Mechanistic Studies on Conjugated Diene Polymerizations Promoted

Aug 9, 2012 - Antonio Buonerba , Maria Fienga , Stefano Milione , Cinzia Cuomo ... a novel family of catalysts for the copolymerization of carbon diox...
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Mechanistic Studies on Conjugated Diene Polymerizations Promoted by a Titanium Complex Containing a Tetradentate [OSSO]-Type Bis(phenolato) Ligand Chiara Costabile, Carmine Capacchione,* Daniela Saviello, and Antonio Proto Dipartimento di Chimica e Biologia and NANOMATES, Research Centre for NANOMAterials and nanoTEchnology, Università di Salerno, via Ponte don Melillo, I-84084 Fisciano (Salerno), Italy S Supporting Information *

ABSTRACT: A thorough experimental and theoretical mechanistic study on the conjugated dienes polymerization promoted by the postmetallocene complex dichloro{1,4dithiabutanediyl-2,2′-bis(4,6-di-tert-butyl-phenoxytitanium, 1, activated by methylaluminoxane (MAO), is presented. Experimental polymerization studies, previously reported on butadiene, isoprene, and 4-methyl-1,3-pentadiene, have been extended to E- and Z-1,3-pentadiene polymerizations and to ethylene-1,3-pentadiene copolymerizations. Complex 1/MAO showed to be unactive in the Z-1,3-pentadiene polymerization, whereas it was quite performing toward E-1,3-pentadiene, leading to a polymer containing a mixture of 1,2- and 1,4-trans units, with a prevalence of 1,2 units at low temperatures. Also for ethylene-1,3-pentadiene copolymerizations, complex 1/MAO showed good activity. The copolymer microstructure can be varied by changing the ratio between the monomers in the copolymerization feed, affording copolymers with 1,3-pentadiene content up to 36%. Density functional theory (DFT) mechanistic studies on butadiene, E- and Z-1,3-pentadiene polymerizations indicate that monomer insertions proceed through an allylic mechanism involving a syn-η3 coordination of the growing chain and a s-trans-η2 monomer insertion.

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INTRODUCTION The stereoselective polymerization of conjugated dienes promoted by transition metal complexes is an active task of research in both academic and industrial environments due to the relevance of these materials as synthetic rubbers. In particular, butadiene and isoprene have been the objective of intensive studies due to the large availability of these monomers and to the mechanical proprieties of the resulting materials while other 1,3-alkadienes have received rather limited attention.1 Among higher conjugated dienes, it is worth mentioning that 1,3-pentadiene exists in two isomeric forms E and Z, having different reactivities and chemoselectivities depending on the type of catalyst used. Notably, while many catalytic systems promote the polymerization of the E-1,3-pentadiene (E-PD)2 the Z-1,3-pentadiene (Z-PD) is less reactive and is polymerized by a rather limited number of catalytic systems.3 The stereoregular polymerization of 1,3-pentadiene has been the objective of many studies not only for the possibility to use as monomer a high-volume by product of petroleum chemistry but also to gain fundamental information about the behavior of a given catalyst in the polymerization of 1,3-alkadienes. In particular, in order to rationalize the behavior of one of the most active catalysts for diene polymerization, CpTiCl3, many experimental and computational work has been devoted to the chemo and stereoselective polymerization of butadiene, E-PD and Z-PD.4 A comparison between chemoselectivity and stereoselectivity of © 2012 American Chemical Society

this catalytic system with E and Z isomers of 1,3-pentadiene is particularly relevant. As a matter of fact E-PD affords at any temperature an irregular product containing substantial amounts of both cis-1,4 and 1,2 units, while Z-PD is stereospecifically polymerized to 1,2-syndiotactic polymer at −20 °C or less4a,b and 1,4-cis-isotactic polymer at temperatures higher than 20 °C.3a,4b More recently, in spite of the mushrooming of new group 4 complexes bearing noncyclopentadienyl ligands, the so-called postmetallocenes, active in the polymerization of 1-olefins5 the example of efficient catalysts for the stereoselective polymerization of 1,3-alkadienes are rather scarce.6 We have recently reported that the titanium catalyst precursors which incorporate a tetradentate ligand having two phenolate units linked through a 1,4-dithiaabutanediyl bridge S(CH2)2S are active in the stereoselective polymerization of various 1,3-alkadiene monomers and in the copolymerization with ethylene.7 Namely, the titanium complex 1 in Scheme 1, having 1,4-dithiabutanediyl bridge, showed to be highly active, after proper activation, in the stereoselective polymerization of butadiene, isoprene7b and 4-methyl-1,3pentadiene(4-MPD)7a and in the copolymerization of these monomers with ethylene.7c−e In particular, highly stereoregular 1,4-trans polybutadiene was obtained, as well as predominantly Received: June 8, 2012 Revised: July 20, 2012 Published: August 9, 2012 6363

