Half-Titanocenes Containing Anionic Ancillary Donor Ligands : Effective Catalyst Precursors for Ethylene / Styrene Copolymerization

This review summarizes recent results for ethylene/styrene copolymerization using half-titanocenes containing anionic donor ligands, Cp’TiX2(Y) (X = halogen, alkyl; Y = aryloxo, ketimide etc.)–cocatalyst systems. The product composition, the styrene incorporation and microstructures in the resultant copolymers are highly influenced by the anionic donor employed. A methodology for an exclusive synthesis of the copolymers even under high temperature and high styrene concentrations has been introduced on the basis of a proposed catalytically-active species in this catalysis.


Introduction
Precise control over macromolecular structure is a central goal in synthetic polymer chemistry, and copolymerization is an important method that usually modifies physical, mechanical and electronic properties by varying the ratio of the individual components.Considerable effort has been devoted to establishing a new synthetic strategy for precise placement of a chemical functionality, and the design and synthesis of efficient transition metal complex catalysts for precise olefin polymerization have been a subject of extensive studies [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18].Ethylene/styrene copolymers, which cannot be prepared by free radical or ordinary Ziegler-Natta processes [19][20][21][22], are promising due to their unique properties [23][24][25].The introduction of styrene into the PE backbone results in drastic changes in both the viscoelastic behavior and thermo-mechanical properties of the polymeric material [24], since the OPEN ACCESS crystallizability of PE chains is gradually inhibited by the incorporation of styrene.The copolymers range from semi-crystalline to amorphous materials, depending on the styrene content [23].Therefore, these copolymers can become effective blend compatibilizers for PS/PE blends and also have potential in foam, film and sheet applications.
Typical 13 C NMR spectra (methylene and methine regions) of the copolymers, including assignment of the monomer sequences, are shown in Figure 1 [53].As described below, the microstructures for the resultant poly(ethylene-co-styrene)s depend on the catalysts used.The glass transition temperature (T g ), as measured by differential scanning calorimetry (DSC) increased with an increase in the styrene content (−8.1-58.3°C), because the crystallizability of PE chains is gradually inhibited by the incorporation of styrene [23].
Copolymerization with the other modified half-titanocenes has also been reported (Scheme 2).Ethylene/styrene copolymerization by Cp*TiCl 2 (N=C t Bu 2 ) (6) took place in a living manner in the presence of MAO cocatalyst, although the homopolymerization of ethylene and styrene did not proceed in a living manner [57].No styrene repeating units were observed in the resultant copolymers, which suggest that a certain degree of styrene insertion inhibits chain transfer in this catalysis.The living nature was maintained under various conditions (Al/Ti molar ratios, ethylene pressure, styrene concentrations, temperature) [58].Scheme 2. Other half-titanocenes employed for ethylene/styrene copolymerization [48][49][50][51].
On the basis of the above results, modified half-titanocenes, Cp'TiX 2 (Y), especially the aryloxo modified half-titanocenes, are better catalyst precursors for the synthesis of ethylene/styrene copolymers, and both the activity and the styrene incorporation are highly affected by the kind of ligands used (both cyclopentadienyl and anionic donor).

