Activity and Microstructure Variations with Temperature in Conjugated

Macromolecules 2005, 38, 6327-6335

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Activity and Microstructure Variations with Temperature in Conjugated Diene Polymerizations Catalyzed by CpTiCl3-MAO Chiara Costabile, Gaetano Guerra,* Pasquale Longo, and Stefania Pragliola Dipartimento di Chimica, Universita` di Salerno, Via Salvador Allende, I-84081 Baronissi, Salerno, Italy Received March 8, 2005; Revised Manuscript Received May 12, 2005

ABSTRACT: The activity and the polymer microstructures obtained for polymerizations of four different conjugated dienes catalyzed by CpTiCl3-MAO have been closely compared for a broad temperature range (-45 to +70 °C). Apparent activation energies of 16.1 and 13.1 kcal/mol have been obtained for the poorly chemoselective butadiene and (E)-1,3-pentadiene polymerizations, while an apparent activation energy close to 8 kcal/mol has been obtained for the highly 1,2-syndiospecific polymerization of 4-methyl-1,3pentadiene. As for (Z)-1,3-pentadiene polymerization, previous experimental evidences of two competing mechanisms have been confirmed. A molecular modeling analysis, comparing three possible polymerization mechanisms for the four considered dienes, contributes to rationalization of the available polymerization results.

1. Introduction

Scheme 1

Several polymerization studies of different conjugated dienes, in the presence of CpTiCl3 (where Cp is cyclopendienyl) and methylalumoxane (MAO) catalytic system, have been reported in the literature.1-8 The chemoselectivity of the polymerization is generally low for monomers with low-energy s-cis-η4 coordination, like butadiene (B) and (E)-1,3-pentadiene (EP), while the chemoselectivity and stereoselectivity can be high for monomers with high-energy s-cis-η4 coordination, like (Z)-1,3-pentadiene (ZP) and 4-methyl-1,3-pentadiene (4MP).1-8 Porri and co-workers have found that temperature has generally no effect on polymerization chemoselectivity, but for ZP for which polymers obtained at +20 °C consist almost exclusively of cis-1,4 units,5 while polymers obtained at -20 °C or below consist almost exclusively of 1,2 units.3 Large differences between polymerization activities, in the presence of the CpTiCl3-MAO catalytic system, of different conjugated dienes have been also reported.3,5,8 In particular, it has been established that the polymerization of 4MP is much faster than that of B, EP, and ZP. Moreover, as for ZP, the polymerization temperature does not only affect polymer microstructure, but it has also a surprising effect on the polymerization rate, which reaches a maximum at subambient temperatures (close to -40 °C).3 However, a comprehensive comparison between polymerization tests of different diene monomers for different temperatures is presently not available. In the first part of this paper, the activity and the polymer microstructures obtained for polymerizations of four different conjugated dienes (B, EP, ZP, 4MP), when catalyzed by CpTiCl3-MAO, have been closely compared for a broad temperature range (-45 °C to +70 °C). Several diene polymerization data have been rationalized by the traditional and generally accepted polymerization mechanism involving monomer-bound intermediates that present an anti-η3 coordination of the * Author to whom correspondence should be addressed. Email: [email protected].

allyl terminal growing chain and a s-cis-η4 monomer coordination, indicated as mechanism I in Scheme 1.9-11 (An analogous scheme was already published in ref 12). However, Porri and co-workers suggested that 1,2syndiospecific polymerizations of ZP and 4MP by CpTiCl3-MAO2-3,5,7 could be rationalized by a different mechanism involving the insertion of a trans-η2coordinated monomer with consequent formation of a syn-η3 allyl group.3 The occurrence of a different mechanism for 1,2-syndiospecific polymerizations of ZP and 4MP was also clearly indicated by copolymerization tests. For instance, B and 4MP copolymerizations tests,