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4, Table 2) was dissolved in toluene (75 mL), p-toluensulfonylhydrazine was added (4 g), and the resulting mixture was refluxed for 48 h under stirring. The reaction was stopped by quenching with ethanol (200 mL). The resulting polymer, recovered by filtration, washed with water and ethanol, was dried in vacuum until constant weight. Characterization of the Polymers. The 13C NMR spectra of the 1,3-pentadiene homopolymers and ethylene−1,3-pentadiene copolymers samples were recorded with a Bruker AVANCE 300 spectrometer (300 MHz for 1H and 75 MHz for 13C). Using 5 mm (o.d.) NMRtubes, polymer samples (30 mg) were dissolved in chloroform-d (0.7 mL) and analyzed at 298 K. Chemical shifts were referenced to TMS and calculated by using the residual isotopic impurities of the deuterated solvent (77.23 ppm for chloroform-d). The 1,3-pentadiene content of the copolymer was calculated from 1H NMR considering the integrals of the broad signal at 1.25 ppm relative to the methylene protons belonging to ethylene and 1,3-pentadiene units and the signal at 5.29 ppm belonging to the protons of unsaturated carbon bonds of the 1,3-pentadiene units inserted along the polymeric chain with 1,2- or 1,4-regiochemistry. The amount of 1,3-pentadiene inserted with 1,2- or 1,4-regiochemistry was calculated by considering the integrals of the methyl carbons at 17.6 and 20.6 ppm belonging, respectively, to the 1,4 and 1,2 units.

Scheme 1. Dichloro{1,4-dithiabutanediyl-2,2′-bis(4,6-di-tertbutyl-phenoxy)}titanium

1,4-trans-polyisoprene and stereoregular isotactic-poly-1,2-(4MPD). Herein, a wide mechanistic study on diene polymerization with postmetallocene complex 1 activated by MAO will be presented. In particular, to get further indications on the possible involved mechanism, polymerizations of E-PD and ZPD were performed as well as the corresponding ethylene-1,3pentadiene copolymerizations. Moreover, DFT calculations on the polymerization of butadiene, E-PD and Z-PD with catalyst 1/MAO have been conducted, to rationalize experimental data. Indeed, we will here report, for the first time, a theoretical study that shows mechanistic insights on conjugated diene polymerization with postmetallocene catalysts.

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COMPUTATIONAL DETAILS

The density functional calculations were performed on all the systems with the Gaussian09 set of programs.9 BP86 was used as a functional and gradient corrections were taken from the work of Becke and Perdew.10−12 The electronic configuration of the molecular systems was described by the split-valence basis set with polarization functions of Ahlirchs and co-worker (standard SVP basis set in Gaussian09), for H, C, O and S.13 For Ti, we used the small-core, quasi-relativistic Stuttgart/Dresden effective core potential basis set (standard SDD basis set in Gaussian09).14 Minimum free energy structures were characterized by the presence of zero imaginary frequency. Solvent effects have been estimated in calculations based on the polarizable continuous solvation model PCM. Toluene was chosen as model solvent.

