Design of Efficient Molecular Catalysts for Synthesis of Cyclic Olefin Copolymers ( COC ) by Copolymerization of Ethylene and α-Olefins with Norbornene or Tetracyclododecene

Selected results for the synthesis of cyclic olefin copolymers (COCs)—especially copolymerizations of norbornene (NBE) or tetracyclododecene (TCD) with ethylene and α-olefins (1-hexene, 1-octene, 1-dodecene)—using group 4 transition metal (titanium and zirconium) complex catalysts have been reviewed. Half-titanocenes containing an anionic ancillary donor ligand, Cp’TiX2(Y) (Cp’ = cyclopentadienyl; X = halogen, alkyl; Y = anionic donor ligand such as aryloxo, ketimide, imidazolin-2-iminato, etc.), are effective catalysts for efficient synthesis of new COCs; ligand modifications play an important role for the desired copolymerization. These new COCs possess promising properties (high transparency, thermal resistance (high glass transition temperature), low water absorption, etc.), thus it is demonstrated that the design of an efficient catalyst plays an essential role for the synthesis of new fine polyolefins with specified properties.


Introduction
Polyolefins such as polyethylene (high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), etc.) and polypropylene, produced by metal-catalyzed olefin coordination polymerization, are important commercial synthetic polymers in our daily life, and the market capacity still increases every year (especially in China, India, and countries in the Middle East).Synthesis of new polymers with specified functions has been considered to be one of the most attractive targets, and precise control in the copolymerization is thus a feature that usually allows the alteration of the (physical, mechanical, and electronic) properties by varying the ratio of individual components.Design of molecular catalysts attracts considerable attention in terms of synthesis of new polymers; recent progress in the newly designed catalysts offers several new possibilities.Certain cyclic olefin copolymers (COCs) and cyclic olefin polymers (COPs) are amorphous materials with a promising combination of high transparency in the UV-vis region along with humidityand thermal-resistance (high glass transition temperature, T g ), and extremely low water absorption (excellent water vapor barrier properties) [1][2][3][4][5][6][7].Due to promising characteristics of COCs, these are used as (i) optical applications such as lenses, liquid crystal display light guide panels, and optical films; (ii) materials in medical and diagnostic equipment and electronic devices; and (iii) packaging applications, and many other uses [8,9].In the practical production process, as shown in Scheme 1, methods such as ring-opening metathesis polymerization (ROMP) of multi-COs and subsequent hydrogenation (for COPs), coordination insertion copolymerization of ethylene with COs methods such as ring-opening metathesis polymerization (ROMP) of multi-COs and subsequent hydrogenation (for COPs), coordination insertion copolymerization of ethylene with COs (for COCs), or homopolymerization of COs (for COPs) are generally employed.The copolymerization approach seems to be more promising, because the desired properties (e.g., Tg) should be tuned by their compositions (e.g., CO content) as well as their microstructures (including tacticity, etc.) [1][2][3][4][5][6][7].COCs are commercialized (as TOPAS ® , APEL ® , etc.) as ultrapure (applicable to advanced pharma packaging or food-contact films), crystal-clear (glass clear, amorphous), high-barrier (resistant to moisture, alcohols, acids) materials [8,9].Scheme 1.Typical process for synthesis of COCs (cyclic olefin copolymers) and COPs (cyclic olefin polymers).

Copolymerization of Ethylene with Norbornene (NBE), Tetracyclododecene (TCD) by Titanium and Zirconium Complex Catalysts
As described in the introduction, many have reported on ethylene copolymerization with norbornene (NBE) , however, successful examples of the efficient synthesis of random, high molecular weight copolymers with high T g values (high NBE content) have been limited.Selected complex catalysts employed for the copolymerization are shown in Scheme 2, and selected typical results are summarized in Table 1.
