Single-component Initiators Research Articles

N-Heterocyclic Carbene-Initiated Epoxide/Anhydride Ring-Opening Copolymerization: Effective and Selective Organoinitiators for the Production of Various Polyesters

The present report describes the first use of unprotected and "free" N-heterocyclic carbenes (NHCs), such as IMes, IPr and air-stable Cl-IPr carbenes, as single component initiators for the alternating ring-opening...

Alternating copolymerization of CO2 and cyclohexene oxide initiated by rare-earth metal complexes stabilized by o-phenylenediamine-bridged tris(phenolate) ligand

A series of o-phenylenediamine bridged tris(phenolate) ligand-stabilized rare-earth metal complexes were synthesized and characterized. Lanthanum (complex 1), neodymium (complex 2tBu), and yttrium (complex 3) complexes stabilized by ligand of bulky tert-butyl substituents are mononuclear, whereas neodymium complexes 2Me and 2Cl bearing smaller methyl and chloro substituents are dinuclear, respectively. They were applied in the alternating copolymerization of CO2 and cyclohexene oxide. The addition of benzyl alcohol is beneficial to improving yields (up to 86%) and selectivity of poly(carbonate) (up to 99%). Stoichiometric reaction of benzyl alcohol with rare-earth metal complexes 1 and 2tBu results in formation of complexes 4 and 5 as dinuclear complexes with benzyl alcohol coordination, which was confirmed via single crystal X-ray diffraction analysis, and nuclear magnetic resonance (NMR) spectroscopy (for 4). Complex 5 shows equally good activity and selectivity of that of complex 2tBu and benzyl alcohol, consolidating that complex 5 is indeed generated during polymerization. The neodymium tris(phenolate) 5 is one of the few examples of rare-earth metal complex as a single-component initiator for copolymerization of epoxide and CO2. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry analysis proves benzyloxy group as the chain end. 1H NMR monitoring suggests that the insertion of CO2 may be the first step in the growth of the polymer chain.

Synthesis of mono- and tetra-nuclear β-pyridylenolate zinc complexes and their catalysis toward the ring-opening copolymerization of ε-caprolactone and lactide

The synthesis of mono-nuclear β-pyridylenolate zinc complexes L12Zn (1) (L1 = (2-C5H4N)–CH=C(CH3)–O–) and L22Zn (2) (L2 = (2-C5H4N)–CH=C (2-CH3C6H5)–O–) were performed by the protonolysis of the corresponding β-pyridylenol bidentate ligands 1-(2-pyridinyl)-2-propanone (L1H) and 1-(2-methylphenyl)-2-(2-pyridinyl)-ethanone (L2H) with diethylzinc in a 2:1 M ratio, respectively. The unanticipated tetra-nuclear β-pyridylenolate zinc complex [L12Zn-μ-O (Et)ZnEt]2 (3) was isolated by the reaction of L1H with an equiv. of ZnEt2 and oxygenation with air. All the complexes were well characterized by single crystal X-ray crystallography. Complexes 1–3 were tested as an initiator for the ring opening polymerization (ROP) of ε-caprolactone and l-lactide. In the same conditions, 1 and 2 are shown inefficient for ROP due to the lack of suitable initiating group for polymerization, however, tetra-nuclear complex 3 is a highly active single-component initiator in the ROP of ε-caprolactone and l-lactide. Complex 3 was also researched for its catalytic activity in block (PCL-b-PLLA) and gradient (LA-grad-CL) copolymerization of ε-caprolactone and l-lactide.

Ph3 C][B(C6 F5 )4 ]: A Highly Efficient Metal-Free Single-Component Initiator for the Helical-Sense-Selective Cationic Copolymerization of Chiral Aryl Isocyanides and Achiral Aryl Isocyanides.

Commercially available [Ph3 C][B(C6 F5 )4 ] served as a highly efficient metal-free and single-component initiator not only for the carbocationic polymerization of polar and bulky aryl isocyanides with extremely high activity up to 1.2×107 g of polymer/(molcat. h), but also for the helical-sense-selective polymerization of chiral aryl isocyanides and copolymerization with achiral aryl isocyanides to afford high-molecular-weight functional poly(aryl isocyanide)s with good solubility as well as AIE characteristics and/or a single-handed helical conformation.

β-Pyridylenolate zinc catalysts for the ring-opening homo- and copolymerization of ε-caprolactone and lactides.