EXPERIMENTAL PART

Materials. All manipulations of air- and/or water-sensitive compounds were performed under a nitrogen atmosphere using standard Schlenk techniques or a MBraun glovebox. Commercial grade toluene (Carlo Erba) was dried over calcium chloride, refluxed 48 h under a nitrogen atmosphere over sodium, and distilled before using. Polymerization grade ethylene, purchased from Societa′ Ossigeno Napoli (S.O.N.), was dried by passing through a column filled with activated molecular sieves (4 Å). 1,3-pentadiene (E/Z mixture) and E-1,3-pentadiene (Sigma-Aldrich) was purified by distillation over calcium hydride under nitrogen atmosphere. Methylalumoxane (MAO), purchased from Aldrich as a 10 wt % solution in toluene, was dried in vacuo at 50 °C to remove toluene and “free” AlMe3 and used as a solid after washing hexane. The catalysts 1 was prepared according to the literature procedures.8 Homopolymerization of 1,3-Pentadiene. A 100 mL flask equipped with a magnetic bar was charged with 0.70 g of MAO (0.01 mol, Al/Ti = 1200), 9 mL of 1,3-pentadiene (E/Z mixture) and 15 mL of toluene in order to reach a total volume of 25 mL. After equilibration of the solution at the desired temperature the reaction was started by injection of a toluene solution (1 mL) of the catalyst (10 μmol). The run was terminated after 1 h by introducing ethanol (15 mL) and antioxidant (Wingstay K; 0.5−0.75 phr). The polymer was coagulated in ethanol (200 mL) acidified with aqueous HCl, recovered by filtration, washed with an excess of ethanol, and dried in vacuum at room temperature. Copolymerization of Ethylene and 1,3-Pentadiene. The copolymerization runs were carried out following a standard procedure. A 100 mL flask equipped with a magnetic bar was charged with 0.70 g of MAO (0.01 mol, Al/Ti=1200), and the proper amount of a 1,3-pentadiene (E/Z mixture) solution in toluene, to reach a total volume of 10 mL. The flask was then evacuated and filled with ethylene (1 atm); after equilibration of the solution at target temperature the reaction was started by injection of a toluene solution (1 mL) of the catalyst (10 μmol). The run was terminated after the desired time by introducing ethanol (15 mL) and antioxidant (Wingstay K; 0.5−0.75 phr). The polymer was coagulated in ethanol (200 mL) acidified with aqueous HCl, recovered by filtration, washed with an excess of ethanol, and dried in vacuum at room temperature. Hydrogenation of Ethylene−1,3-Pentadiene Copolymer. A sample (1 g) of the copolymer (containing 34% of 1,3-pentadiene (run

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RESULTS AND DISCUSSION The performances of 1/MAO in the polymerization of the 1,3pentadiene (E−Z mixture) and E-1,3-pentadiene are reported in Table 1. Table 1. 1,3-Pentadiene Polymerization Results runa

T (°C)

yield (g)

activityb

Mw/Mn

Mn/103

% 1,4c

% 1,2c

1 2 3 4 5d

0 20 50 70 20

0.05 0.12 2.27 1.88 0.18

1.6 2.4 45 38 3.6

1.7 2.3 1.9 2.1 −

17 20 52 40 −

21 29 40 44 35

79 71 60 56 65

Polymerization conditions: Ti complex (10.0 μmol), Al/Ti = 1200, toluene (10 mL), 1,3-pentadiene (E/Z mixture) = 5 mL; polymerization time, 3 h. bActivity = kg/(mol Ti)·h. cCalculated from 13C NMR. dE-1,3-pentadiene; polymerization time: 5 h. a

Complex 1, activated by MAO, promotes the polymerization of E-PD while the Z-PD is completely unreactive.15 Because of the large availability and low cost of the mixture of isomers we then used such mixture in which the E-isomer is the major component (2/1 ratio). The activity is quite depending from the polymerization temperature giving a maximum at 50 °C in the range 0−70 °C. The polymers have a narrow molecular weight distribution (1.7−2.3) indicating a single site catalyst. The 13C NMR analysis of the polymers shows that the microstructure of the polymer consists mainly of trans-1,2 units 6364

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mers obtained with the same catalytic system7d,e and are reported in Table 3 (the numbering of carbon atoms follows the Scheme 2).

