Naofumi Naga

Recent developments in olefin polymerizations with transition metal catalysts

Abstract Discovery of effective homogeneous transition metal catalysts for olefin polymerization must be one of the most dramatic technologies for polymer synthesis in the 1980s. Especially, development of group 4-metallocene catalysts not only improved the property of manufactured polyolefins but also made us possible to synthesize new type polyolefins. A large number of studies have also been made on structure and property of the resultant polyolefins with the advance of catalyst technology. Over the past few years, trend in catalyst development has moved from modification of group 4-metallocene catalysts to search for new generation catalysts, such as non-metallocene catalysts and late transition-metal catalysts. Some of the new catalysts can proceed specific polymerizations, which are not achieved with existent olefin polymerization catalysts. Although, there are some subjects for the manufacturing of new catalysts and polyolefins, the technologies are progressing rapidly. In this review, we highlight not only the development of polymerization catalysts but also the polyolefins obtained with the new catalysts in the last decade.

Biocompatible and Biodegradable Dual-Drug Release System Based on Silk Hydrogel Containing Silk Nanoparticles

We developed a facile and quick ethanol-based method for preparing silk nanoparticles and then fabricated a biodegradable and biocompatible dual-drug release system based on silk nanoparticles and the molecular networks of silk hydrogels. Model drugs incorporated in the silk nanoparticles and silk hydrogels showed fast and constant release, respectively, indicating successful dual-drug release from silk hydrogel containing silk nanoparticles. The release behaviors achieved by this dual-drug release system suggest to be regulated by physical properties (e.g., β-sheet contents and size of the silk nanoparticles and network size of the silk hydrogels), which is an important advantage for biomedical applications. The present silk-based system for dual-drug release also demonstrated no significant cytotoxicity against human mesenchymal stem cells (hMSCs), and thus, this silk-based dual-drug release system has potential as a versatile and useful new platform of polymeric materials for various types of dual delivery of bioactive molecules.

Chain transfer reaction by trialkylaluminum (AIR3) in the stereospecific polymerization of propylene with metallocene - AIR3/Ph3CB(C6F5)4

Abstract Stereospecific polymerization of propylene was carried out with rac-ethylenebis(indenyl)zirconium dichloride (rac-Et(Ind)2ZrCl2) (1), rac-dimethylsilylenebis(indenyl)zirconium dichloride (rac-Me2Si(Ind)2ZrCl2) (2) and isopropylidene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride (i-Pr(Cp)(Flu)ZrCl2) (3) combined with trialkyl-aluminum (AIR3: R = C2H5, i-C4H9)/triphenylcarbenium tetrakis(pentafluorophenyl)borate (Ph3CB(C6F5)4) (4). In isospecific polymerization with 1 and 2, the molecular weight of polypropylenes decreased with increase in the molar ratio of AlEt3 (Et = C2H5)/Zr, whereas, an effect of AliBu3 (iBu = i-C4H9) concentration on molecular weight was not observed. The microstructures of resulting polypropylenes were studied by 13C n.m.r. and an increase in the molar ratio of ethyl end groups (derived from chain transfer to AlEt3) to n-propyl end groups (derived from β-hydrogen transfer) was observed with increase in the molar ratio of AlEt 3 Zr (1 and 2). The chain transfer reactions by both AlEt3 and AliBu3 were also detected in syndiospecific polymerization with 3. The molar ratio of alkyl (R) end groups (derived from chain transfer to AIR3) to n-propyl end groups was higher in the polypropylene obtained with AlEt3 than that obtained with AliBu3. The relative constants k trA k p (ktrA = rate constant of chain transfer to AIR3, kp = rate constant of propagation) were determined by kinetic study.

Copolymerization of ethylene and cyclopentene with zirconocene catalysts: Effect of ligand structure of zirconocenes

Copolymerization of ethylene and cyclopentene (CPE) was carried out with various non-bridged and bridged zirconocene catalysts using methylaluminoxane as a cocatalyst. Non-bridged metallocene catalysts effected ethylene homopolymerization without copolymerization. On the other hand, bridged zirconocene catalysts produced copolymers containing cis-1,2-cyclopentane units. Among the used catalysts, rac-ethylenebis(indenyl)zirconium dichloride [Et(Ind) 2 ZrCl 2 ] gave copolymers containing the highest amount of CPE units by 1,2- insertion or 1,3-insertion. Temperature-rising elution fractionation of the copolymers using 1,2-dichlorobenzene as a solvent showed a broad distribution of copolymer composition in copolymers obtained by specific zirconocene catalysts.

