. Thus, on polymerization of butadiene in the cerium catalytic system fall in the content of 1,4-cis-units in the polymer is not more than 3% in the range from 3 to 70°C [15]. Only weak change in the stereospecificity of the catalyst is noted at temperatures above 70°C [6]. The M of the polydienes appreciably falls with rise in the polymerization temperature [15, 26, 36]. Like other catalysts of the Ziegler-Natta type the formation of the lanthanide-containing catalytic complex in presence of diene additives enhances its activity [9, 17, 19]. Some impurities in butadiene lower the rate of polymerization on Ln*-cytalysts and increase the M of polybutadiene [34]. The conversion falls from 90 to 70% in presence of < 100 p.p.m, methylacetylene, acetonitrile and acetaldehyde. A certain effect is noted in presence of 300-700 p.p.m, acetone, methylvinylketone, acrolein, ~t-methacrolein, butenal and cyclo-octadiene. The effect of 4-vinylcyclohexane and furane is insignificant. It has been established [42] that a strong inhibitor of polymerization on Ln*-systems is cyclopentadiene. Nature of the diene. The type of stereoregulation on polymerization on Ln*-catalysts depends on the structure of the monomer. A more complex structure of the diene promotes increase in the dependence of the stereoregulating capacity of Ln*-catalysts on their composition and probably on the conditions of polymerization. On polymerization of butadiene the microstructure of the polymer practically depends neither on the composition of the systems of groups I-III nor on the conditions of polymerization with fairly wide variation of them. The polybutadiene chain almost entirely (>99 %) consists of 1,4-units. The stereospecificity of the Ln*-catalysts on polymerization of isoprene already to a certain degree is due to the nature both of the lanthanide and halogen. If a chlorine-containing neodymium system is taken as a guide then the content in the chain of units of 1,4-attachment is 95 %. On polymerization of piperylene on the same system the number of units of the 1A-attachment in the chain falls already to 80% [27]. It may be that on polymerization of piperylene the dependence of the stereospecificity of the catalyst on the nature of the lanthanide and the conditions of polymerization will be more clearly manifest. Comparison of the results of studies [18] and [27] (Table 1) gives grounds for such an assumption. Only the trans-isomer of piperylene is polymerized [11, 14]. The neodymium catalyst which gives 1,4-cis-polymers of butadiene, isoprene and

N.G. MARINAet al.

1264

piperylene, in the case of polymerization of trans-, trans-2,4-hexadiene leads to the formation of a high molecular weight crystalline polymer in the chain of which there is ,~100~o 1,4-trans-units [28]. The authors of [28] considered two possibilities (anti,~yn-isomerization of the active centre on coordination of the monomer by two double bonds or coordination only through one double bond), which might have produced such a microstructure, but they gave preference to neither. It should be noted that the various systems of the Ziegler type based on Ti, Co, Cr, Ni, Mn and V also lead to the formation of polymers not containing cis-units [49]. Thus, in the given case the sterically hindered structure of this monomer becomes an essential factor determining the type of stereoregulation. COPOLYMERIZATION

The most notable feature of the copolymerization of butadiene with isoprene on c/s-regulating Ln*-catalysts is the formation of polymers almost entirely consisting of units of 1,4-cis structure [5, 9, 10, 50, 51, 52]. Figure 5 shows that the total content of 1,4-cis units in the butadiene and isoprene fragments stays at a high level over a wide range of compositions of the copolymer with use of an Ln*catalyst. H:rein lies the main difference from the other cis-regulating systems of the Ziegler type, copolymerization on which is accompanied by appreciable worsening of the microstructure of the butadiene fragments and fall in the content of the cis-units in the chain [53]. To illustrate this Fig. 5 also gives the results of the treatment of the data [54] on copolymerization of butadiene with isoprene on the cis-regulating system TiI4-(Aliso-C4Hg) 3 . This titanium-containing system possesses quite high stereospecificity on homopolymerization of the dienes mentioned although copoiymers with a high content of units of cis-structure do not form on it. A tribenzylneodymium etherate gives copolymers of butadiene with isoprene consisting essentially of 1,4-trans units [32]. Some increase is noted in the fraction of 1,2units in the butadiene fragments on enrichment of the copolymer with isoprene; the content of the 3,4-units in the isoprene part is appreciably higher than in the homopoTABLE 5. COPOLYMERIZATION CONSTANTS OF BUTADIENE rz WITH ISOPRENE r2 ON LN-CATALYSTS

Catalytic system

Solvent

NdSt3-AI(C2Hs)zC1-AI(iso-C4H9)3 Toluene Ln*Naph3-AJ2(C2Hs)sCI3 Hydrogenated Al(iso-C4Hg)3, where Ln*: benzine La Ce Pr Nd (C6H~CH2)3Nd'nEtzO o-Xylene Note.