10.1021/ma0504926 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/28/2005

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in the presence of CpTiCl3-MAO, produce only polybutadiene, although 4MP homopolymerization is much faster than B homopolymerization.8 It is also worth adding that several experimental data obtained by Zambelli and co-workers have clearly shown that the high chemo- and stereoselectivities observed for polymerizations of ZP and of 4MP involve a relevant role of the growing chain.4,6-7 In fact, both 1,2 and cis1,4 chemoselectivities are heavily reduced for monomer insertion steps involving an attack on an allyl growing chain that does not present a double bond in the penultimate inserted unit. In particular, the study of the initiation step for the polymerization of 4MP (in the presence of a catalytic system prepared with CpTiCl3MAO and Al(13CH3)3) showed that the first monomeric unit can be both 1,2 and 1,4 inserted.4 Moreover, copolymerizations with ethylene of ZP7,12 as well as of 4MP6 give irregular copolymers containing substantial amounts of both 1,2 and cis-1,4 units adjacent to ethylene-inserted units. In recent years, molecular modeling studies have confirmed that mechanism I of Scheme 1 would be of minimum energy for monomers presenting a low energy of s-cis-η4 coordination, like B and EP.13-16 Moreover, these studies have indicated that mechanism I would be generally poorly chemoselective.13-16 Molecular modeling studies have also allowed identification of an alternative mechanism involving the insertion of a trans-η2-coordinated monomer with consequent formation of a syn-η3 allyl group (mechanism II in Scheme 1),15 which includes the essential features anticipated by Porri and co-workers for 1,2-syndiospecific polymerization of ZP and 4MP. According to that molecular modeling analysis, monomer-bound and monomer-free intermediates of mechanism II of Scheme 1 would be of minimum energy for monomers presenting a high energy of s-cis-η4 coordination, like ZP and 4MP. Moreover, for mechanism II, as suggested by Zambelli and co-workers,7 the occurrence of the high 1,2-syndiospecificity for 4MP and low-temperature polymerization of ZP2-3,5,7 would be determined by the steric constraint associated with the maintenance, in monomerbound intermediates, of the back-biting coordination of the syn-η3-allyl growing chain.16 A recent preliminary molecular modeling study has suggested the possible occurrence of an additional polymerization mechanism (mechanism III in Scheme 1), which would be cis-1,4 stereoselective, whose selectivity would be determined by the steric constraint associated with the maintenance, in monomer-bound intermediates, of the back-biting coordination of an antiη3-allyl growing chain.12 This new mechanism III could rationalize the occurrence of cis-1,4 selectivity, which is observed for ZP homopolymerization at high temperatures.12 In the second part of this paper, a molecular modeling analysis is reported that compares intermediates and, when possible, transition states of the three polymerization mechanisms for the four considered dienes. This analysis, completing a previous one relative to mechanisms I and II only for B, ZP, and 4MP,15 is aimed at contributing to rationalization of the reported polymerization kinetic data. 2. Experimental Section Polymerization. General Procedure. All the operations were performed under nitrogen atmosphere by using conven-

Macromolecules, Vol. 38, No. 15, 2005 tional Schlenk-line techniques. Toluene was refluxed over sodium diphenylketyl for 48 h and distilled before use. Methylaluminoxane (10% in toluene, Witco) was used as a solid after distillation of solvent. CpTiCl3 (>99%), 1,3-butadiene (>99%), (E)-1,4-pentadiene (98%), (Z)-1,3-pentadiene (98%), and 4-methyl-1,3-pentadiene (98%) were purchased from Aldrich and purified by distillation in the presence of Al(i-Bu)3. The CpTiCl3-MAO catalyst was preformed in toluene and kept to age at 50 °C for 10 minutes to avoid the formation of catalytic species that may be blocked or slowed for polymerizations performed at low temperature. Runs 1-6. Polymerizations of 1,3-butadiene were performed by introducing 19 mL of dry toluene into 100-mL glass flasks equipped with a magnetic stirrer. The flasks were cooled with liquid nitrogen, and the inert gas was evacuated. 1,3Butadiene (0.8 g, 0.015 mol) was condensed into the flask. Then, the reactors were quickly thermostated at the reaction temperature, and polymerizations were started by injection of 1 mL of toluene solution of preformed catalyst CpTiCl3 (1.3 mg, 6.0 × 10-6 mol)-MAO (116 mg, 2.0 × 10-3 mol, based on Al). Polymerizations were stopped by introducing a few milliliters of ethanol. Then, the polymers were coagulated in an excess of acidified ethanol, washed several times with fresh ethanol, and dried in vacuo at room temperature. Runs 7-27. Polymerizations of (E)-1,4-pentadiene, (Z)-1,3pentadiene, and 4-methyl-1,3-pentadiene were carried out in 50-mL glass flasks equipped with a magnetic stirrer by sequentially introducing toluene and monomers. After thermostating at polymerization temperature, reactions were started by injecting 1 mL of toluene solution of preformed catalyst CpTiCl3-MAO. The amounts of reagents are reported in Table 2-4. Polymerizations were stopped by introducing a few milliliters of ethanol. Then, the polymers were coagulated in an excess of acidified ethanol, washed several times with fresh ethanol, and dried in vacuo at room temperature. Repeated polymerization tests have shown that our experimental error relative to the polymerization activities is definitely larger for 4MP than for that of the other diene monomers. This is mainly due to the low catalyst concentrations (typically 10-5 mol/l) and to the short times (5-10 min, see Table 4) required to get low conversions (less than 10%), which are suitable for activity evaluations. Polymer Analysis. 13C NMR spectra were recorded on an AX 400 Bruker spectrometer operating at 100 MHz at 298 K. The samples were prepared by dissolving 40 mg of polymer in 0.5 mL of CDCl3. TMS was used as an internal chemical shift reference. The absence of signals relative to the terminal carbons in our 13C NMR spectra clearly indicates that, for all the polymers obtained by our polymerization tests, the molecular weight never decreases below 5 × 103 uma.