Scheme 2. Selected complex catalysts for copolymerization of ethylene with norbornene .In the copolymerization of ethylene with NBE using metallocene (exemplified as 1) and linked half-titanocene (3), the activity generally decreases upon increasing the NBE content (NBE concentration charged).Aryloxo-modified half-titanocenes (exemplified as 5 and 6) have been known as effective catalysts in ethylene copolymerization, not only with α-olefin, but also with sterically encumbered olefins and cyclic olefins [39,41,45]; an efficient catalyst for the desired copolymerization can be tuned by the cyclopentadienyl fragment [39,41].The indenyl-aryloxo analogue (6) showed efficient NBE incorporation with high catalytic activity, whereas the Cp*-aryloxo analogue (5) showed less efficient NBE incorporation than 6 [22,23], however, both the activity and the M n value decreased upon increasing the NBE content.The Cp'-pyrazolato analogue (10) exhibited efficient NBE incorporation with rather low NBE concentration (Table 1) to yield ultrahigh molecular weight copolymers with unimodal molecular weight distributions, but the activity decreased upon increasing the NBE concentration [25].
We demonstrated that the Cp-ketimide analogue, CpTiCl 2 (N=C t Bu 2 ) (7), exhibited remarkable catalytic activities, and a decrease in the activity was not observed, even after 30 min [24].The activity increased upon increasing the initial NBE concentration, whereas, as described above, a decrease in activity was observed in other catalysts upon increasing NBE concentration.The NBE content in the copolymers was thus higher than those prepared by other catalysts under the same conditions.It is noteworthy that neither activity nor NBE incorporation were strongly affected by the Al cocatalyst employed (methylaluminoxane (MAO), modified MAOs (MMAOs)), a fact that is in distinct contrast to that reported for the copolymerization by [Me 2 Si(fluorenyl)(N t Bu)]TiMe 2 (4) in which both the catalytic activity and NBE incorporation are highly dependent upon the Al cocatalyst employed [21].Both the activity (of 7) and the NBE content in the copolymer increased at a higher temperature (40-80 • C), and the resultant copolymers possessed high molecular weights with unimodal molecular weight distributions (M n = 3.38 × 10 5 to 6.20 × 10 5 , M w /M n = 2.20-2.37)[24].The efficient synthesis of high molecular weight copolymers with high NBE content (58.8-73.5 mol %) is possible upon increasing the NBE concentration at low ethylene pressure (2 atm).This may also be explained by the fact that 7 showed remarkable catalytic activity for NBE homopolymerization.The resultant polymers possessed uniform compositions, confirmed by differential scanning calorimetry (DSC) thermograms as well as gel permeation chromatography (GPC) traces, and a linear relationship between glass transition temperatures and NBE content in the resultant poly(ethylene-co-NBE)s was observed [22][23][24][25].
The Cp'-imidazolin-2-iminato analogue exhibits remarkable catalytic activities for ethylene (co)polymerization [49][50][51], and ethylene/NBE copolymerization by CpTiCl 2 [1,3-t Bu 2 (CHN) 2 C=N] (9) afforded ultrahigh molecular weight copolymer with efficient NBE incorporation and a catalytic activity that did not decrease at 80 • C [26]; both the activity and the M n value decreased upon increasing the NBE concentration under low ethylene pressure.Although the observed activities by 9 were lower than those by 7, 9 seems to be a promising candidate as an efficient catalyst that affords high molecular weight polymers at high temperature.In contrast to the imidazolin-2-iminato analogue [26], the Cp'-imidazolidin-2-iminato analogue showed less efficient NBE incorporation in the ethylene/NBE copolymerization [52].In these catalysts, a significant effect of the ligand substituents on both the catalytic activity and the comonomer incorporation was observed.
As described above, ethylene copolymers with tetracyclododecene (TCD, dimethanooctahydronaphthaline) and poly(ethylene-co-TCD)s are promising materials, possessing higher glass transition temperatures compared to poly(ethylene-co-NBE)s with the same cyclic olefin content; catalyst(s) development/design for the efficient copolymerization (high catalytic activity and efficient TCD incorporation) has thus been a promising target in the field of catalysis and polymer chemistry.These are produced by using classical Ziegler-type vanadium catalyst systems (VOCl 3 , VO(OEt)Cl 2 -EtAlCl 2 •Et 2 AlCl, etc.) [9,31].In contrast to many reports of the ethylene/NBE copolymerization , there have been reports for the copolymerization using metallocene catalysts (Scheme 3) [3,[32][33][34][35] and others [36], however, these catalysts generally exhibited low catalytic activities and/or less efficient TCD incorporation [35].and others [36], however, these catalysts generally exhibited low catalytic activities and/or less efficient TCD incorporation [35].