Polymerization Itemperature, °C I

25 50

50

NdSt-neodymlum steal'ate; Ln*Napl~-Lanthanlde naphthenato.

rt '

r2

Literart r2 ture

1-94 0.62

1.2

1-5 1"3 1"2 1"4 2.1

1"3 1.3 1"1

0"84

0"75 0"89 0"60 0"33

[5Ol [io1

0"8

0"7

[32]

Polymerization of dienes

1265

lymer obtained on the same catalyst. In this connexion it should be noted that with use of c/s-regulating Ln*-catalysts the fraction of 3,4-units in the isoprene part of the copolymers by contrast falls as compared with the homopolymer and reaches a level of less than 1% [5, 9, 50]. Butadiene is more reactive than isoprene on copolymerization both on cis- and trans-regulating Ln*-catalysts (Table 5). The product of the copolymerization constants are close to unity, which corresponds to statistical copolymers. Monotonic change is observed in the glass transition point of the samples obtained with change in their composition [50]. The statistical character of the distribution of the butadiene and isoprene units along the chain of the copolymers obtained on the cis-regulating cerium system is confirmed by 13C-NMR spectroscopy. It has been shown that even for a 10-fold excess of one comonomer in the chain the microblocks of the second comonomer persist [55]. As noted, on polymerization on Ln*-catalysts the termination and transfer of the chain in certain conditions occur only to an insignificant degree. The realization of "live" polymerization gives block-copolymers of butadiene with isoprene [10]; at first one monomer was polymerized and after reaching 100~ conversion the other was introduced into the polymerization medium. The authos of [10] assume that a large part of the product thus obtained is a block-copolymer of the A-B type. PROPERTIES OF POLYDIENES

The results achieved revealed certain distinguishing features of the polydienes obtained with the aid of cis-regulating Ln*-catalysts. 1. The high content of the 1,4-cis units (up to 98-99 ~ in polybutadiene and up to 98 ~o in polyisoprene). 2. In the sequences of several 1,4-units in polypiperylene only "head-to-tail'" attachment of the units takes place [27]. It may be supposed that such uniformity in the construction of the chain is also peculiar to polyisoprene. 3. On Ln*-catalyst polydienes form with a linear structure of the chain [10, 36, 561. 4. The rubbers do not contain a gel fraction and are completely soluble in the usuaI solvents [15, 30, 36, 40]. 5. The polymerization products of the dienes have no impurities of volatile oligomers as a result of which the rubbers do not have an unpleasant odour [15] 6. Easy control of the M of the polydienes is one of the specific features of polymerization on Ln*-catalysts [6]. The value of the characteristic viscosity of the polybutadiene could be adjusted from 2 to 16 dl/g by varying the conditions of polymerization [30]. In line with this the Money viscosity of butadiene rubber is easily regulated over a wide range from < 20 to > 70 [6]. On polymerization of isoprene one may obtain M,-- 3 x × l0 s [30], the Mooney viscosity of the isoprene rubber reaching values of 80-120

[61. 7. The polydienes possess a wide MD: for polybutadiene and polyisoprene a value

1266

N.G. MAI~INAet aL

of y-~4 has been noted [10, 15, 30], for polypiperylene this value reaches ~8-10 [44]. The MD may be varied within wide limits [30, 44]. The possibility of regulating the M and MD gives elastomers with molecular-mass characteristics optimal for a particular commercial rubber product and the conditions of its exploitation. 1"7

~100 i

1 l'l

05

?

~ 6o I

I

#0 80 Butadiene , mole%

FIG. 5

O.1

I I

I

I00

I

I

300 500

700

~,%

FIG. 6

FxG, 5. Total content of l'4-cis-units as a function of the composition of the copolymersof butadiene with isoprene obtained on catalytic systemsNd(St)3-Al(iso C4Hg)a-AI(C2Hs)2C1(1) [50] and on Til,-AI(iso-C4Hg)a (2) [54]. FIG. 6. Cohesivestrength of raw filledrubber mixtures: 1- Ti-polybutadiene;2 - Ln*-polybutadien¢; 3-natural rubber [8]; e-stretching. 8. The wide MD and high stereoregularity promote the good workability of the rubbers. High adhesiveness, rapid formation of a smooth even skin on the rollers and ease of extrusion with good dispersion of the ingredients of the rubber mixtures are noted [8, 15, 30]. In the formulation of the usual sulphur vulcanization "lanthanide" polybutadiene is readily milled even at 70°C whereas the commercial "titanium" rubber processes far worse, especially at temperatures above 40°C [8, 20, 21]. 9. "Lanthanide" polybutadiene displays higher erystallizability on stretching than the standard commercial samples [8, 22, 23], which helps to improve the strength properties both of the raw rubber (cohesive strength) and its vulcanizates (Fig. 6). A substantial increase in resistance to rupture is noted at raised temperatures in the vulcanizates of polybutadiene obtained on a cerium catalyst as compared with standard "'titanium" samples [15]. An adverse consequence of the enhanced capacity for crystallization is the worsening of the freeze resistance of the vulcanizates of polybutadiene obtained on Ln*-catalysts [8]. 10. From comparison of the dynamic properties of the vulcanizates of Ce- and Tipolybutadienes it has been assumed that the vulcanization network is less dense in the Ce-sample in the same regime of vulcanization [15]. Therefore it cannot be excluded that for an optimal regime of vulcanization the lanthanide rubbers will show their advantages in physicomechanical properties to a greater degree. 11. The copolymers of butadiene with isoprene have very good low temperature properties. Thus, the vulcanizates of the copolymer containing 2 0 ~ isoprene retain elasticity peculiar to room temperature down to -50°C [10]. Further widening of

Polymerization of dienes

1267

study o f t h e " l a n t h a n i d e " r u b b e r s and t h e i r vulcanizates will w i t h o u t d o u b t help to reveal m o r e fully all t h e i r a d v a n t a g e s p r i m a r i l y d e t e r m i n e d by t h e high u n i f o r m i t y o f t h e s t r u c t u r e o f t h e p o l y m e r chains.