3. Models and Computational Details 3.1. Models. The models considered correspond to both monomer-free and monomer-bound intermediates. The occurrence of both kinds of intermediates is widely accepted in the literature and is on the basis of NMR and X-ray diffraction characterizations of π-allyl complexes.17 The kinds of units which can, in principle, be achieved as a consequence of diene monomer insertion have been labeled as cis-1,4-like or 1,2-unlike. Of course, 1,4 and 1,2 refer to the constitution of diene units, and cis refers to the configuration of double bonds along the chain in 1,4 units. As for the like and unlike nomenclature, it indicates that the corresponding intermediate would possibly lead to isotactic and syndiotactic units, respectively. In particular, isotactic and syndiotactic stereoregularity could occur for all diene monomers in the case of 1,2 polymerization, while it would occur only for 4-alkyl-substituted (like EP and ZP) in the case of 1,4 polymerization.9-10,13-16

Conjugated Diene Polymerizations Catalyzed by CpTiCl3-MAO

Macromolecules, Vol. 38, No. 15, 2005

Table 1. Polymerizations of 1,3-Butadiene

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Table 2. Polymerizations of (E)-1,3-Pentadiene

runa

temperature (°C)

time (h)

yield (mg)

activityb

runa

temperature (°C)

time (h)

yield (mg)

activityb

1 2 3 4 5 6

-45 -20 0 20 50 70

102 15 1 0.5 0.033 0.083

11 29 31 77 126 134

2.4 × 10 4.4 × 102 7.0 × 103 3.5 × 104 8.5 × 105 3.5 × 105

7 8 9 10 11 12

-45 -20 0 20 50 70

96 48 8 0.33 0.33 0.33

14 30 42 32 111 55

4.7 × 10 2.0 × 102 1.7 × 103 3.5 × 104 1.2 × 105 6.0 × 104

a All the runs were performed utilizing 6.0 × 10-6 mol of CpTiCl3, 2 × 10-3 mol of MAO, 19 mL of toluene, 0.8 g of monomer. b Activity in (g polymer)/[(mol catalyst) × (h) × (mol/L monomer)].

Transition states relative to the monomer insertion reactions (i.e., connecting the monomer-bound intermediates to the corresponding monomer-free intermediates) have been evaluated only for mechanism I because the absence of the back-biting coordination of the growing chain makes modeling easier. 3.2. Computational Details. Stationary points on the potential energy surface were calculated with the Amsterdam density functional (ADF) program system developed by Baerends et al.18 The electronic configurations of the molecular systems were described by a triple-ζ STO basis set on Ti for 3s, 3p, 3d, 4s, and 4p. Double-ζ STO basis sets were used for C (2s, 2p) and H (1s). The basis sets on C are augmented with a single 3d polarization function except for H, where a 2p function was used. The 1s22s22p6 configuration on titanium and 1s2 configuration on carbon were assigned to the core and treated within the frozen core approximation. Energetics and geometries were evaluated by using the local exchange-correlation potential by Vosko et al.,19 augmented in a self-consistent manner with Becke’s20 exchange-gradient correction and Perdew’s21 correlation-gradient correction. First-order scalar relativistic corrections were added to the total energy because a perturbative relativistic approach is sufficient for 3d metals. Because of the open-shell character of the systems under study, an unrestricted formalism has been used. All the structures that will follow are stationary points on the potential energy surface. Geometry optimizations were terminated if the largest component of the Cartesian gradient was smaller than 0.002 au. A considerable amount of related computational studies have contributed to the comprehension of fine details of olefin polymerizations with both early and late transition metals,22 of styrene polymerization with Cpbased titanium catalysts,23 and of butadiene polymerization with Ni(II)-based catalysts,24-28 and finally, butadiene polymerization with CpTiCl3-MAO in our previous paper.13-14,16 Furthermore, a comparative study has shown that the DFT functional we have chosen is in excellent agreement with one of the best wave function-based methods available today to investigate polymerization reactions with Ziegler-Natta catalysts.29 4. Results 4.1. Polymerization Tests at Different Temperatures. Polymerization tests for four different conjugated diolefins were carried out in the presence of CpTiCl3-MAO at different reaction temperatures (from -45 up to 70 °C) and at constant monomer concentration. The polymer yield, the catalyst activity, and the microstructure of the obtained polymers, for B, EP, ZP, and 4MP are reported in Tables 1-4, respectively.

a All the runs were performed utilizing 6 × 10-6 mol of CpTiCl , 3 2 × 10-3 mol of MAO, 19 mL of toluene, 0.7 g of monomer. b Activity in (g polymer)/[(mol catalyst) × (h) × (mol/L monomer)].

Table 3. Polymerizations of (Z)-1,3-Pentadiene runa

temperature (°C)

yield (mg)

activityb

structure

13 14 15

-60 -45 -20

10 70 31

2.9 × 10 2.3 × 102 1.0 × 102

16

0

15

4.8 × 10

17

20

27

8.8 × 10

18

50

63

2.1 × 102

19

70

28

9.2 × 10

1,2 (>99%) 1,2 (>99%) 1,2 (94%) 1,4-cis (6%) 1,2 (42%) 1,4-cis (55%) 1,4-trans (3%) 1,4-cis (96%) 1,4-trans (4%) 1,4-cis (92%) 1,4-trans (8%) 1,4-cis (92%) 1,4-trans (8%)

a All the runs were performed utilizing 6 × 10-6 mol of CpTiCl , 3 2 × 10-3 mol of MAO, 20 mL of toluene, 0.73 g of monomer. Time 96 h. b Activity in (g polymer)/[(mol catalyst) × (h) × (mol/L monomer)].