More recently, we realized that the ketimide-modified half-titanocenes, which are effective catalysts for ethylene copolymerization with NBE [24], especially the ( t BuC 5 H 4 )TiCl 2 (N=C t Bu 2 ) (8)-MAO catalyst, exhibited remarkable catalytic activities and efficient comonomer incorporation in ethylene/TCD copolymerization, affording high molecular weight polymers with uniform molecular weight distributions (Table 2) [53].The activity of 8 was significantly higher than that of the Cp analogue (7), whereas the activity of the Cp analogue ( 7) was much higher than that of 8 for α-olefin/TCD copolymerization [54].The resultant polymers prepared by 7-or 8-MAO catalysts possess high molecular weights with uniform molecular weight distributions.The resultant polymers possess sole glass transition temperatures (T g ), suggesting that these are poly(ethylene-co-TCD)s with uniform compositions.  1C NMR spectra; g estimated on the basis of the plots of T g and TCD content.
In contrast, attempted copolymerizations by the imidazolin-2-iminato analogues, CpTiCl 2 [1,3-t Bu 2 (CHN) 2 C=N] (Cp' = t BuC 5 H 4 , Cp (9)), afforded polymers possessing melting temperature (ca.110 • C, probably corresponding to polyethylene with low TCD content) and/or glass transition temperature (ca.160 • C, corresponding to ethylene/TCD copolymer containing moderate TCD content).These suggest that the resultant polymers are a mixture of at least two compositions and that the catalytically active species are not uniform during the polymerization.
The linked half-titanocene (3) exhibited high catalytic activity under the same conditions, affording a high molecular weight copolymer with low TCD content (estimated by the T g value).
Note that the activity of the 8-MAO catalyst increased at higher temperature (activity of 8: 23,900 kg-polymer/mol-Ti•h (25 • C) < 27,800 (40 • C) < 33,300 (60 • C)) upon increasing the TCD content in the copolymers.As observed in poly(ethylene-co-NBE)s, a linear relationship between the TCD content and glass transition temperature (T g ) was observed, suggesting that the resultant copolymers possess uniform composition.Since the reported metallocene catalysts showed low catalytic activities (and/or had less efficient TCD incorporations), and ordinary Ziegler-type vanadium catalyst systems are generally sensitive to polymerization temperature, the present catalyst would be a promising candidate (as a thermally robust catalyst) for efficient synthesis of ethylene/TCD copolymers (highly transparent, thermal-resistant materials).
As shown in Table 3, the copolymerizations of NBE with 1-hexene (HX), 1-octene (OC), and 1-dodecene (DD) by 7-or 8-MAO catalysts afforded high molecular weight copolymers with unimodal molecular weight distributions and with uniform compositions confirmed by DSC theromograms (observed as a sole glass transition temperature (T g )).Moreover, the T g values increased upon increasing the NBE content, as estimated by 13 C NMR spectra.The copolymerization of NBE with HX by ( t BuC 5 H 4 )TiCl 2 (N=C t Bu 2 ) (8)-MAO catalyst proceeded with moderate catalytic activity that is comparable to that of the reported Flu-CGC (4)-Ph 3 CB(C 6 F 5 ) 4 catalyst [62], and the activity increased upon increasing the NBE concentration charged.The Cp analogue (7) showed >10 times higher activity and better NBE incorporation than the t BuC 5 H 4 analogue (8) under the same conditions, and the T g values (corresponding to NBE contents in the resultant copolymers) were not affected by the kind of Al cocatalyst employed (MAO or MMAO).Good correlation between T g values and NBE content in poly(NBE-co-HX)s was observed, irrespective of the cyclopentadienyl fragment (Figure 1a); this would be in distinct contrast to that reported previously (exemplified by 4), in which the plots (slope) are affected by substituents on the fluorenyl ligand as well as by the cocatalyst employed [62,63].Similarly, the copolymerization of NBE with OC by 7-or 8-MAO catalysts proceeded with high catalytic activities, and both the activity and the NBE incorporation by 7 were higher than that by 8 under the same conditions.The activity of 7 (on the basis of polymer yields) was not strongly affected by the