(methyl signal at 17.9 ppm) and trans-1,4 units (methyl signal at 20 ppm)16 and the 1,4 content increases by increasing the temperature. Such behavior is intermediate between butadiene and 4-MPD that respectively afford highly 1,4-trans-polybutadiene and isotactic poly-1,2-(4-MPD). All the polymers are not crystalline showing only a Tg at −15 °C in agreement with miscrostructural features observed at 13C NMR. Because of the versatility of this catalytic system in the reaction of copolymerization of diene (butadiene, isoprene, 4methyl-1,3-pentadiene) with ethylene and in order to gain insights about the polymerization mechanism we decided to study also the behavior of 1,3-pentadiene (E and Z mixture) in the copolymerization reaction with ethylene. In Table 2, the

Table 3. 13C NMR Assignments for the Ethylene−1,3Pentadiene Copolymers

Table 2. Ethylene−1,3-Pentadiene Copolymerization

runa

% 1,3PD in the feed

% 1,3-PD in the copolymerb

yield/g

activityc

Mw/ Mn

Mn/ 103

% 1,4d

% 1,2d

1 2 3 4 5

30 38 42 45 50

27 30 31 34 36

0.38 0.85 1.05 2.00 1.74

25 57 70 134 116

1.9 1.7 1.9 2.0 1.9

4.8 5.3 7.6 6.9 5.6

37 43 49 59 65

63 57 51 41 35

a

a Polymerization conditions: Ti complex (10.0 μmol), Al/Ti = 1200, toluene (15 mL), ethylene pressure = 1 atm, T = 20 °C; polymerization time, 1.5 h. bCalculated from 1H NMR. cActivity = kg/(mol Ti)·h. dCalculated from 13C NMR.

carbon typea

chemical shift (ppm)

C10 C5 D B EEE A F C7 C6 C1 C4 C2−C3 C8 C9

17.6 20.6 24.6 27.2 29.5 32.5 35.5 36.2 36.6 37.4 42.2 123.6 135.8 137.3

See Scheme 2

Scheme 2. Copolymers Microstructure

data relative to the copolymers are reported. The activity of the catalyst in the copolymerization reactions is increased as observed for isoprene suggesting a so-called “co-monomer effect”.7e The Tg value for the run 4 is −50 °C and the endothermic peak corresponding to the melting point of the polyethylene block was observed at 110 °C. The molecular weight of the polymers is sensibly lower that those reported in the homopolymerization reactions and the molecular weight distributions are monomodal for both systems ranging between 1.7 and 2.0 indicating a single-site catalyst and the material being copolymeric in nature. To shed more light into the microstructural features of the copolymers a thorough analysis by means of 13C NMR spectroscopy was carried out. In Figure 1, the spectrum of

The chemical shift assignments were further supported by hydrogenation of the copolymer that confirmed the presence of alternating ethylene-1,3-pentadiene segments with the pentadiene units in 1,4 or 1,2 arrangement.17

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MOLECULAR MODELING STUDIES In order to shed more light on the mechanisms involved in the diene polymerization promoted by Ti complexes, bearing [OSSO]-type bis(phenolato) ligands, a DFT study on butadiene, E-PD and Z-PD homopolymerizations was performed. A first screening on the possible involved species and transition states was reported on butadiene. As generally accepted1 conjugated diene polymerization usually proceeds through a mechanism involving an η3-allyl intermediate and a scis or s-trans monomer coordination. In particular, as reported in Scheme 3, cis-1,4 units would be achieved from an anti-η3allyl terminal growing chain generated by a s-cis insertion of the monomer, while trans-1,4 units would be achieved from a synη3-allyl terminal growing chain generated by a s-trans insertion of the monomer or by isomerization of the less stable anti-η3allyl terminal growing chain. According to Scheme 3, 1,2 units can be in principle obtained both starting from a s-cis and s-trans monomer insertion. It was

Figure 1. 13C NMR spectrum of the ethylene−1,3-pentadiene copolymer (Table 2, run 4).

the copolymer containing 34% of E-PD units (Table 2, run 4) is reported. No Z-PD inserted unit can be observed, being this monomer unreactive in copolymerization with ethene as well. The spectrum clearly shows the presence of the ethylene homosequences at 29.5 ppm (E) and additional signals due to the presence of the E-PD units along the polymer chain 1,2 and trans-1,4 enchained alternated with ethylene units. The assignments have been made comparing this spectrum with those of the ethylene−isoprene and ethylene−4-MPD copoly6365