Table 4. Polymerizations of 4-Methyl-1,3-Pentadiene runa

temperature (°C)

time (min)

yield (mg)

activityb

20 21 22 23 24 25 26 27

-45 -20 -10 0 10 20 50 70

60 60 10 10 10 5 5 5

25 60 83 21 20 167 58 46

5.1 × 105 1.2 × 106 1.0 × 107 2.5 × 106 2.4 × 106 4.0 × 107 8.4 × 107 6.1 × 107

a All the runs were performed utilizing 1 × 10-7 mol of CpTiCl , 3 2 × 10-3 mol of MAO, 15 mL of toluene, 0.82 g of monomer. b Activity in (g polymer)/[(mol catalyst) × (h) × (mol/L monomer)].

The activity data for polymerization tests with the four monomers, expressed as (g polymer)/[(mol catalyst) (h) (mol/L monomer)] and evaluated for conversions lower than 10%, are reported versus polymerization temperature in Figure 1. For all the considered monomers, the polymerization activity slightly decreases in the temperature range 5070 °C. This clearly suggests the possible occurrence of a partial deactivation of the catalyst above 50 °C. The observed reactivities of B and EP are somewhat similar. In fact, their polymerization activities are close, and by increasing the temperature, they increase with a quite regular trend up to nearly 50 °C. Moreover, for both monomers, the polymerization is poorly chemoselective and the achieved microstructure is poorly dependent on the polymerization temperature. For instance, the concentrations of 1,4-cis, 1,2-, and 1,4-trans units, evaluated by 13C NMR, for B were 79, 10, and 11% at -45 °C and 79, 13, and 8% at 70 °C, while for EP they were 51, 46, and 3% at -45 °C and 45, 52, and 3% at 70 °C, respectively. Because of the regular activity increase with temperature and because of the independence of the polymer

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Figure 1. Polymerization activity data, expressed as (g polymer)/[(mol catalyst) × (h) × (mol/L monomer)] versus polymerization temperature for the four monomers: B (butadiene); EP ((E)-pentadiene); ZP ((Z)-pentadiene) and 4MP (4methyl-pentadiene).

microstructure on the polymerization temperature, it is reasonable to assume the occurrence of a same mechanism for the whole temperature range. In particular, by using the activity data as obtained for the temperature range -45 to +50 °C for polymerizations catalyzed by CpTiCl3-MAO of B and EP, apparent activation energies of 16.1 and 13.1 kcal/mol are obtained, respectively. These data are in the range of values of activation energy (12-20 kcal/mol), which have been measured30 or calculated26 for Ziegler-Natta 1,4-cis polymerizations of butadiene. The activity of the CpTiCl3-MAO catalytic system for polymerization of ZP does not follow a regular trend in the examined temperature range (Figure 1). In fact, at -45 °C, the polymerization activity is higher than for that of B and EP of an order of magnitude, while it significantly decreases as the polymerization temperature increases up to 0 °C, where a minimum of activity is observed. In agreement with previous literature reports,3,5 the polymers obtained for ZP present a microstructure strongly dependent on the temperature: 1,2-syndiotactic below -20 °C and substantially 1,4-cis above +20 °C (Table 3). The polymers obtained by 4MP polymerization at all temperatures present a highly stereoregular 1,2-syndiotactic structure. For the whole range of polymerization temperature, the activity of 4MP is higher than that for all other dienes. In particular, at -45 °C, the activity of 4MP is higher by 3-4 orders of magnitude, while the increase of the activity with increasing temperature is less steep than that observed for B and EP. Correspondingly, by using the activity data as obtained for the temperature range -45 °C to +50 °C, an apparent activation energy of 8 kcal/mol has been obtained. 4.2. Molecular Modeling. 4.2.1. Monomer-Bound Intermediates. Minimum-energy monomer-bound intermediates, for the four considered conjugated dienes and for the three polymerization mechanisms described in Scheme 1, have been modeled by DFT techniques. In particular, monomer-bound intermediates I (corresponding to mechanism I) present an anti-η3 coordination of the allyl terminal of the growing chain and a s-cis-η4 monomer coordination. Monomer-bound intermediates II (corresponding to mechanism II) present a