initial NBE/OC feed ratios, whereas the activity of 8 increased upon increasing the initial NBE concentration charged under certain conditions.Similar trends (for both the activities and NBE incorporations) were observed in the copolymerization of NBE with DD.Good linear relationships between T g values and NBE content in poly(NBE-co-HX)s (Figure 1a) and poly(NBE-co-OC)s (Figure 1b) were observed, irrespective of kind of titanium catalysts (7 and 8) employed.The T g values were affected by the α-olefin employed; T g values with the same NBE content increased in the following order: poly(NBE-co-DD) < poly(NBE-co-OC)s < poly(NBE-co-HX)s (Figure 1b).Moreover, the NBE incorporation was not strongly affected by the nature of α-olefin employed, and relatively linear correlations between NBE content in the copolymers and NBE/α-olefin feed molar ratios were observed, irrespective of kind of α-olefins employed, especially under high NBE/α-olefin feed ratio conditions.Synthesis of high molecular weight copolymers of (long chain) α-olefins with various NBE content (with uniform molecular weight distributions and compositions) can be achieved by adopting these catalyses.The activity of 7 increased at high temperature, especially at 50 • C, and no significant differences in the T g values (NBE content) in the resultant copolymers were observed by varying the temperature (25-70 • C).This would suggest that there has not been a significant temperature dependence observed for the NBE incorporation, as has been observed in ethylene/α-olefin copolymerization by ordinary ansa metallocene catalysts [68,69].The copolymerization of TCD with HX by the Cp-ketimide analogue ( 7) afforded the copolymers with unimodal molecular weight distributions, with uniform compositions confirmed by DSC thermograms (Table 3) [54].The copolymerizations with OC or DD by 7 afforded copolymers with high T g values.In contrast, 8 showed low activity in the copolymerization with HX, OC, and DD.The activity in the TCD/HX copolymerization decreased upon increasing the TCD concentration, with increasing TCD content in the copolymer (Table 4) [54].The resultant copolymers possessed rather high molecular weights with unimodal molecular weight distributions (M n = 12,000-27,600, M w /M n = 1.20-1.76),and the TCD content (estimated by 13 C NMR spectra) increased upon increasing TCD concentration with an increasing glass transition temperature (T g ); synthesis of the copolymers with high T g (e.g., 286 • C) could be thus attained by adopting this catalysis.The activity was also affected by MAO (Al/Ti molar ratio), especially under high TCD concentration conditions, and no significant differences in either the molecular weights or T g values of the copolymers were observed.Similarly, the copolymerizations of TCD with OC or DD afforded copolymers possessing rather high molecular weights with unimodal molecular weight distributions (M n = 9900-25,600, M w /M n = 1.61-1.93);the TCD content increased upon increasing TCD concentration with an increase in the T g values.Moreover, the TCD incorporation by the 7-MAO catalyst was not strongly affected by the nature of the α-olefin employed (HX, OC, or DD) under high TCD/α-olefin feed conditions; relatively linear correlations were observed between TCD content in the copolymers and TCD/α-olefin feed molar ratios.The copolymerization of TCD with HX by the Cp-ketimide analogue (7) afforded the copolymers with unimodal molecular weight distributions, with uniform compositions confirmed by DSC thermograms (Table 3) [54].The copolymerizations with OC or DD by 7 afforded copolymers with high Tg values.In contrast, 8 showed low activity in the copolymerization with HX, OC, and DD.The attempted copolymerizations of TCD with HX by [Me2Si(C5Me4)(N t Bu)]TiCl2 (3)-MAO catalyst afforded a negligible amount of polymers in most cases; low molecular weight oligomer (Mn = 5300) with low TCD content (estimated by DSC thermogram, Tg = 9.8 °C) was obtained under specified conditions.The copolymerizations with HX, OC, and DD by [Et(indenyl)2]ZrCl2 (2) or Cp2ZrCl2 (11) yielded a small amount of oligomer under certain conditions; this is in distinct contrast to that observed for CpTiCl2(N= t Bu2) (7).
