Cubane‐Type Polynuclear Zinc Complexes Containing Tridentate Schiff Base Ligands: Synthesis,Characterization, and Ring‐Opening Polymerization Studies of rac‐Lactide and ε‐Caprolactone

New polynuclear zinc complexes containing tridentate Schiff base ligands were successfully synthesized and fully characterized. The solid‐state structure of the complexes was determined using single crystal X‐ray diffraction. The complexes display a tetranuclear cubane‐like core structure [Zn₄O₄] and sowed good catalytic activity towards the ring‐opening polymerization (ROP ) of rac‐lactide (rac‐LA ) and ε‐caprolactone (ε‐CL ) under solvent‐free conditions. The polylactic acid (PLA ) obtained from rac‐LA showed isotactic enrichment, as proved by homonuclear decoupled ¹H‐NMR analysis. These complexes also showed good activity and superior control towards the ROP of rac‐LA and ε‐CL in the presence of benzyl alcohol as a co‐initiator. Furthermore, kinetic studies demonstrated that the ROP of rac‐LA and ε‐CL has a first order dependence on both monomer (rac‐LA and ε‐CL ) and catalyst concentration.     

Aluminum salen and salan complexes in the ring‐opening polymerization of cyclic esters: Controlled immortal and copolymerization of rac‐β‐butyrolactone and rac‐lactide

Aluminum‐based salen and salan complexes mediate the ring‐opening polymerization (ROP) of rac‐β‐butyrolactone (β‐BL), rac‐lactide, and ε‐caprolactone. Al‐salen and Al‐salan complexes exhibit excellent control over the ROP of rac‐β‐butyrolactone, yielding atactic poly(3‐hydroxybutyrate) (PHB) with narrow PDIs of <1.15 for Al‐salen and <1.05 for Al‐salan. Kinetic studies reveal pseudo‐first‐order polymerization kinetics and a linear relationship between molecular weight and percent conversion. These complexes also mediate the immortal ROP of rac‐β‐BL and rac‐lactide, through the addition of excess benzyl alcohol of up to 50 mol eq., with excellent control observed. A novel methyl/adamantyl‐substituted Al‐salen system further improves control over the ROP of rac‐lactide and rac‐β‐BL, yielding atactic PHB and highly isotactic poly(lactic acid) (P_m = 0.88). Control over the copolymerization of rac‐lactide and rac‐β‐BL was also achieved, yielding poly(lactic acid)‐co‐poly(3‐hydroxybutyrate) with narrow PDIs of <1.10. ¹H NMR spectra of the copolymers indicate a strong bias for the insertion of rac‐lactide over rac‐β‐BL. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Ring‐opening polymerization of rac‐ and meso‐lactide initiated by indium bis(phenolate) isopropoxy complexes

Ring‐opening polymerization of rac‐ and meso‐lactide initiated by indium bis(phenolate) isopropoxides {1,4‐dithiabutanediylbis(4,6‐di‐tert‐butylphenolate)}(isopropoxy)indium ( 1 ) and {1,4‐dithiabutanediylbis(4,6‐di(2‐phenyl‐2‐propyl)phenolate)}(isopropoxy)indium ( 2 ) is found to follow first‐order kinetics for monomer conversion. Activation parameters ΔH^? and ΔS^? suggest an ordered transition state. Initiators 1 and 2 polymerize meso‐lactide faster than rac‐lactide. In general, compound 2 with the more bulky cumyl ortho‐substituents in the phenolate moiety shows higher polymerization activity than 1 with tert‐butyl substituents. meso‐Lactide is polymerized to syndiotactic poly(meso‐lactides) in THF, while polymerization of rac‐lactide in THF gives atactic poly(rac‐lactides) with solvent‐dependent preferences for heterotactic (THF) or isotactic (CH₂Cl₂) sequences. Indium bis(phenolate) compound rac‐(1,2‐cyclohexanedithio‐2,2′‐bis{4,6‐di(2‐phenyl‐2‐propyl)phenolato}(isopropoxy)indium ( 3 ) polymerizes meso‐lactide to give syndiotactic poly(meso‐lactide) with narrow molecular weight distributions and rac‐lactide in THF to give heterotactically enriched poly(rac‐lactides). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4983–4991     