back-biting syn-η3-allyl growing chain (obtained by a previous 1,2-unlike enchainment) and a trans-η2 coordination of the monomer. Finally, monomer-bound intermediates III (corresponding to mechanism III), present a back-biting anti-η3-allyl growing chain (obtained by a previous 1,4-like enchainment) and a s-cisη2 coordination of the monomer. The minimum energies of the monomer-bound intermediates for the three mechanisms are listed in the second column of Tables 5-8, for B, EP, ZP, and 4MP, respectively. For each monomer, the zero energy has been fixed to the minimum-energy monomer-bound intermediate. Butadiene Monomer-Bound Intermediates (Second Column of Table 5). The monomer-bound intermediate I, which presents a lower energy with respect to intermediates II and III (∆E ) +2.1 and +2.5 kcal/ mol, respectively), is shown in the middle of Figure 2. The lower energy of I is in part associated with a reduction of steric hindrance because of the removal of the back-biting, which allows extension of the rest of the growing chain out of the metal coordination sphere. Moreover, the loss of coordination energy associated with the removal of the back-biting is largely compensated by the energy gain corresponding to the s-cis-η4 monomer coordination rather than to the η2 monomer coordination (typical of intermediates II and III). (E)-Pentadiene Monomer-Bound Intermediates (Second Column of Table 6). As for the case of B, the monomer-bound intermediate I, presenting an antiη3 coordination of the allyl terminal of the growing chain and a s-cis-η4 monomer coordination, is the most favored. (Z)-Pentadiene Monomer-Bound Intermediates (Second Column of Table 7). As already described by previous preliminary calculations,14,15 the intermediate II (shown in the upper part of Figure 3), involving a back-biting (syn-η3)-η2 allyl coordination of the growing chain, is energetically strongly favored. In fact, both I and III (the latter shown in the lower part of Figure 3) present a cisoid anti-η3 coordination of the allyl terminal of the growing chain, and hence, both C5′ and C1′ are anti with respect to the allyl group and generate high steric interactions. Moreover, III presents a lower energy (-2.5 kcal/mol) with respect to I. In fact, the loss of the back-biting in I is not compensated by the coordination of the monomer, which for ZP cannot be perfectly s-cis-η4 because of the cisoid interaction of the methyl group (C5). 4-Methyl-pentadiene Monomer-Bound Intermediates (Second Column of Table 8). As in the case of ZP and for the same reasons, the most favored intermediate is the monomer-bound intermediate II, presenting a back-biting (syn-η3)-η2 allyl growing chain and a trans-η2 coordination of the monomer (shown in Figure 4). 4.2.2. Insertion Products (Monomer-Free Intermediates). Minimum-energy monomer-free intermediates, corresponding to the completion of monomer insertion reactions for the four considered conjugated dienes and for the three polymerization mechanisms described in Scheme 1, have also been modeled by DFT techniques. Both monomer insertion products corresponding to the formation of cis-1,4 or 1,2 units, can be generally achieved, starting from the monomer-bound intermediate I that is in the absence of back-biting of the allyl



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Figure 2. Butadiene-bound intermediate (center) and monomer-free intermediates corresponding to cis-1,4 (left) and 1,2syndiotactic (right) butadiene insertion, in the framework of mechanism I. The cis-1,4 or 1,2 insertions occur by formation of a bond between a terminal diene carbon atom (1) and a terminal (4′) or internal (2′) carbon atom of the coordinated allyl group. The Cp ligand is indicated only by sticks. The carbon gray atoms are directly bonded to the metal. Distances are in Å. Table 5. Calculated Energies of Monomer-Bound and Postinsertion Intermediates for Butadiene Polymerization Mechanisms I, II, and III, and Insertion Transition States Relative to Mechanism I butadiene mechanisms I II III

monomer-bound

TS

0/0 2.1 2.5

3.0/4.3

postinsertion -12/-11.5 -8.5 -12

enchainment cis-1,4-like/1,2-unlike 1,2-unlike cis-1,4-like

Table 6. Calculated Energies of Monomer-Bound and Postinsertion Intermediates for (E)-Pentadiene Polymerization Mechanisms I, II, and III, and Insertion Transition States Relative to Mechanism I (E)-pentadiene mechanisms I II III

monomer-bound

TS

0/1.1 2.8 2.5

4.7/5.9

postinsertion -7.3/-8.5 -3.3 -7.3


Table 7. Calculated Energies of Monomer-Bound and Postinsertion Intermediates for (Z)-Pentadiene Polymerization Mechanisms I, II, and III, and Insertion Transition States Relative to Mechanism I (Z)-pentadiene mechanisms I II III

monomer-bound

TS

7.9/8.0 0 5.5

12.0/12.0

post-insertion -3.2/-3.2 -9.0 -3.2


Table 8. Calculated Energies of Monomer-Bound and Postinsertion Intermediates for 4-Methyl-pentadiene Polymerization Mechanisms I, II, and III, and Insertion Transition Sstates Relative to Mechanism I 4-methyl-pentadiene mechanisms I II III

monomer-bound

TS

5.7/5.6 0 8.1

12.1/13.3

terminal of the growing chain. In fact, cis-1,4 or 1,2 units are obtained, depending on whether the incoming monomer reacts at the terminal or internal allyl carbon, respectively.9-11 Just as an example, models of products corresponding to cis-1,4 and 1,2 insertions of butadiene are shown in right and left parts of Figure 2, respectively. On the other hand, monomer-bound intermediates II and III, which present a back-biting coordination of the allyl growing chain because of the associated geometrical constraints, are expected to be highly chemoselective. In fact, intermediate II (e.g, models shown for ZP in the upper part of Figures 3 and for 4MP in Figure 4) presents the reactive terminal carbon atom 1 of the

postinsertion -6.3/-7.3 -12 -6.3


diene, with orientation and proximity suitable for reaction with the internal allyl carbon 2 and unsuitable for reaction with the terminal allyl carbon 4. Instead, intermediate III (e.g, model shown for ZP in the lower part of Figure 3) presents the reactive terminal carbon atom 1 of the diene, with orientation and proximity suitable for reaction with the terminal allyl carbon 4 and unsuitable for reaction with the internal allyl carbon 2. These simple geometrical considerations on monomer-bound intermediates, as well as rough molecular modeling of transition states of the insertion reactions, suggest that reaction routes starting from intermediates II and III lead selectively to insertion products corresponding to the formation of 1,2 and cis-