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Moreover, the allyl group can direct the rest of the polymer chain (P in Scheme 5) in the direction of the free site, where the monomer is supposed to coordinate, or in the opposite direction. As a consequence the allyl group is arranged to give a 1,2 or a 1,4 enchainment after monomer insertion whether the polymer chain is oriented toward the free site or in the opposite direction, respectively. As for butadiene, the polymer chain was simulated by a methyl group. Internal and free energies in gas phase and toluene of the located four minimum energy structures (allbut12-exo, all-but14-exo, all-but12-endo, all-but14-endo) are reported in Table 4, and structures are shown in Figure 2. No

Scheme 3

Table 4. Internal and Free Energies in Gas Phase and Toluene of the Located Minimum Energy Structures for Allyl Terminal Growing Chain Coordination and Butadiene 1,2, 1,4 s-trans,1,4 s-cis, and 1,2-Primary Insertion Transition Statesa

also shown that in some cases 1,2 units can be achieved by a primary insertion of the conjugated diene, that behaves as an αolefin, following the classical Cossee mechanism (Scheme 4)18,4h

structure all-but12-exo all-but14-exo all-but12-endo all-but14-endo

Scheme 4

but12-Tre-exo≠ but12-Tsi-exo≠ but12-Tre-endo≠ but12-Tsi-endo≠ but14-Tre-exo≠ but14-Tsi-exo≠ but14-Tre-endo≠ but14-Tsi-endo≠ but14-Cre-exo≠ but14-Csi-exo≠ but14-Cre-endo≠ but14-Csi-endo≠ but12-Tre-prim≠ but12-Cre-prim≠

To rationalize polymerization data, intermediates and transition states of mechanisms, reported in Scheme 3 and 4, were modeled for butadiene with DFT calculations. Ligand of complex 1, was modeled with a p-methyl and a o-tert-butyl aryl substituents in place of ligand with p and o-tert- butyl aryl substituents, being the para substituent non relevant for experimental results. Four possible intermediates presenting a syn-η3 coordination of the growing chain were located. Indeed, the concavity of the allyl can be oriented in different ways with respect to the ligand. In particular, as sketched in Scheme 5, we will call exo the allyl group presenting the concavity oriented in the closest tert-butyl direction, and endo the allyl with the concavity oriented in the opposite direction.

E(gas)

G(gas)

E(tol)

G(tol)

0 0.8 0.1 0.6

0 0.4 −0.1 0.6

0 0.9 0.0 0.7

0 0.6 −0.1 0.6

4.7 2.6 3.9 2.9 2.6 0 1.2 2.0 5.1 1.6 3.2 3.3 8.7 10.6

5.0 3.2 4.0 3.9 3.0 0 1.6 1.7 5.0 1.1 3.7 3.4 6.1 8.2

4.6 2.6 3.9 3.0 2.5 0 1.1 1.8 5.0 1.7 3.1 3.1 8.9 10.7

4.9 3.2 4.0 3.9 2.9 0 1.5 1.5 4.9 1.1 3.6 3.2 6.4 8.2

a

Energies are in kcal/mol. All insertion transition state energies are related to structure but14-Tsi-exo≠.

Scheme 5. Anti η3 Coordination of the Growing Chaina

significant energy differences among the four reported structures can be observed. It is worth noting that no stable structures presenting different orientation of the allyl concavity were found. Calculation were also run to locate the corresponding anti-η3 coordinated growing chain intermediates, but were not reported being over 5 kcal/mol less stable than the corresponding syn-η3 intermediates. According to the polymerization mechanism reported in Scheme 3, the butadiene monomer can give an s-trans or an scis coordination and insertion. Since, we were unable to locate any monomer coordination intermediate, only insertion transition states (TS) were calculated. It is worth underlining that butadiene can, in principle, react with the allyl terminal chain presenting all four possible arrangements of Figure 2. Moreover, η2 s-trans as well as an η2 s-cis diene can coordinate to the metal with a si or re face of the double bond. As a consequence, all possible TS were calculated considering both allyl and diene arrangements. In particular, in Table 4, internal and free energies in gas phase and toluene were reported for 1,2 insertion TS of s-trans butadiene (but12-Tre-exo≠, but12-Tsiexo≠, but12-Tre-endo≠, but12-Tsi-endo≠), for 1,4 insertion

a

P indicates the rest of the polymer chain. As for molecular modeling studies, P = −CH3, R = −H for butadiene, whereas P = −CH2CH3, R = −CH3 for E-1,3-pentadiene. 6366

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Figure 2. Minimum energy structures of coordinated allyl terminal growing chain (all-but12-exo, all-but14-exo, all-but12-endo, allbut14-endo).