The activity in the TCD/HX copolymerization decreased upon increasing the TCD concentration, with increasing TCD content in the copolymer (Table 4) [54].The resultant copolymers possessed rather high molecular weights with unimodal molecular weight distributions (Mn = 12,000-27,600, Mw/Mn = 1.20-1.76),and the TCD content (estimated by 13 C NMR spectra) increased upon increasing TCD concentration with an increasing glass transition temperature (Tg); synthesis of the copolymers with high Tg (e.g., 286 °C) could be thus attained by adopting this catalysis.The activity was also affected by MAO (Al/Ti molar ratio), especially under high TCD concentration conditions, and no Plots of glass transition temperature (T g ) vs NBE content (mol %) in (a) poly(NBE-co-1-hexene)s by Cp'TiCl 2 (N=C t Bu 2 ) (Cp' = Cp (7, ) or t BuC 5 H 4 (8, ))-MAO catalysts; (b) poly(NBE-co-1-hexene)s (marked with ) and poly(NBE-co-1-octene)s (marked with ); (c) plots of T g values vs TCD content (mol %) in poly(TCD-co-1-hexene)s and poly(TCD-co-1-dodecene)s; (d) plots of transmittance vs wavelength in poly(TCD-co-1-hexene) [54].Fairly good linear relationships between the T g values and the TCD content in poly(TCD-co-HX)s, poly(TCD-co-OC)s, and poly(TCD-co-DD)s were observed (Figure 1c), and the T g values in the resultant copolymers are affected by the α-olefin employed.It is clear that the T g values in poly(TCD-co-HX)s, [poly(TCD-co-OC)s, and poly(TCD-co-DD)s] are higher than those in poly(NBE-co-HX)s [poly(NBE-co-OC)s, and poly(NBE-co-DD)s] with the same α-olefin content (Figure 1c).Synthesis of high molecular weight copolymers of (long chain) α-olefins with various TCD content (with uniform molecular weight distributions and compositions) can be achieved by using these catalysts.Moreover, the resultant copolymers, poly(TCD-co-HX)s and poly(TCD-co-OC)s, show high transparency as thin films (Figure 1d), demonstrating a possibility that these polymers are promising materials with high transparency as well as with high glass transition temperatures (T g > 230 • C).

Efficient Norbornene (NBE) Incorporation in Ethylene/NBE Copolymerization by Half-Titanocene Catalysts Containing Chlorinated Aryloxo Ligands
As shown in Table 5, the Cp-chlorinated phenoxy analogues, CpTiCl 2 (OAr) (Ar = 2,6-Cl 2 C 6 H 3 (12), 2,4,6-Cl 3 C 6 H 2 (13), or C 6 Cl 5 (14)), showed higher catalytic activity (2010-3090 kg-polymer/mol-Ti•h) than the reported (Ind)TiCl 2 (O-2,6-i Pr 2 C 6 H 3 ) (6) and CpTiCl 2 (O-2,6-i Pr 2 C 6 H 3 ) (15) under the same conditions in the presence of the MAO cocatalyst (Scheme 5) [70].The activities of 12-14 were also higher than those of the indenyl analogues.The resultant polymers are poly(ethylene-co-NBE)s possessing rather high molecular weights (M n = 45,400-82,500) with uniform molecular weight distributions (M w /M n = 1.29-1.63),and their compositions are uniform, as confirmed by DSC thermograms.These are similar observations to those of copolymerization by the 6-MAO catalyst, but the resultant polymer prepared by 15 possessed bimodal molecular weight distributions, as previously observed in ethylene/1-hexene copolymerization [71], suggesting that several catalytically active species are present in the solution.Note that the NBE content in the resultant copolymers (estimated by glass transition temperature (Tg)) prepared by 12-14 are higher than those prepared by the others under the same conditions; the NBE content by 