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Figure 3. Upper part: ZP-bound intermediate (center) and monomer-free intermediate corresponding to 1,2-syndiotactic (right) ZP insertion, in the framework of mechanism II. The 1,2 insertion occurs by formation of a bond between the terminal diene carbon atom (1) and the internal (2′) carbon atom of the coordinated allyl group. Lower part: ZP-bound intermediate (center) and monomer-free intermediate corresponding to cis-1,4 (left) ZP insertion, in the framework of mechanism III. The cis-1,4 insertion occurs by formation of a bond between the terminal diene carbon atom (1) and terminal (4′) carbon atom of the coordinated allyl group. The Cp ligand is indicated only by sticks. The carbon gray atoms are directly bonded to the metal. Distances are in Å.

Figure 4. 4MP-bound intermediate (left) and monomer-free intermediate corresponding to 1,2-syndiotactic (right) 4MP insertion, in the framework of mechanism II. The 1,2 insertion occurs by formation of a bond between the terminal diene carbon atom (1) and the internal (2′) carbon atom of the coordinated allyl group. The Cp ligand is indicated only by sticks. The carbon gray atoms are directly bonded to the metal. Distances are in Å.

1,4 units, respectively (as shown by the models of Figures 3 and 4 and as sketched in Scheme 1). The minimum energies of insertion products for the three mechanisms are listed in the fourth column of Tables 5-8, for B, EP, ZP, and 4MP, respectively. For each monomer, the zero energy has been fixed to the minimum-energy monomer-bound intermediate, indicated in the second column of Tables 5-8. Butadiene Insertion Products. (Fourth Column of Table 5). The cis-1,4 and 1,2 insertion products

obtained from the monomer-bound intermediate I for B polymerization, shown in the left and right sides of Figure 2, present free energies that are much lower than that for the starting monomer-bound intermediate. As already discussed in ref 14, the energy differences between the two intermediates are small (in favor of the cis-1,4 unit of nearly 0.5 kcal/mol). (E)-Pentadiene Insertion Products (Fourth Column of Table 6). As for B, the cis-1,4 and 1,2 insertion products obtained from the monomer-bound intermedi-



ate I for EP polymerization, present free energies that are similar and much lower than that for the starting monomer-bound intermediate. (Z)-Pentadiene Insertion Products. (Fourth Column of Table 7). Starting from the minimum-energy monomer-bound intermediate II, only the ZP insertion product corresponding to the formation of a 1,2 unit (shown in the upper right part of Figure 3) can be easily achieved. Its free energy is nearly 9 kcal/mol lower than that for the starting monomer-bound intermediate.15 Starting from the minimum-energy monomer-bound intermediate III, only the ZP insertion product corresponding to the formation of a cis-1,4 unit (shown in the lower-left part of Figure 3) can be easily achieved. Its free energy is nearly 8 kcal/mol lower than that for the starting monomer-bound intermediate. 4-Methyl-pentadiene Insertion Products (Fourth Column of Table 8). Starting from the minimumenergy monomer-bound intermediate II, only the 4MP insertion product corresponding to the formation of a 1,2 unit (shown in the right part of Figure 4) can be easily achieved. Its free energy is nearly 12 kcal/mol lower than that for the starting monomer-bound intermediate. 4.2.3. Transition States for Monomer Insertion. In the framework of mechanism I, the monomercoordinated species (like that one shown for B in Figure 2) could give both 1,4 and 1,2 insertions by approaching C1 of butadiene to the terminal and to the internal allyl carbons, respectively. As for B, the transition states for 1,4 and 1,2 insertions have already been calculated and presented in Figure 4a and 4b of ref 14, respectively. The analogous insertion transition states have now been calculated for EP, ZP, and 4MP, and their energies are shown in Tables 6-8 (1st row, 3rd column). All transition states present a distance between the two reactant carbons close to 2.1 Å. For all the considered monomers, the 1,2-unlike transition state presents only slightly larger energy (≈1 kcal/mol). The energy increases with respect to the monomer-coordinated intermediates are small. For instance, the calculated energy barriers for the 1,4 insertion is only 3 and 4.7 kcal/mol for B and EP, respectively. As for diene insertions, according to mechanisms II and III, it is much more difficult to locate the transition states because of geometrical constraint given by the back-biting of the growing chain. In fact, because we were not able to precisely define the minimum-energy reactions paths, we have limited our analyses to the monomer-bound and monomer-free intermediates of the previous sections. 4.2.4. Polymerization Internal Energy. For the minimum-energy insertion mechanisms of the four monomers, we have also evaluated the energy difference between intermediates presenting n or n + 1 monomeric units in the growing chain. In particular, this difference has been evaluated as -19.3, -17.1, -17.8, and -17.4 kcal/mol, for B, EP, ZP, and 4MP, respectively. In our framework, these calculated values correspond to the reduction of internal energy associated with each polymerization step and well compare with experimental values of heat of polymerization of butadiene (17.5-18.5 kcal/mol).31 5. Discussion 5.1. Information on Diene Polymerization Mechanisms from Activity and Microstructure Varia-