Figure 3. Most favored transition state structures for 1,4 s-trans (but14-Tsi-exo≠), 1,4 s-cis (but14-Csi-exo≠), and 1,2 s-trans (but12Tsi-exo≠) butadiene insertions.

TS of s-trans butadiene (but14-Tre-exo≠, but14-Tsi-exo≠, but14-Tre-endo≠, but14-Tsi-endo≠) and for 1,4 insertion TS of s-cis butadiene (but14-Cre-exo≠, but14-Csi-exo≠, but14Cre-endo≠, but14-Csi-endo≠). Most favored TS structure (but14-Tsi-exo≠) presents a strans monomer with a si face coordination and an exo allyl orientation. ΔG insertion barrier in toluene with respect to the corresponding allylic intemediate all-but14-exo is 19.1 kcal/ mol (see Supporting Information, Table S2, for all best TS barriers located for butadiene, E-PD and Z-PD). All TS energies in Table 4 are related to but14-Tsi-exo≠ energy. In Figure 3 most stable structures for 1,4 s-trans (but14-Tsiexo≠), 1,4 s-cis (but14-Csi-exo≠) and 1,2 s-trans (but12-Tsiexo≠) butadiene insertion have been reported. All three structures present a si face coordination of the reacting double bond and an exo allyl orientation, that minimize the interactions with tert-butyl groups. 1,2 s-trans butadiene energy insertion barrier is quite higher than 1,4 s-trans (ΔΔG(tol) = 3.2 kcal/ mol) due to the polymer chain orientation toward the inserting monomer. This result rationalizes the absence of 1,2 units in the polymer chain experimentally observed.7b On the other hand, 1,4 s-cis free energy insertion barrier in toluene is only 1.1 kcal/mol unfavored with respect to the 1,4 s-trans. The energy difference is probably determined by the higher s-cis conformation energy of the monomer, that does not show an η4 coordination, as usually observed for other systems, as CpTiCl3 and Ni(II) based complexes.4c−h,19 Indeed, for butadiene polymerization, promoted by catalytic systems as CpTiCl3-MAO, the high content of cis-1,4 units (about 85%) was shown to arise from an s-cis η4 monomer coordination, favored with respect to the η2 s-trans coordination, where the

monomer can only coordinate one double bond.4c−h The product of a 1,4 s-cis insertion would be an anti-η3 coordinated growing chain intermediate, that, as reported above, is strongly unfavored for this system, and would probably rapidly isomerize to the corresponding syn-η3 isomer. The high energy of the anti-η3 isomer together with the higher insertion energy of the s-cis monomer explains the presence of negligible amount of cis-1,4 units in the polymer chain. Finally, it is worth noting that in the modeled structures, involving a postmetallocene catalytic system, the concavity of the allyl group of the terminal growing chain is preferentially oriented toward the closest tert-butyl aryl substituent of the ligand and never toward the incoming monomer, as it was, on the contrary, previously reported for different catalysts, as CpTiCl3 and Ni(II) based complexes.4c−h,19 To exclude the competing pathway of Scheme 4 for the formation of 1,2 units, energy barriers for primary butadiene insertion was also calculated. As reported in Table 4, free energy of insertion TS in toluene of primary s-trans (but12Tre-prim≠)or s-cis (but12-Cre-prim≠) butadiene are 6.4 and 8.2 kcal/mol higher than the most favored insertion TS of the allylic mechanism (but14-Tsi-exo≠). The computational study has been extended to E-PD and ZPD. For these two monomers the growing chain on the allyl group was simulated by an ethyl group to differentiate it from methyl group of the monomer. Moreover, to simplify and compare the E-PD and Z-PD behavior, in both cases the monomer insertion was modeled on a syn-η3 terminal growing chain coming from an E-PD inserted unit. As for E-PD, preliminary syn-η3 terminal growing chain intermediates (all-epd12-exo, all-epd14-exo, all-epd12-endo, 6367