Alkaline Earth Metal‐Mediated Highly Iso‐selective Ring‐Opening Polymerization of rac‐Lactide

Alkaline earth (Ae) metal complexes of the aminophosphine borane ligand are highly active and iso‐selective catalysts for the ring‐opening polymerization (ROP) of rac‐lactide (LA). The polymerization reactions are well controlled and produce polylactides with molecular weights that are precise and narrowly distributed. Kinetic studies reveal that the ROP of rac‐LA catalyzed by all Ae metal complexes had a first‐order dependency on LA concentration as well as catalyst concentration. A plausible reaction mechanism for Ae metal complex‐mediated ROP of rac‐LA is discussed, based on controlled kinetic experiments and molecular chain mobility.     

ɛ‐caprolactone and lactide polymerization promoted by an ionic titanium(IV)/iron(III) polynuclear halo‐alkoxide

The ionic [Ti₃(µ₃‐OPrⁱ)₂(µ‐OPrⁱ)₃(OPrⁱ)₆][FeCl₄] halo‐alkoxide ( A ) was investigated for its activity towards the bulk polymerization of rac‐lactide (rac‐LA) and ?‐caprolactone (?‐CL) in various temperatures, monomer/ A molar proportions, and reaction times. The reactivity of A in the ring‐opening polymerization (ROP) of both monomers is mainly due to the cationic [Ti₃(OPrⁱ)₁₁]⁺ unity and proceeds through the coordination–insertion mechanism. Molecular weights ranging from 6,379 to 13,950 g mol^?1 and PDI values varying from 1.22 to 1.52 were obtained. Results of ROP kinetic studies for both ?‐CL and rac‐LA confirm that the reaction rates are first‐order with respect to monomers. The production of poly(?‐caprolactone) shows a higher sensitivity of the reaction rate to temperature, while the polymerization of rac‐LA is slower and more dependent on the thermal stability of the active species during the propagation step. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2509–2517     

Ring‐opening polymerization of cyclic esters promoted by phosphido‐diphosphine pincer group 3 complexes

Phosphido‐diphosphine Group 3 metal complexes 1–4 [(o‐C₆H₄PR₂)₂P‐M(CH₂SiMe₃)₂; R = Ph, 1 : M = Y, 2 : M = Sc; R = iPr, 3 : M = Y, 4 : M = Sc] are very efficient catalysts for the ring‐opening polymerization (ROP) of cyclic esters such as ε‐caprolactone (ε‐CL), L ‐lactide, and δ‐valerolactone under mild polymerization conditions. In the ROP of ε‐CL, complexes 1–4 promote quantitative conversion of high amount of monomer (up to 3000 equiv) with very high turnover frequencies (TOF) (～4 × 10⁴ mol_CL/mol_I h) showing a catalytic activity among the highest reported in the literature. The immortal and living ROP of ε‐CL and L ‐lactide is feasible by combining complexes 1–4 with 5 equiv of 2‐propanol. Polymers with controlled molecular parameters (M_n, end groups) and low polydispersities (M_w/M_n = 1.05–1.09) are formed as a result of fast alkoxide/alcohol exchange. In the ROP of δ‐valerolactone, complexes 1–4 showed the same activity observed for lactide (L ‐ and D ,L ‐lactide) producing high molecular weight polymers with narrow distribution of molar masses. Complexes 1–4 also promote the ROP of rac‐β butyrolactone affording atactic low molecular weight poly(hydroxybutyrate) bearing unsaturated end groups probably generated by elimination reactions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Highly Stereocontrolled Ring‐Opening Polymerization of Racemic Alkyl β‐Malolactonates Mediated by Yttrium [Amino‐alkoxy‐bis(phenolate)] Complexes