Translated by A. CROZY

REFERENCES 1. 1. M. ROBINSON and R. C. SCHREYER, (U.S.A.) Pat. 3118864, Printed in Chem. Abstv. 60: 10908, 1964 2. W. C. V a N DOHLEN, T. P. WILSON and E. G. CALFISCH, Belg., Pat. 644291, Printed in Chem. Abstr. 63: 58774b, 1965 3. C.-C. SHEN, C.-Y. KUNG, C.-C. CHUNG and Y.-C. OU, Chem. Abstr. 61: 12091L 1964 4. T.-C. SHEN, C.-Y. GUNG, C.-C. CHUNG and J. OUYANG, Sci. Sin. 13: 1339, 1964 5. C. CAPITANI, M. BRUZZONE and A. MAZZEI, Chimica e Industria 59: 33, 1977 6. A. MAZZEI, Makromolek. Chem. 182: 61, 1981 7. G. MARWEDE and G. SYLVESTER, Chem. Abstr. 95: 188384, 1981 8. J. WITTE, Angew. Makromolek. Chemic 94: 119, 1981 9. J. YANG, J. HU, S. FENG, E. PAN, D. XIE, C. ZHONG and J. OUYANG, Sci. Sin. 23: 734, 1980 10. Z. SHEN, J. OUYANG, F. WANG, Z. HU, F. YU. and N. QIAN, J. Polymer Sci. Polymer Chem. Ed. 18: 3345, 1980 I 1. D.-M. XIE, C.-G. ZHONG, N.-A. YUAN, Y.-F. SUN, S.-X. XIAO and J. OUYANG, Chem. Abstr. 92: 77008r, 1980 12. J. H. YANG, M. TSUTSUI, Z. CHEN and D. E. BERGBREITER, M.acromolecules 15: 230, 1982 13. F.-S. WANG, R.-Y. SHA, Y.-T. JIN, Y.-L, WANG and Y.-L. ZHENG, Sci. Sin. 23: 172, 1980 14. Yu. M. MONAKO, S. R. RAFIKO'V, A. S. BEZGINA, G. A. TOLSTIKOV, N. V. DUVAKINA, N. G. MARINA, Yu. I. NIKITIN, A. A. BERG, A. A. PANASENKO, V. G. KOZLOV and N. F. KOVALEV, U.S.S.R. Pat. 726110, Byul. izobr., No. 13, 131, 1980 15. M. C. THROCKMORTON, Kautschuk und Gummi Kunstoffe 22: 293, 1969 16. FRG Pat. 1812935, printed in Chem. Abstr. 71:82385 1969 17. M. C. THROCKMORTON and R. E. MOURINGAN, FRG Pat. 2011543, Printed in Chem. Abstr. 74: 13981y, 1971 18. M. C. THROCKMORTON, USA. Pat. 3657205, printed in Chem. Abstr. 77: 21203y, 1972 19. M. C. THROCKMORTON and R. E. MOURIGHAN, USA. Pat. 3794604, Publ. KZhKhim. No. 35279P, 1975 20. G. SYLVESTER, J. WITTE and G. MARWEDE, FRG Pat. 2830080, Printed in Chem. Abstr. 9 2 : 130367k, 1980 21. G. SYLVESTER, J. WITTE and G. MARWEDE FRG Pat. 2848964, Printed in Chem. Abstr. 9 3 : 965554, 1980 22. G. SYL'VESTER and W. WIEDER, Polymer Preprints 22: 142, 1981 23. M. A. BRUZZONE, Polymer Preprints 22: 142, 1981 24. SHUNGO KESYUS, Sci. Sin. 17: 656, 1974 25. S. R. RAFIKO'V, Yu. B. MONAKOV, Ya. Kh. BIYESHEV, I. F. "VALITOVA, Yu. 1. MURINO'V, G. A. TOLSTIKOV and Yu. Ye. NIKITIN, Dokl, Akad. Nauk SSSR.. 229: 1174, 1976 26. Yu. B. MONAKOV, Ya. Kh. BIYESHEV, A. A. BERG and S. R. RAFIKO'V, Ibid. 234: 1125, 1977 27. A. A. PANASENKO, V. N, ODINOKO'V, Yu. B. MONAKO'V, L. M. KI-IAL1][JO'V,A. S. BEZGINA, V. K. IGNATYUK and S. R. RAFI'KOV, Vysokomol. soyed. BI9 : 656, 1977 (Not translated in Polymer Sci. U.S.S.R..)

I PolYmer" N [

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N . G . MARINA et al.

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