Role of Anionic Donor Ligand (Y) and Mechanistic Considerations
Three half-titanocene complexes containing Cp* ligand of type, Cp*TiX (12), Me (13)]-cocatalyst systems were used for the copolymerization to explore the effect of anionic donor ligands under the same conditions (Tables 2,3) [58,85,86].The ketimide analogue (6)-MAO catalyst showed moderate catalytic activities, and the styrene content increased at higher temperature.The resultant copolymers prepared with 6 even at 55 and 70 °C still possessed relatively low PDI values, suggesting that the living nature was maintained under these conditions (Table 2).In contrast, the polymerizations by the aryloxide analogue (4) gave copolymers with high styrene contents (31.9-34.3mol%), and a significant increase in the activity was not observed at high temperature.The resultant copolymers possessed lower M n values with unimodal, rather large PDI values (M w /M n = 1.50-1.62),which strongly suggests that a chain transfer reaction occurred to some degree.Note that the polymers prepared with the trichloride analogue (11) showed bimodal molecular weight distributions consisting of a mixture of PE and SPS, and the proportion of SPS increased at high temperature, due to an increase in the activity for syndiospecific styrene polymerization [58,87,88].Copolymerization using the aryloxo-dimethyl analogue (12)-MAO or [PhN(H)Me 2 ][B(C 6 F 5 ) 4 ] (AFPB) catalyst system afforded the copolymer (Table 3); no distinct differences in the microstructures were seen between MAO and AFPB in the 13 C NMR spectra.In contrast, note that the polymer prepared with the Cp*TiMe 3 (13)-AFPB catalyst was PE (containing a trace amount of the copolymer with low styrene content) or the copolymer with an extremely low styrene content, whereas the copolymerization in the presence of MAO afforded a mixture of PE and SPS, as seen with the trichloride analogue (11).The fact that no SPS was formed in polymerization with 13-AFPB catalyst was analogous to the fact that Cp*Ti(CH 2 Ph) 3 -AFPB catalyst did not afford SPS in an attempted styrene polymerization (under dark conditions), and only poly(propylene-co-styrene) oligomer was formed in the propylene/styrene copolymerization [89].These results strongly suggest that cationic Ti(IV) species play an important key role in ethylene polymerization, as well as ethylene/styrene copolymerization.These results also suggest that another catalytically-active species [likely Ti(III)] for syndiospecific styrene polymerization is formed in the presence of MAO [58].The exclusive formation of copolymers without formation of SPS as a by-product was observed with the introduction of ethylene into a solution of syndiospecific styrene polymerization using Cp'TiCl 2 (O-2,6-i Pr 2 C 6 H 3 )-MAO catalysts (Scheme 3) [90].Moreover, the activities and the M w values, as well as the styrene contents in the latter copolymerizations, were identical to those in independent runs (Table 4), clearly indicating that the catalytically active species for the syndiospecific styrene polymerization can be tuned to the active species for copolymerization by exposure of ethylene.
Since the reports concerning the efficient synthesis of the copolymers with high styrene contents have been limited so far, we explored a possibility for the exclusive synthesis of copolymers under high temperature conditions.Development of the catalyst systems for the exclusive synthesis should be thus considered as potentially important in terms of not only polymer synthesis, but also better understanding of polymerization mechanism, although the copolymers with high styrene contents can be easily prepared when the polymerizations were conducted at 25 ºC under high styrene concentrations [52,53].On the basis of our proposed assumption shown in Scheme 4 that two catalytically active species (for the syndiospecific styrene polymerization and ethylene/styrene copolymerization) are present in the reaction mixture, we chose an approach in the presence of borate (in place of MAO) as the cocatalyst in combination with Al alkyls to generate the cationic species preferentially.6) [55].The exclusive synthesis has been achieved even at 70 °C (run 13), and the styrene content exceeded 50 mol% under high styrene concentration conditions (run 12).An optimization of Al/Ti molar ratios was a prerequisite for the exclusive synthesis (runs 8-10, Figure 3), and the observed activities were affected by the Al/Ti molar ratios.Moreover, the observed catalytic activity could be improved when partially fluorinated Cp*TiCl[OCH(CF 3 ) 2 ](O-2,6-i Pr 2 C 6 H 3 ) ( 14) was employed as the catalyst precursor [56].The high activity, as well as the high selectivity, could be thus maintained even at 70 °C (Table 7).We have shown that an exclusive synthesis of ethylene/styrene copolymers has been achieved by using modified half-titanocene, Cp*TiCl 2 (O-2,6-i Pr 2 C 6 H 3 ), in the presence of [PhN(H)Me 2 ][B(C 6 F 5 ) 4 ] and Al i Bu 3 /Al(n-octyl) 3 .The fact presented here should be promising not only from academic, but also from practical, viewpoints.We are exploring a possibility for synthesis of new styrene copolymers by efficient copolymerization.An exclusive synthesis of the copolymers at high temperature should be important in terms of better process application (viscosity, mass transport), especially from the practical viewpoint.The exclusive synthesis of copolymers with high styrene contents by adopting the borate cocatalyst should also be important.This is because the fact may suggest a presence of two catalytically active species [for the syndiospecific styrene polymerization and ethylene/styrene copolymerization, neutral Ti(III) and cationic Ti(IV)], as well as preferred formation of the cationic species in situ.

Summary and Outlook
Promising results have been reported regarding copolymerization using modified half-titanocenes [Cp'TiX 2 (Y), Y = anionic ancillary donor ligands], affording a random copolymer with various styrene contents, and both the catalytic activities and the styrene incorporation are highly affected by both the cyclopentadienyl fragments and anionic donor ligands employed.The efficient synthesis of random copolymers could be achieved by using half-titanocene-containing aryloxo ligands.Cationic Ti(IV) species play an important role as catalytically active species in copolymerization, whereas (neutral and/or cationic) Ti(III) species play roles in syndiospecific styrene polymerization, and these findings may suggest why the polystyrene structure in random copolymer prepared with half-titanocenes was atactic.Various copolymers, which differ with regard to their compositions, microstructures and properties, and these unique characteristics are dependent upon the nature of the complex catalysts used.I hope that the development of other new polyolefin materials would follow the use of this synthetic technique, which can be applied not only to styrene, but also to styrene-containing reactive functionalities (in combination with other polymerization techniques, such as ATRP, RAFT, ROP etc.).

AlMe 3
and toluene from commercially available PMAO-S, Tosoh Finechem Co.) 3.0 mmol, (Al/Ti = 1500, molar ratio), 10 min.b Initial molar ratio of styrene/ethylene in the reaction mixture.c Activity in kg-polymer/mol-Ti•h, polymer yields were based on a mixture of SPS (syndiotactic polystyrene) and E-S (copolymer) as acetone insoluble fraction.dComposition estimated by13 C NMR spectra (and confirmed by 1 H NMR spectra).e GPC data in o-dichlorobenzene vs. polystyrene standards.f Styrene content in mol% estimated by 1 H NMR spectra.
[PhN(H)Me 2 ][B(C 6 F 5 ) 4 ] was chosen, because the cationic polymerization of styrene (probable accompanying reaction in the mixture) by this borate did not proceed rapidly as that by [Ph 3 C][B(C 6 F 5 ) 4 ] in the presence of Al alkyls [93].Although the observed catalytic activities were lower than those in the presence of MAO, poly(ethylene-co-styrene)s became the sole reaction product by 4 when the polymerizations were conducted in the presence of [PhN(H)Me 2 ][B(C 6 F 5 ) 4 ] and Al i Bu 3 /Al(n-octyl) 3 (Table
e GPC data in O-dichlorobenzene vs. polystyrene standards.
a e GPC data in O-dichlorobenzene vs. polystyrene standards.f Styrene content in mol% estimated by 1 H NMR spectra.g Styrene 15 mL, toluene 15 mL.