Yttrium [amino‐alkoxy‐bis(phenolate)]amido complexes have been used for the ring‐opening polymerization (ROP) of racemic alkyl β‐malolactonates (4‐alkoxycarbonyl‐2‐oxetanones, rac‐MLA^Rs) bearing an allyl (All), benzyl (Bz) or methyl (Me) lateral ester function. The nature of the ortho‐substituent on the phenolate rings in the metal ancillary dictated the stereocontrol of the ROP, and consequently the syndiotactic enrichment of the resulting polyesters. ROP promoted by catalysts with halogen (Cl, Br)‐disubstituted ligands allowed the first reported synthesis of highly syndiotactic PMLA^Rs (P_r ≥ 0.95); conversely, catalysts bearing bulky alkyl and aryl ortho‐substituted ligands proved largely ineffective. All polymers have been characterized by ¹H and ¹³C{¹H} NMR spectroscopy, MALDI‐ToF mass spectrometry and DSC analyses. Statistical and thermal analyses enabled the rationalization of the chain‐end control mechanism. Whereas the stereocontrol of the polymerization obeyed a Markov first‐order (Mk1) model for the ROP of rac‐MLA^Bz and rac‐MLA^All, the ROP of rac‐MLA^Me led to a chain end‐control of Markov second‐order type (Mk2). DFT computations suggest that the high stereocontrol ability featured by catalysts bearing Cl‐ and Br‐substituted ligands does not likely originate from halogen bonding between the halogen substituent and the growing polyester chain.     

Zinc(II) complexes of Triaza and Amidinate ligands: Efficient initiators for the ring‐opening polymerization of ε‐Caprolactone and rac‐Lactide

Zinc complexes supported by tertiary 1,3,5‐triazapenta‐1,3‐dienate ligand (L¹) and N ‐benzoyl‐N′ ‐arylbenzamidinate [aryl =2,6‐diisopropylphenyl (L²), phenyl (L³)] ligands have been synthesized and characterized. The reaction of L¹H with ZnEt₂ affords a mononuclear zinc complex [L¹ZnEt] ( 1 ) in good yield. Tetra nuclear zinc complex [(L¹)₂Zn₄O(OAc)₄] ( 2 ) is prepared by treating L¹H with one equivalent of Zn(OAc)₂ in toluene. Further, dinuclear zinc complexes [L²ZnEt]₂ ( 3 ) and [L³ZnEt]₂ ( 4 ) are obtained in good yields from L²H and L³H with ZnEt₂ in toluene respectively_. The complexes 1–4 have been characterized by ¹H/¹³C NMR spectroscopy and single crystal X‐ray diffraction studies. All of the complexes have been explored for their catalytic activity toward the ring‐opening polymerization (ROP) of ε ‐caprolactone. It has been found that complex 1 is an active catalyst for the polymerization of ε ‐caprolactone in presence of a cocatalyst benzyl alcohol (BnOH). While complex 2 is as active as 1 there is no need for a cocatalyst for the polymerization to proceed. Dinuclear zinc complexes 3 and 4 show very high activity for the ROP of ε ‐caprolactone (CL) and rac ‐lactide (LA) without requiring a cocatalyst. The resultant polymers are found to have very high molecular weight (M _n = 296 X 10³ g mol⁻¹) and relatively narrow polydispersity index compared to 1 and 2 .     