Copolymerization of ethylene and 1,7-octadiene, 1,9-decadiene with zirconocene catalysts

Copolymerizations of ethylene and 1,7-octadiene (OD) and 1,9-decadiene (DD) were investigated with various non-bridged and bridged zirconocene catalysts using methylisobutylaluminoxane as a cocatalyst. The resulting copolymers were extracted with boiling o-dichlorobenzene (ODCB), and the structure of the boiling ODCD-soluble part was studied by 1 H, 13 C NMR and DEPT (distortionless enhancement of polarization transfer) spectroscopy. In the case of ethylene/OD copolymerization, the ligand structure of the zirconocene catalysts strongly affected the propagation mode of OD. The zirconocene catalysts having cyclopentadienyl or pentamethylcyclopentadienyl ligands gave copolymers having 1-hexenyl and 1,3-disubstituted cycloheptane units, derived from 1,2-addition propagation and addition-cyclization propagatin of OD, respectively. On the other hand, the zirconocene catalysts with indenyl ligand produced the copolymers having exclusively 1,3-disubstituted cyclo-heptane units. Furthermore, the copolymer prepared by diphenylmethylene(cyclopentadienyl)(0-fluorenyl)zirconuim dichloride was crosslinked. The diastereostructure of the 1,3-disubstituted cycloheptane units in the copolymers was not influenced by the stereospecificity of the catalysts used, and a cis-structure was preferentially formed. In the case of the copolymerization of ethylene and DD, the C 2v -symmetric zirconocene catalysts produced the copolymers with 1-octenyl branches derived from 1,2-addition propagation of DD. Other C 2 - and C s -symmetric zirconocene catalysts with bulky ligands yielded copolymers with cross-linking structures derived form addition propagation of side-chain unsaturated bond of 1,2-added DD units.

Effect of ligand in ethylene/styrene copolymerization by [Me2Si(C5Me4)(NR)]TiCl2 (R = tert-Bu, cyclohexyl) and (1,3-Me2C5H3)TiCl2(O-2,6-iPr2C6H3)-MAO catalyst system

Abstract Copolymerization of ethylene with styrene using linked cyclopentadienyl-amide titanium(IV) complexes, [Me 2 Si(C 5 Me 4 )(R)]TiCl 2 [R= tert -Bu ( 1 ), cyclohexyl ( 2 )], and non-bridged (1,3-Me 2 C 5 H 3 )TiCl 2 (O-2,6- i Pr 2 C 6 H 3 ) ( 3 )-MAO catalysts have been explored. Although the catalytic activity by 2 was lower than 1 , 2 showed more efficient styrene incorporation than 1 under the same conditions. Moreover, the resultant copolymer prepared by 2 possessed completely different microstructure from those by 1 , indicating that the nature of amide ligand affects both styrene incorporation and monomer sequence.

Stereochemical control in propylene polymerization with non-bridged metallocene dichloride/methylaluminoxane

Abstract Propylene polymerization was carried out with non-bridged bis (substituted-cyclopentadienyl)zirconium dichloride ((RCp) 2 ZrCl 2 /methylaluminoxane (MAO) and bis (substituted indenyl)zirconium dichloride ((RInd) 2 ZrCl 2 )/MAO catalyst at various polymerization temperature. In the case of (RCp) 2 ZrCl 2 , the pentad meso sequence ([mmmm]) increased with lowering polymerization temperature. The isotactic sequence of polypropylene increased with an increase of formula weight of substituents in the case of mono-alkyl substituted bis (cyclopentadienyl)zirconium dichloride. The number of methyl (Me) substituents in [(CH 3 ) n −(C 5 H 5−n )] 2 ZrCl 2 effected the stereo control of polymerization of propylene, and one or two Me substitution resulted in higher isotacticity than the usage of non- or full substituted ligands. Lowering polymerization temperature gave higher isotacticity as observed in Cp 2 TiCl 2 . The (RInd) 2 ZrCl 2 (R = non or 2-Me) showed minimum meso sequence ([mmmm]) at 0°C. Chain-end control was predominant for producing the isotactic portion. The stereoregulation energies were evaluated from the stereochemical dyad composition of the obtained polypropylenes by the Arrhenius plot of ln ([m]/[r]) versus 1/ T ( T = polymerization temperature (K)).