12 and 13 (NBE content 49.6 and 45.3 mol %, respectively) is much higher than [Me2Si(C5Me4)(N t Bu)]TiCl2 (CGC, 3, 29.5 mol%), [Me2Si(indenyl)2]ZrCl2 (DSBI, 1, 26.5 mol%), and CpTiCl2(N=C t Bu2) (7) under the same conditions.Although the activities of 12 and 13 were lower than that of 7 (but at a higher or similar level compared to CGC and DSBI), the results thus clearly indicate that the chlorinated phenoxy-modified Cp analogues, especially 12 and 13, showed the most efficient NBE incorporation compared with the others under these conditions.The 2,4,6-Cl3C6H2 analogue (13) showed higher catalytic activity than that of the 2,6-Cl2C6H3 analogue (12), whereas 12 showed better NBE incorporation than 13 under the same conditions (e.g., run 1 vs run 4).The activity (on the basis of polymer yields) of 13 at 40-60 °C was higher than that conducted at 25 °C; the activity of 12 at 40 °C was higher than that conducted at 25 °C.A linear relationship between the NBE content in the resultant copolymers and glass transition temperature (Tg) was observed.
As far as we know, this is the first demonstration that an introduction of electron-withdrawing substituents leads to more efficient comonomer incorporation in ethylene/cyclic olefin copolymerization.This fact would be in distinct contrast to the catalyst design in ordinary olefin Scheme 5. Copolymerization of ethylene with norbornene with half-titanocenes containing chlorinated aryloxo ligands [70].
Note that the NBE content in the resultant copolymers (estimated by glass transition temperature (T g )) prepared by 12-14 are higher than those prepared by the others under the same conditions; the NBE content by 12 and 13 (NBE content 49.6 and 45.3 mol %, respectively) is much higher than [Me 2 Si(C 5 Me 4 )(N t Bu)]TiCl 2 (CGC, 3, 29.5 mol%), [Me 2 Si(indenyl) 2 ]ZrCl 2 (DSBI, 1, 26.5 mol%), and CpTiCl 2 (N=C t Bu 2 ) (7) under the same conditions.Although the activities of 12 and 13 were lower than that of 7 (but at a higher or similar level compared to CGC and DSBI), the results thus clearly indicate that the chlorinated phenoxy-modified Cp analogues, especially 12 and 13, showed the most efficient NBE incorporation compared with the others under these conditions.The 2,4,6-Cl 3 C 6 H 2 analogue (13) showed higher catalytic activity than that of the 2,6-Cl 2 C 6 H 3 analogue (12), whereas 12 showed better NBE incorporation than 13 under the same conditions (e.g., run 1 vs run 4).The activity (on the basis of polymer yields) of 13 at 40-60 • C was higher than that conducted at 25 • C; the activity of 12 at 40 • C was higher than that conducted at 25 • C. A linear relationship between the NBE content in the resultant copolymers and glass transition temperature (T g ) was observed.
As far as we know, this is the first demonstration that an introduction of electron-withdrawing substituents leads to more efficient comonomer incorporation in ethylene/cyclic olefin copolymerization.This fact would be in distinct contrast to the catalyst design in ordinary olefin (ethylene and styrene) polymerization, in which stabilization of the active site by more electron-donating substituents is important, leading to the high activity.The results from this study demonstrate a possibility that control of π-donation (which would affect the ability of accepting π-electrons from olefins) plays a role for the efficient incorporation of (sterically encumbered) cyclic olefins in this catalysis.