[2‐(2‐{Bis[2‐(1H‐imidazol‐2‐ylmethyleneamino)ethyl]amino}ethyliminomethyl)imidazolido]cobalt(III) bis(tetrafluoridoborate) monohydrate

The title compound, [Co(C₁₈H₂₃N₁₀)](BF₄)₂·H₂O, is the result of complexing a Co cation (initially in a Co^II state) with tris[2‐(1H‐imidazol‐2‐ylmethyleneamino)ethyl]amine (L), obtained by a condensation process involving imidazole‐2‐carbaldehyde and tris(2‐aminoethyl)amine. Both the Co cation and the ligand were modified in the synthesis process, the cation via oxidation to Co^III, and the ligand via deprotonation to convert it into the 2‐(2‐{bis[2‐(1H‐imidazol‐2‐ylmethyleneamino)ethyl]amino}ethyliminomethyl)imidazolide anion (L⁻). The ligand chelates the metal centre in a hexadentate fashion, forming a slightly distorted octahedral CoN₆ chromophore. Packing is governed by N—H...N hydrogen bonds defining zigzag chains. A similar structure in the literature is discussed, and the wrong assignment of the oxidation state, given therein to the Co cation, is corrected.     

Crystallization experiments with the dinuclear chelate ring complex di‐μ‐chlorido‐bis[(η2‐2‐allyl‐4‐methoxy‐5‐{[(propan‐2‐yloxy)carbonyl]methoxy}phenyl‐κC1)platinum(II)]

Crystallization experiments with the dinuclear chelate ring complex di‐μ‐chlorido‐bis[(η²‐2‐allyl‐4‐methoxy‐5‐{[(propan‐2‐yloxy)carbonyl]methoxy}phenyl‐κC¹)platinum(II)], [Pt₂(C₁₅H₁₉O₄)₂Cl₂], containing a derivative of the natural compound eugenol as ligand, have been performed. Using five different sets of crystallization conditions resulted in four different complexes which can be further used as starting compounds for the synthesis of Pt complexes with promising anticancer activities. In the case of vapour diffusion with the binary chloroform–diethyl ether or methylene chloride–diethyl ether systems, no change of the molecular structure was observed. Using evaporation from acetonitrile (at room temperature), dimethylformamide (DMF, at 313 K) or dimethyl sulfoxide (DMSO, at 313 K), however, resulted in the displacement of a chloride ligand by the solvent, giving, respectively, the mononuclear complexes (acetonitrile‐κN)(η²‐2‐allyl‐4‐methoxy‐5‐{[(propan‐2‐yloxy)carbonyl]methoxy}phenyl‐κC¹)chloridoplatinum(II) monohydrate, [Pt(C₁₅H₁₉O₄)Cl(CH₃CN)]·H₂O, (η²‐2‐allyl‐4‐methoxy‐5‐{[(propan‐2‐yloxy)carbonyl]methoxy}phenyl‐κC¹)chlorido(dimethylformamide‐κO)platinum(II), [Pt(C₁₅H₁₉O₄)Cl(C₂H₇NO)], and (η²‐2‐allyl‐4‐methoxy‐5‐{[(propan‐2‐yloxy)carbonyl]methoxy}phenyl‐κC¹)chlorido(dimethyl sulfoxide‐κS)platinum(II), determined as the analogue {η²‐2‐allyl‐4‐methoxy‐5‐[(ethoxycarbonyl)methoxy]phenyl‐κC¹}chlorido(dimethyl sulfoxide‐κS)platinum(II), [Pt(C₁₄H₁₇O₄)Cl(C₂H₆OS)]. The crystal structures confirm that acetonitrile interacts with the Pt^II atom via its N atom, while for DMSO, the S atom is the coordinating atom. For the replacement, the longest of the two Pt—Cl bonds is cleaved, leading to a cis position of the solvent ligand with respect to the allyl group. The crystal packing of the complexes is characterized by dimer formation via C—H…O and C—H…π interactions, but no π–π interactions are observed despite the presence of the aromatic ring.     