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Figure 5. Activity, expressed as (g polymer)/[(mol catalyst) × (h) × (mol/L monomer)] (left scale) and polymer microstructure expressed as percent of 1,2 units (right scale) for ZP polymerizations in the presence of CpTiCl3-MAO vs the polymerization temperature.

tions with Temperature. The similar activation energies measured for B and EP polymerizations, as well as the invariance with temperature of irregular polymer microstructures, indicate the occurrence of a common poorly chemoselective mechanism. The much lower activation energy measured for 4MP polymerization, as well as the invariance with temperature of the highly stereoregular 1,2-syndiotactic microstructure, can suggest the occurrence of a different highly selective mechanism. The anomalous temperature dependence of the activity data, and in particular, the significant decreases of reactivity observed in the polymerization of ZP for temperature increases up to 0 °C, can be rationalized by assuming the competition between two monomer insertion mechanisms. In particular, an energetically favored mechanism would operate at low temperatures, and an entropically favored mechanism would operate at high temperatures, and the reduction of activity with increasing temperature could depend on a lower reactivity of the insertion product of the entropically favored mechanism. This hypothesis of two competing mechanisms (already suggested in refs 3, 7, 8) is confirmed by a close comparison between the variation of activity (left scale) and microstructure (right scale) with temperature, shown in Figure 5, being somewhat similar to Figure 3 of ref 3. It is clearly shown that the occurrence of 1,4 insertions heavily reduces the ZP polymerization activity and only for temperatures higher than +20 °C; when all the monomer units are 1,4 inserted, the activity starts again to increase, as usual, with temperature. In summary, these results clearly indicate the occurrence for ZP polymerization of a competition between two insertion mechanisms, an energetically favored one, leading to 1,2-syndiotactic selectivity and an entropically favored one, leading to cis-1,4 selectivity. 5.2. Competition Between Polymerization Mechanisms. The molecular modeling calculations (reported in the second part of this paper) can help to rationalize the experimental results realtive to activity and polymer microstructure for polymerization tests conducted in a broad temperature range (reported in the first part of this paper). The poorly chemoselective mechanism that would operate for polymerization of B and EP for the whole

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examined temperature range would be the traditional mechanism I, which for these monomers presents minimum-energy monomer-bound as well as monomerfree intermediates (Tables 5 and 6). As for 4MP, the highly 1,2-syndiospecific mechanism would be instead mechanism II, which for this monomer with a high-energy s-cis-η4 coordination presents minimum-energy monomer-bound as well as monomer-free intermediates (Table 8). As for polymerization of ZP, the energetically favored mechanism, which would operate at low temperatures and is stereoselective in favor 1,2-syndiotactic insertion, would be again mechanism II, which for this monomer (with a high-energy s-cis-η4 coordination as for 4MP) presents minimum-energy monomer-bound as well as monomer-free intermediates (Table 7). The competing polymerization mechanism that would operate at high temperatures, being stereoselective in favor cis-1,4 insertion, would be mechanism III. In this respect, it is worth noting that, only for ZP, the monomer-bound intermediate for mechanism III is lower in energy with respect to that of mechanism I (of nearly 2.5 kcal/mol), while for the other three monomers (B, EP, and 4MP), the monomer-bound intermediate for mechanism I is lower in energy with respect to that of mechanism III (again, of nearly 2.5 kcal/mol). As discussed in section 5.2, the reduction of ZP polymerization activity with increasing temperature could be rationalized by a lower reactivity of the insertion product of the high-temperature mechanism. In this respect, it is worth noting that the cis-1,4 insertion product of the high-temperature mechanism III (lower part of Figure 3) compared with the 1,2 insertion product of the low-temperature mechanism II (upper part of Figure 3) could present a higher energy barrier for monomer coordination because of the shorter distance between C5 and C1′ (3.63 Å) versus the distance between C5 and C5′ (4.14 Å) (see Figure 3), as well as for the alkyl substitution of C1′. 5.3. Reaction Pathways. The easy modeling, in the framework of mechanism I, of the transition state of the diene insertion step and the experimental evaluation of the activation energy of B and EP polymerization reactions (see Section 4.1), allow attempting a more detailed description of the polymerization pathway relative to these monomers. The calculated internal energy reaction profiles for B and EP are shown by continuous lines in Figures 6A and B, respectively. To get a free energy profile, the T∆S contribution has been based for all monomers on the experimental entropy of polymerization of butadiene (nearly 20 cal/Kmol).31 Because the measured activation energies (16 and 13 kcal/mol) are much larger than the activation energies calculated for monomer insertion (3 and 4.7 kcal/mol), reaction paths compatible with our experimental and modeling results can be obtained by assuming that the observed activation energies correspond to the monomer coordination steps, as shown by dashed lines in Figure 6A and B. This assumption is not unreasonable because diene monomer coordination generally can require replacement of solvent or counterions and, in the framework of mechanism I, also requires the removal of the back-biting coordination of the growing chain. This assumption can also rationalize the much lower polymerization activation energy measured for 4MP (≈8 kcal/mol). In fact, the activation energy for 4MP coor-