Summary and Outlook
We have shown that efficient incorporations of norbornene (NBE) and tetracyclododenece (TCD) have been achieved in the copolymerization with ethylene, 1-hexene (HX), 1-octene (OC), and with 1-dodecene (DD) by using Cp TiCl2(N=CtBu2) (Cp = Cp (7), tBuC5H4 (8))-MAO catalysts.The resultant copolymers possess rather high molecular weights, unimodal molecular weight distributions, and uniform compositions as confirmed by DSC thermograms.This is, as far as we know, the first demonstration for the synthesis of TCD/α-olefin copolymers, in addition to preparation of a series of random poly(NBE-co-α-olefin)s with various NBE content.Moreover, the efficient synthesis of poly(ethylene-co-TCD)s has also been attained in this catalysis.These copolymers possess a high glass transition temperature (thermal resistance) with high transparency and should show unique properties (e.g., humidity resistance).It is thus highly expected that these polymers are promising materials as a new class of cyclic olefin copolymers (COCs).
As described in the introduction, design of molecular catalysts that precisely control the copolymerization has attracted considerable attention for the synthesis of new polymers with specified functions.The ligand design plays an essential role, and we demonstrated in so-called nonbridged half-titanocene catalysts-Cp'TiX 2 (Y) (Cp' = cyclopentadienyl; X = halogen, alkyl etc.; Y = anionic ancillary donor such as aryloxy, ketimide, imidazolin-2-iminato, etc.)-that efficient catalysts for the desired copolymerization can be tuned by the ligand modification on both the cyclopentadienyl fragment and nature (and substituents) of the anionic donor ligand employed, as demonstrated in the ethylene copolymerization with NBE [22][23][24], TCD [53], cyclohexene [72], cyclopentene [73], 2-methyl-1-pentene [74], and the others [39,41,45], as summarized in Scheme 6.We thus believe that the results demonstrated here are promising in terms of the synthesis of new polymers, and provides a better understanding not only for the polymer design (combination of monomers), but also for the catalyst design for efficient synthesis.
distributions, and uniform compositions as confirmed by DSC thermograms.This is, as far as we know, the first demonstration for the synthesis of TCD/α-olefin copolymers, in addition to preparation of a series of random poly(NBE-co-α-olefin)s with various NBE content.Moreover, the efficient synthesis of poly(ethylene-co-TCD)s has also been attained in this catalysis.These copolymers possess a high glass transition temperature (thermal resistance) with high transparency and should show unique properties (e.g., humidity resistance).It is thus highly expected that these polymers are promising materials as a new class of cyclic olefin copolymers (COCs).
As described in the introduction, design of molecular catalysts that precisely control the copolymerization has attracted considerable attention for the synthesis of new polymers with specified functions.The ligand design plays an essential role, and we demonstrated in so-called nonbridged half-titanocene catalysts-Cp'TiX2(Y) (Cp' = cyclopentadienyl; X = halogen, alkyl etc.; Y = anionic ancillary donor such as aryloxy, ketimide, imidazolin-2-iminato, etc.)-that efficient catalysts for the desired copolymerization can be tuned by the ligand modification on both the cyclopentadienyl fragment and nature (and substituents) of the anionic donor ligand employed, as demonstrated in the ethylene copolymerization with NBE [22][23][24], TCD [53], cyclohexene [72], cyclopentene [73], 2-methyl-1-pentene [74], and the others [39,41,45], as summarized in Scheme 6.We thus believe that the results demonstrated here are promising in terms of the synthesis of new polymers, and provides a better understanding not only for the polymer design (combination of monomers), but also for the catalyst design for efficient synthesis.
The attempted copolymerizations of TCD with HX by [Me 2 Si(C 5 Me 4 )(N t Bu)]TiCl 2 (3)-MAO catalyst afforded a negligible amount of polymers in most cases; low molecular weight oligomer (M n = 5300) with low TCD content (estimated by DSC thermogram, T g = 9.8 • C) was obtained under specified conditions.The copolymerizations with HX, OC, and DD by [Et(indenyl) 2 ]ZrCl 2 (2) or Cp 2 ZrCl 2 (11) yielded a small amount of oligomer under certain conditions; this is in distinct contrast to that observed for CpTiCl 2 (N= t Bu 2 ) (7).

Ti/μmol TCD b /mol/L temp./°C Activity c Mn d × 10 −5 Mw/Mn d Tg e (Tm e )/°C TCD f /mol% 3
g estimated on the basis of the plots of Tg and TCD content.