Dual zinc catalysts for L‐lactide polymerization and reaction of CO2 with cyclohexene oxide

A series of zinc complexes, [ L ^X ZnEt] ( 1–5 ) and [ L ^X Zn ₂ (OAc) ₃ ] (6–9) , associated with NNO‐tridentate Schiff base ligands (2‐(((2‐((cyclohexyl[methyl]amino)methyl)phenyl)imino)methyl)phenolate (CAP) derivatives), were synthesized, and their activity toward ring‐opening polymerization (ROP) of L‐lactide (LA) and the reaction of CO₂ with cyclohexene oxide were also investigated. All of [ L ^X ZnEt] revealed excellent catalytic activity to ring‐opening polymerization (ROP) of LA in the presence of benzyl alcohol. Among them, [ L ^H ZnEt] (1) showed the highest activity with 82% conversation within 45 s. In contrast, [L ^X Zn ₂ (OAc) ₃ ] (6–9) were inactive in ROP of L‐lactide. In addition, all of these Zn complexes demonstrated moderate activity in the reaction of CO₂ with cyclohexene oxide in the presence of Bu₄NCl.     

Kinetics of propylene polymerization initiated by rac‐Me2Si(1‐C5H2‐2‐Me‐4‐tBu)2Zr(NME2)2/MAO catalyst

The kinetics of propylene polymerization initiated by ansa‐metallocene diamide compound rac‐Me₂Si(CMB)₂Zr(NMe₂)₂ (rac‐1, CMB = 1‐C₅H₂‐2‐Me‐4‐^tBu)/methylaluminoxane (MAO) catalyst were investigated. The formation of cationic active species has been studied by the sequential NMR‐scale reactions of rac‐1 with MAO. The rac‐1 is first transformed to rac‐Me₂Si(CMB)₂ZrMe₂ (rac‐2) through the alkylation mainly by free AlMe₃ contained in MAO. The methylzirconium cations are then formed by the reaction of rac‐2 and MAO. Small amount of MAO ([Al]/[Zr] = 40) is enough to completely activate rac‐1 to afford methylzirconium cations that can polymerize propylene. In the lab‐scale polymerizations carried out at 30°C in toluene, the rate of polymerization (R_p) shows maximum at [Al]/[Zr] = 6,250. The R_p increases as the polymerization temperature (T_p) increases in the range of T_p between 10 and 70°C and as the catalyst concentration increases in the range between 21.9 and 109.6 μM. The activation energies evaluated by simple kinetic scheme are 4.7 kcal/mol during the acceleration period of polymerization and 12.2 kcal/mol for an overall reaction. The introduction of additional free AlMe₃ before activating rac‐1 with MAO during polymerization deeply influences the polymerization behavior. The iPPs obtained at various conditions are characterized by high melting point (approximately 155°C), high stereoregularity (almost 100% [mmmm] pentad), low molecular weight (MW), and narrow molecular weight distribution (below 2.0). The fractionation results by various solvents show that iPPs produced at T_p below 30°C are compositionally homogeneous, but those obtained at T_p above 40°C are separated into many fractions. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 737–750, 1999     

MAO‐free polymerization of propylene by rac‐ Me2Si(1‐C5H2‐2‐Me‐4‐tBu)2Zr(NMe2)2 compound