Isothermal crystallization of syndiotactic poly(propylene-co-olefin)s

Abstract Isothermal crystallization of syndiotactic poly(propylene-co-olefin)s (olefin=ethylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene) was studied. The melting behavior of isothermally crystallized polymers was investigated by differential scanning calorimetry, and the equilibrium melting temperature (Tm0) was determined by Hoffman–Weeks plot. Crystallization rate and Avrami exponent of isothermally crystallized copolymers from the melt were measured by depolarized light intensity technique. The half time of crystallization (t1/2) was found to be dependent on the degree of supercooling (ΔT), but independent of the comonomer. The Avrami exponents (n) of copolymers were detected by the Avrami plot and the average n ranged from 2.0 to 2.8 for the primary crystallization process in any copolymer. The effect of isothermal crystallization temperature (Tc) on the crystalline structure of copolymers was studied by wide-angle X-ray diffraction (WAXD). The WAXD patterns of all the copolymers were similar to the pattern of Cell II in the syndiotactic polypropylene. The WAXD pattern of 1-butene copolymer with small amount of 1-butene indicated the existence of Cell III.

Structure of cyclopentene unit in the copolymer with propylene obtained by stereospecific zirconocene catalysts

Abstract Copolymerization of propylene and cyclopentene (CPE) was carried out using as a catalyst isospecific rac -ethylenebis(indenyl)zirconium dichloride ( 1 ), rac -dimethylsilylenebis(indenyl)zirconium dichloride ( 2 ), rac -dimethylsilylenebis(2-methylindenyl)zirconium dichloride ( 3 ), or syndiospecific diphenylmethylene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride ( 4 ) with methylaluminoxane as a cocatalyst. Isospecific zirconocene catalysts 1 – 3 produced copolymers having narrow molecular weight distribution, while syndiospecific catalyst 4 effected propylene homopolymerization. Microstructures of the copolymers were studied by 13 C NMR and distortionless enhancement of polarization transfer (DEPT) spectroscopy. CPE was found to be incorporated in the copolymer preferentially via 1,2-insertion mechanism in the copolymerization with the catalyst 3 . The catalyst 1 and 2 gave copolymers containing CPE units formed by either 1,2-insertion or 1,3-insertion mechanism. The proportion of 1,3-insertion units increased with increasing CPE content in the copolymers. The isomerization reaction from 1,2-insertion to 1,3-insertion CPE units was discussed on the basis of kinetic parameters.

Polymerization behavior of α-olefins with rac- and meso- type ansa-metallocene catalysts: Effects of cocatalyst and metallocene ligand

Polymerizations of propene, 1-butene and 1-hexene were conducted with a mixture of rac- and meso-[dimethylsilylenebis(2-methylindenyl)]zirconium dichloride (1) combined with methylaluminoxane (MAO), triethylaluminium (AlEt 3 ))/triphenylcarbenium tetrakis(pentafluorophenyl)borate (2) or triisobutylaluminium (AliBu 3 )/2 as a cocatalyst. The polymerization profiles of propene with rac-1 and meso-1 were determined from the rate of overall propene consumption and the fractions of isotactic and atactic polymers which were sampled during polymerization. An induction time to reach the maximum R p (rate of polymerization) followed by gradual decay was observed in the case of using the systems rac-, meso-1-MAO and rac-1-AliBu 3 /2. Besides, a rapid drop of R p from the initial value was found when using AlEt 3 /2. Molecular weights of the isotactic and atactic polymers sampled do not change during polymerization, and it is suggested that the change of [C * ] (number of active centers) is reflected in the profiles of R p . The rate ratio of rac-1 to meso-1 (R p (rac)/R p (meso)) in propene and 1-butene polymerizations decreases in the following order: AlEt 3 /2 > AliBu 3 /2 > MAO. In the case of 1-hexene polymerization, the highest R p (rac)/R p (meso) value was obtained. This result indicates that the coordination of 1-hexene to the sterically hindered site of meso-1 is difficult compared with propene and 1-butene.