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°C. These differences are too small to be well appreciated by DFT calculations. Nevertheless, one could argue that the higher pecentage of 1,2 units at lower temperatures could be related to an energy pathway slightly enthalpically favored with respect to 1,4-trans insertion, and that this condition is qualitatively reproduced by DFT calculations, being epd12-Tsi-exo‡ internal energies lower than that of epd14-Tsi-exo‡, while the corresponding free energies are higher. Finally, to rationalize the absence of Z-PD units in polymer chain, obtained during the polymerization of a mixture of E-PD and Z-PD, discussed in the previous section, we also performed DFT calculations on the 1,2 and 1,4-trans insertion of Z-PD on a poly-E-1,3-pentadienyl chain. Internal and free energy for minimum energy structures are reported in Table 6. As in the

all-epd14-endo) were modeled, showing small energy differences as in the case of butadiene (see Table 5). The corresponding anti-η3 intermediates were not considered being already high energy structures for the less hindered butadiene. Table 5. Internal and Free Energies in Gas Phase and Toluene of the Located Minimum Energy Structures for Allyl Terminal Growing Chain Coordination and the E-PD 1,2 and 1,4 s-trans Insertion Transition Statesa E(gas)

G(gas)

E(tol)

G(tol)

all-epd12-exo all-epd14-exo all-epd12-endo all-epd14-endo

0.1 0.3 0 0.2

−0.4 −0.7 0 0.2

0.1 0.2 0 0.0

−0.3 −0.7 0 0.0

epd12-Tre-exo≠ epd12-Tsi-exo≠ epd12-Tre-endo≠ epd12-Tsi-endo≠ epd14-Tre-exo≠ epd14-Tsi-exo≠ epd14-Tre-endo≠ epd14-Tsi-endo≠

2.8 0 1.7 1.1 3.0 0.4 1.9 1.3

2.8 0 2.0 1.4 3.5 −0.5 1.7 1.5

2.7 0 1.6 1.0 3.0 0.5 1.8 1.4

2.7 0 2.0 1.4 3.5 −0.5 1.6 1.5

structure

Table 6. Internal and Free Energies in Gas Phase and Toluene of Z-PD 1,2 and 1,4 s-trans Insertion Transition Statesa

a

Energies are in kcal/mol. All insertion transition state energies are related to structure epd12-Tsi-exo≠.

TS were modeled for 1,2 (epd12-Tre-exo≠, epd12-Tsi-exo≠, epd12-Tre-endo≠, epd12-Tsi-endo≠) and 1,4 (epd14-Treexo≠, epd14-Tsi-exo≠, epd14-Tre-endo≠, epd14-Tsi-endo≠) s-trans monomer insertions, and energies, related to structure epd12-Tsi-exo≠ are reported in Table 5. Lowest energy TS structures for 1,2 (epd12-Tsi-exo≠) and 1,4-trans (epd14-Tsiexo≠) insertions are depicted in Figure 4, and present an overall

structure

E(gas)

G(gas)

E(tol)

G(tol)

zpd12-Tre-exo≠ zpd12-Tsi-exo≠ zpd12-Tre-endo≠ zpd12-Tsi-endo≠ zpd14-Tre-exo≠ zpd14-Tsi-exo≠ zpd14-Tre-endo≠ zpd14-Tsi-endo≠

2.6 0 1.6 1.1 2.8 0.2 2.0 1.4

2.5 0 1.2 1.0 2.8 −0.8 1.3 0.7

2.5 0 1.6 1.1 2.7 0.3 1.9 1.4

2.5 0 1.1 1.0 2.7 −0.8 1.2 0.7

a

Energies are in kcal/mol. All insertion transition state energies are related to structure zpd12-Tsi-exo≠.

case of E-PD, favored energy structures for Z-PD 1,2 and 1,4trans insertions are zpd12-Tsi-exo≠ and zpd14-Tsi-exo≠ (structures in Figure 5). Nevertheless, overall free energy

Figure 5. Most favored transition state structures for 1,4-trans (zpd14Tsi-exo≠) and 1,2 (zpd12-Tsi-exo≠) Z-PD insertions.