Ansa‐zirconocene diamide complex rac‐Me₂Si(CMB)₂Zr(NMe₂)₂ (rac‐1, CMB = 1‐C₅H₂‐2‐Me‐4‐^tBu) reacts with AlR₃ (R = Me, Et, i‐Bu) and then with [CPh₃]⁺[B(C₆F₅)₄]⁻ (2) in toluene in order to in situ generate cationic alkylzirconium species. In the sequential NMR‐scale reactions of rac‐1 with various amount of AlMe₃ and 2, rac‐1 transforms first to rac‐Me₂Si(CMB)₂Zr(Me)(NMe₂) (rac‐3) and rac‐Me₂Si(CMB)₂ZrMe₂ (rac‐4) by the reaction with AlMe₃, and then to [rac‐Me₂Si(CMB)₂ZrMe]⁺ (5⁺) cation by the reaction of the resulting mixtures with 2. The activities of propylene polymerizations by rac‐1/Al(i‐Bu)₃/2 system are dependent on the type and concentration of AlR₃, resulting in the order of activity: rac‐1/Al(i‐Bu)₃/2 > rac‐1/AlEt₃/2 > rac‐1/MAO ≫ rac‐1/AlMe₃/2 system. The bulkier isobutyl substituents make inactive catalytic species sterically unfavorable and give rise to more separated ion pairs so that the monomers can easily access to the active sites. The dependence of the maximum rate (R_{p, max}) on polymerization temperature (T_p) obtained by rac‐1/Al(i‐Bu)₃/2 system follows Arrhenius relation, and the overall activation energy corresponds to 0.34 kcal/mol. The molecular weight (MW) of the resulting isotactic polypropylene (iPP) is not sensitive to Al(i‐Bu)₃ concentration. The analysis of regiochemical errors of iPP shows that the chain transfer to Al(i‐Bu)₃ is a minor chain termination. The 1,3‐addition of propylene monomer is the main source of regiochemical sequence and the [mr] sequence is negligible, as a result the meso pentad ([mmmm]) values of iPPs are very high ([mmmm] > 94%). These results can explain the fact that rac‐1/Al(i‐Bu)₃/2 system keeps high activity over a wide range of [Al(i‐Bu)₃]/[Zr] ratio between 32 and 3,260. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1071–1082, 1999     

Controlled Ring‐Opening Polymerization of O‐Carboxyanhydrides Using a β‐Diiminate Zinc Catalyst

Recently O‐carboxyanhydrides (OCAs) have emerged as a class of viable monomers which can undergo ring‐opening polymerization (ROP) to prepare poly(α‐hydroxyalkanoic acid) with functional groups that are typically difficult to achieve by ROP of lactones. Organocatalysts for the ROP of OCAs, such as dimethylaminopyridine (DMAP), may induce undesired epimerization of the α‐carbon atom in polyesters resulting in the loss of isotacticity. Herein, we report the use of (BDI‐IE)Zn(OCH(CH₃)COOCH₃) ((BDI)Zn‐1, (BDI‐IE)=2‐((2,6‐diethylphenyl)amino)‐4‐((2,6‐diisopropylphenyl)imino)‐2‐pentene), for the controlled ROP of various OCAs without epimerization. Both homopolymers and block copolymers with controlled molecular weights, narrow molecular weight distributions, and isotactic backbones can be readily synthesized. (BDI)Zn‐1 also enables controlled copolymerization of OCAs and lactide, facilitating the synthesis of block copolymers potentially useful for various biomedical applications. Preliminary mechanistic studies suggest that the monomer/dimer equilibrium of the zinc catalyst influences the ROP of OCAs, with the monomeric (BDI)Zn‐1 possessing superior catalytic activity for the initiation of ROP in comparison to the dimeric (BDI)Zn complex.     

Synthesis of lactide/ɛ‐caprolactone quasi‐random copolymer by using rationally designed mononuclear aluminum complexes with modified β‐ketiminato ligand

A series of novel aluminum complexes containing bulky aryl‐β‐ketiminato ligands [ArNCH C₁₀H₇C₆H₅O]Al(CH₃)₂ ( 3a , Ar = C₆F₅; 3b , Ar = C₆H₅; 3c , Ar = 2,6‐ⁱPr₂C₆H₃) have been synthesized in high yields. These complexes were identified by ¹H and ¹³C NMR spectroscopy, elemental analysis, and X‐ray structural analysis. All the aluminum complexes could efficiently catalyze the ROP of ɛ‐caprolactone (ɛ‐CL) and Lactide (LA) in a controlled manner. It was found that the steric effect of the ligand has less effect on the ROP of CL, while the polymerization rate of L‐LA was suppressed significantly. More interestingly, this kind of catalysts can promote the random copolymerization of ɛ‐CL and L‐LA. The transesterification side reaction and the polymer composition could be adjusted by modulating the electronic and steric effects of the ligand. In paticular, compound 3c could produce quasi‐random copolymers without transesterification side reactions, as indicated by both the values of the reactivity ratios of the two monomers (r_LA = 1.31; r_CL = 0.99) and the similar average lengths of the caproyl and lactidyl sequences (L_CL = 2.34; L_LA = 2.44). Finally, a drug‐random copolymer conjugates could be easily prepared by using 3c , indicating a potential application of 3c in pharmacutical and biomedical field. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 203–212     