Figure 6. Polymerization free energy profiles as calculated for B and EP. The dashed lines correspond to the possible assumption that the experimentally observed activation energies could correspond to the activation energy of the monomer coordination steps.

dination is expected to be smaller, not only because of the trans-η2 rather than s-cis-η4 monomer coordination, but also because of the maintenance of the back-biting coordination of the growing chain in the framework of mechanism II. 6. Conclusions The activity and the polymer microstructures obtained for polymerizations of four different conjugated dienes (B, EP, ZP, 4MP), when catalyzed by CpTiCl3MAO, have been closely compared for a broad temperature range (-45 to +70 °C). A molecular modeling analysis, which compares intermediates and, when possible, transition states of the three polymerization mechanisms of Scheme 1 for the four considered dienes, has also been reported. On the basis of our experimental and modeling analyses we have several conclusions relative to possible mechanisms of diene polymerization in the presence of the catalytic system CpTiCl3-MAO: (i) The poorly chemoselective mechanism that would operate for polymerization of B and EP would be the traditional mechanism I, for which the monomer-bound intermediate presents anti-η3 and s-cis-η4 coordination of growing chain and monomer, respectively.



(ii) The 1,2-syndiospecific mechanism that would operate for 4MP and for low-temperature polymerizations of ZP would be mechanism II, for which the monomer-bound intermediate presents back-biting synη3-η2 and trans-η2 coordination of growing chain and monomer, respectively. (iii) The cis-1,4 mechanism, that would operate for high-temperature polymerizations of ZP would be mechanism III, for which the monomer-bound intermediate presents back-biting anti-η3-η2 and s-cis-η2 coordination of growing chain and monomer, respectively. (iv) The significant decreases of reactivity, observed in the polymerization of ZP by increasing the temperature up to 0 °C, can be rationalized by assuming a competition between mechanisms II and III. In particular, the reduction of activity with increasing temperature would depend on a lower reactivity of the insertion product of the high-temperature mechanism III. (v) Polymerization activation energies as measured for B and EP (16 and 13 kcal/mol) are much higher than that for 4MP (8 kcal/mol). These results could be rationalized by the assumption that the rate-determining step would be the diene monomer coordination (see, e.g., the reaction profiles of Figure 6). Acknowledgment. We thank Prof. Paolo Corradini of the University of Naples, Prof. Lido Porri of the Polytechnic of Milan, and Prof. Adolfo Zambelli, Prof. Luigi Cavallo, and Dr. Giuseppe Milano of the University of Salerno for useful discussions. This work was supported by the Italian National Research Council (CNR) and by the Ministry of University of Italy (Grant PRIN-2004, FISR). References and Notes (1) Oliva, L.; Longo, P.; Grassi, A.; Ammendola, P.; Pellecchia, C. Makromol. Chem., Rapid Commun. 1990, 11, 519-524. (2) Ricci, G.; Italia, S.; Porri, L. Macromolecules 1994, 27, 868869. (3) Ricci, G.; Porri, L.; Giarrusso, A. Macromol. Symp. 1995, 89, 383-392. (4) Longo, P.; Proto, A.; Oliva, P.; Zambelli, A. Macromolecules 1996, 29, 5500-5501. (5) Longo, P.; Oliva, P.; Proto, A.; Zambelli, A. Gazz. Chim. Ital. 1996, 126, 377-382. (6) Longo, P.; Grisi, F.; Proto, A.; Zambelli, A. Macromol. Rapid Commun. 1997, 18, 183-190. (7) Longo, P.; Guerra, G.; Grisi, F.; Pizzuti, S.; Zambelli, A. Macromol. Chem. Phys. 1998, 199, 149-154. (8) Porri, L.; Giarrusso, A.; Ricci, G. Macromol. Symp. 2002, 178, 55-68. (9) Porri, L.; Giarrusso, A. Comprehensive Polymer Science; Eastmond, G. C., Ledwith, A., Russo, S., Sigwalt, P., Eds.; Pergamon Press: Oxford, U.K., 1989; Vol. 4, Part II, p 53). (10) (a) Porri, L.; Giarrusso, A.; Ricci, G. Prog. Polym. Sci. 1991, 16, 405-441. (b) Porri, L.; Ricci, G.; Giarrusso, A. In Metallorganic Catalysts for Synthesis and Polymerization; Kamin-

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Activity and Microstructure Variations with Temperature in Conjugated

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