Figure 4. Most favored transition state structures for 1,4-trans (epd14Tsi-exo≠) and 1,2 (epd12-Tsi-exo≠) E-PD insertions.

barrier in toluene, with respect to the corresponding allylic intermediates, are in this case 23.6 and 23.5 kcal/mol for 1,2 and 1,4-trans insertions, respectively, making Z-PD unfavored with respect to E-PD insertion. This difference is mainly related to the steric interation between the methyl group of the Z-PD and the sulfur atom of the ligand (black distance in Figure 5) in the TS structures, that is absent in case of E-PD.

free energy barrier in toluene with respect to the corresponding allyl intermediates of 22.4 and 22.3 kcal/mol, respectively. Internal and free energy differences for those TS are very small (≤0.5 kcal/mol) indicating that pathways for the generation of 1,2 and trans-1,4 units along the chain are competing. This is not surprising, being the allylic carbons, involved in the formation of 1,2 and trans-1,4 units, similarly encumbered, and is in agreement with the experimental results reported in the previous section. Indeed, percentages of 1,2 and trans-1,4 units are close to 50% for polymerization run at 70 °C, while for lower temperatures 1,2 unit percentage grows up to 79% at 0

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CONCLUSIONS The polymerization of conjugated dienes in the presence of postmetallocene complex 1 activated by MAO have been extensively investigated both by experimental and theoretical 6368

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assistance. Computational support from CINECA−High Performance Computing Portal is also gratefully acknowledged.

studies. In particular, the catalyst behavior has been examinated in the polymerization of E- and Z-1,3-pentadiene and in the copolymerization ethylene-1,3-pentadiene, to get information on the involved polymerization mechanisms. Computational studies have been devoted to the rationalization of the experimental results in order to provide further mechanistic insights of conjugated polymerization promoted by postmetallocene complexes. According to experimental results complex 1/MAO was able to promote the polymerization of E-1,3-pentadiene, while it revealed to be unactive toward Z-1,3-pentadiene. The activity showed to be dependent on temperature, giving a maximum at 50 °C in the range 0−70 °C. The poly-E-1,3-pentadiene microstructure mainly consists of 1,2 and trans-1,4 units, differently from microstructure of polybutadiene previously obtained with same catalyst, that showed highly trans-1,4 enchainments.7b The content of 1,2 units in the poly-E-1,3pentadiene increases by decreasing the temperature, reaching 78% at 0 °C. The activity of the catalyst is increased in the copolymerization reactions with ethylene suggesting a so-called “co-monomer effect”. 13C NMR analysis showed copolymer microstructures with E-1,3-pentadiene content up to 36% and enchained as a mixture of 1,2 and trans-1,4 units, similarly to homopolymer. The catalyst revealed to be unable to insert Z1,3-pentadiene in the copolymerization with ethylene as well. According to DFT studies, butadiene and E-1,3-pentadiene polymerizations proceed through an allylic mechanism involving a syn-η3 coordination of the growing chain and a strans η2 monomer insertion. The concavity of the allyl group of the terminal growing chain is preferentially oriented toward the closest tert-butyl aryl substituent of the ligand and never toward the incoming monomer, as it was, on the contrary, previously reported for different catalysts, as Ni(II) based and CpTiCl3 complexes. As for butadiene, 1,4-trans insertion is favored over 1,2 insertion, whereas in case of E-1,3-pentadiene 1,2 and 1,4trans insertion transition states present similar energies, as 1,2 insertion is slighlty enthalpically favored and entropically unfavored. These findings would be able to rationalize the loss of chemoselectivity moving from butadiene to E-1,3pentadiene and the role played by temperature on the 1,4 and 1,2 unit percentages in the poly-E-1,3-pentadiene chain. Finally, the higher energy barriers for Z-1,3-pentadiene insertions with respect to E-1,3-pentadiene would partially rationalize the unactivity of the catalyst toward this monomer.

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ASSOCIATED CONTENT

S Supporting Information *

13

C NMR and DEPT 135 spectra and data for the copolymers and Cartesian coordinates and energies of all calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Ministero dell’Universita′ e della Ricerca Scientifica (MURST, Roma, Italy) FARB 2009 and PRIN 2008 and Dr. M. Napoli and Dr. P. Oliva for technical 6369

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