Higher α‐olefin polymerizations catalyzed by rac‐Me2Si(1‐C5H2‐2‐CH3‐4‐tBu)2Zr(NMe2)2/Al(iBu)3/[Ph3C][B(C6F5)4]

Polymerizations of higher α‐olefins, 1‐pentene, 1‐hexene, 1‐octene, and 1‐decene were carried out at 30 °C in toluene by using highly isospecific rac‐Me₂Si(1‐C₅H₂‐2‐CH₃‐4‐^t Bu)₂Zr(NMe₂)₂ (rac‐1) compound in the presence of Al(iBu)₃/[CPh₃][B(C₆F₅)₄] as a cocatalyst formulation. Both the bulkiness of monomer and the lateral size of polymer influenced the activity of polymerization. The larger lateral of polymer chain opens the π‐ligand of active site wide and favors the insertion of monomer, while the large size of monomer inserts itself into polymer chain more difficultly due to the steric hindrance. Highly isotactic poly(α‐olefin)s of high molecular weight (MW) were produced. The MW decreased from polypropylene to poly(1‐hexene), and then increased from poly(1‐hexene) to poly(1‐decene). The isotacticity (as [mm] triad) of the polymer decreased with the increased lateral size in the order: poly(1‐pentene) > poly(1‐hexene) > poly(1‐octene) > poly(1‐decene). The similar dependence of the lateral size on the melting point of polymer was recorded by differential scanning calorimetry (DSC). ¹H NMR analysis showed that vinylidene group resulting from β‐H elimination and saturated methyl groups resulting from chain transfer to cocatalyst are the main end groups of polymer chain. The vinylidene and internal double bonds are also identified by Raman spectroscopy. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1687–1697, 2000     

Heterolepic β-Ketoiminate Zinc Phenoxide Complexes as Efficient Catalysts for the Ring Opening Polymerization of Lactide

Zinc phenoxide complexes L¹ZnOAr 1 – 4 (L¹=Me₂NC₂H₄NC(Me)CHC(Me)O) and L²ZnOAr 5 – 8 (L²=Me₂NC₃H₆NC(Me)CHC(Me)O) with donor-functionalized β-ketoiminate ligands (L^1/2) and OAr substituents (Ar=Ph 1 , 5 ; 2,6-Me₂-C₆H₃ 2 , 6 ; 3,5-Me₂-C₆H₃ 3 , 7 ; 4-Bu-C₆H₄ 4 , 8 ) with tuneable electronic and steric properties were synthesized and characterized. 1 – 8 adopt binuclear structures in the solid state except for 5 , while they are monomeric in CDCl₃ solution. 1 – 8 are active catalysts for the ring opening polymerization (ROP) of lactide (LA) in CH₂Cl₂ at ambient temperature and the catalytic activity is controlled by the electronic and steric properties of the OAr substituent, yielding polymers with high average molecular weight (M_n) and moderately controlled molecular weight distribution (MWDs). 1 and 5 showed a living polymerization character and kinetic studies on the ROP of L–LA with 1 and 5 proved first order dependencies on the monomer concentration. Homonuclear decoupled ¹H-NMR analyses of polylactic acid (PLA) formed with rac-LA proved isotactic enrichment of the PLA microstructure.     

Ring‐opening polymerization of lactides initiated by magnesium and zinc complexes based on NNO‐tridentate ketiminate ligands: Activity and stereoselectivity studies

Four metal benzylalkoxides, [L₂M₂(μ‐OBn)₂] (M = Mg or Zn), based on NNO‐tridentate ketiminate ligands are synthesized and characterized. X‐ray crystal structural studies of [(L¹)₂Mg₂(μ‐OBn)₂] ( 1a ) and [(L¹)₂Zn₂(μ‐OBn)₂] ( 1b ) (L¹‐H = (Z)‐4‐((2‐(dimethylamino)ethylamino)(phenyl)methylene)‐3‐methyl‐1‐phenyl‐pyrazol‐5‐one) reveal that both complexes 1a and 1b are dinuclear species whereas the geometry around the metal center is penta‐coordinated bridging through the benzylalkoxy oxygen atoms in the solid structure. The activities and stereoselectivities of these four complexes toward the ring‐opening polymerization of L ‐lactide and rac‐lactide are investigated. Polymerization of L ‐lactide initiated by these four metal benzyloxides proceeds rapidly with good molecular weight control and yields polymer with a very narrow molecular weight distribution. The kinetic studies for the polymerization of L ‐lactide with compound 1a show first order in both compound 1a and lactide concentrations with the polymerization rate constant, k, of 6.94 M/min. Besides, experimental results demonstrate that among these metal benzylalkoxides, complex 1a exhibits the highest stereoselectivity with a Pr up to 87% and complex 1b possesses the highest activity indicating that the terminal group of NNO‐tridentate ketimine ligands exerts a significant influence on both the reactivity and stereoselectivity of these complexes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2318–2329, 2009     

New tethered ansa‐bridged zirconium catalysts: Insights into the “self‐immobilization” mechanism

New ω‐alkenyl‐substituted ansa‐bridged bisindenyl zirconium complexes are prepared and tested as self‐immobilized catalysts for ethene polymerization. But, even at very high concentration of the tethered complexes and low pressure of ethene, there is no evidence of their insertion into the polyethene chain. A “cross polymerization” test, performed by copolymerizing the tethered complexes with ethene using rac‐Me₂Si(2‐MeBenzInd)₂ZrCl₂ ( MBI ), does not lead to their incorporation into the polyethene chain. However, the corresponding ligand proves to be a suitable comonomer for ethene, and, through copolymerization promoted by MBI, innovative poly(ethene‐co‐2,2′‐bis[(1H‐inden‐3′‐yl)‐hex‐5‐ene) copolymers are prepared and characterized by ¹³C NMR. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

3,5‐Bis{4‐[(benzimidazol‐1‐yl)methyl]phenyl}‐4H‐1,2,4‐triazol‐4‐amine and its one‐dimensional polymeric complex with HgCl2

The molecule of 3,5‐bis{4‐[(benzimidazol‐1‐yl)methyl]phenyl}‐4H‐1,2,4‐triazol‐4‐amine (L), C₃₀H₂₄N₈, has an antiperiplanar conformation of the two terminal benzimidazole groups and forms two‐dimensional networks along the crystallographic b axis via two types of intermolecular hydrogen bonds. However, in catena‐poly[[[dichloridomercury(II)]‐μ‐3,5‐bis{4‐[(benzimidazol‐1‐yl)methyl]phenyl}‐4H‐1,2,4‐triazol‐4‐amine] dichloromethane hemisolvate], {[HgCl₂(C₃₀H₂₄N₈)]·0.5CH₂Cl₂}_n, synthesized by the combination of L with HgCl₂, the L ligand adopts a synperiplanar conformation. The Hg^II cation lies in a distorted tetrahedral environment, which is defined by two N atoms and two Cl atoms to form a one‐dimensional zigzag chain. These zigzag chains stack via hydrogen bonds which expand the dimensionality of the structure from one to two.