A family of zinc β-pyridylenolates with varying α,β-substituents has been synthesized. A direct reaction of β-pyridyl ketone ligands with ZnEt2 in toluene afforded dimeric [LZnEt]2 (L = (2-C5H4N)-C(R1)[double bond, length as m-dash]C(R2)-O-). X-ray structural data revealed that all the [LZnEt]2 complexes exist as μ-O-bridged dimers in the solid state, although they adopt a different geometry. Each of the [LZnEt]2 complexes is a highly active catalyst for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) and racemic lactide (rac-LA), respectively. The donating group on the enolate ligand can accelerate the ROP reaction and control the polymerization process well. All complexes also showed different abilities in the block and random copolymerization of ε-CL and l-LA. Diblock PCL-b-PLLA and PLLA-b-PCL copolymers were easily prepared via sequential addition of ε-CL and l-LA by using complex 10, respectively. Random copolymers of PCL-ran-PLLA were also synthesized from complex 10. The copolymers of PCL-ran-PLLA were shown to possess monomer contents which matched well with their composition in the monomer feed. NMR and DSC characterization confirmed a highly random structure of these copolymers, resulting in closely average lengths of the caproyl and lactidyl sequences (LCL = 2.1; LLA = 1.9). All zinc complexes as efficient single-component initiators in both homo- and copolymerizations showed that polymers were probably susceptible to chain transfer and transesterification due to the close ZnZn proximity in the dimer. The polymers initiated by [LZnEt]2 in the absence of BnOH showed high molecular weight and narrow molecular weight distributions.

HL2][P(1,2-O2C6Cl4)3] (L = THF, DMF): Brønsted acid initiators for the polymerization of n-butyl vinyl ether and p-methoxystyrene.

The development of solid, weighable Brønsted acids featuring the hexacoordinated phosphorous(v) anion [TRISPHAT]- are reported. H(DMF)2[1] and H(THF)2[1] {[1]- = [P(1,2-O2C6Cl4)3]-} were synthesized and fully characterized by 1H, 31P, 13C and 2D-NOESY NMR spectroscopy, X-ray crystallography, mass spectrometry and elemental analysis. Both, H(DMF)2[1] and H(THF)2[1] are found to be suitable initiators for the polymerization of n-butyl vinyl ether and p-methoxystyrene. Optimal polymerization results for these single-component initiators were generally obtained at -50 °C where high molecular weight polymers were isolated in high yield. 1H NMR spectroscopic analysis and dynamic light scattering of solutions of the isolated polymers provide evidence for a branched structure which presumably arises from chain transfer.

Copolymerization of Cyclic Esters Controlled by Chiral NNO-Scorpionate Zinc Initiators

Reaction of the chiral alcohol-scorpionate compound bpzteH [bpzteH = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-para-tolylethanol] with [ZnR2] (R = Me, CH2SiMe3) in a 1:2 molar ratio afforded the dinuclear chiral alkyl zinc complexes [Zn(R)(κ2-NNμ-O)Zn(R)2] (1 and 2) [κ2-NNμ-O = bpzte, R = Me 1, CH2SiMe3 2]. Subsequent alcoholysis reaction with ArOH (1 equiv; Ar = 2,6-C6H3Me2) yielded the chiral dinuclear zinc complexes [(ZnR)2(κN:κN-μ-O)(μ-OAr)] (3 and 4) [κN:κN-μ-O = bpzte, R = Me 3, CH2SiMe3 4]. The X-ray crystal structures of 1 and 3 confirmed a dinuclear structure in all complexes, with the alkoxide of the heteroscorpionate ligands in a μ-bridging mode between the two Zn(II) centers. The zinc complexes 1–4, bearing trisalkyl and alkyl-aryloxide ligands, are shown to function as highly efficient single-component initiators for the ring-opening homopolymerization of e-caprolactone and l-/rac-lactide. These initiators enable rapid polymerization at ambient temperatures, affording materials with low molecular w...

A multimetallic iron(ii)-lithium complex as a catalyst for ε-caprolactone polymerization.

Treatment of the lithium salt of β-ketimine with FeCl2(THF)1.5 in the presence of LiN(SiMe3)2 and water affords the multimetallic iron(ii)-lithium complex 1, [(LMe)2Fe]3Li2O [where LMe = MeC(O)CHC(NMe)Me]. In complex 1 three separated (LMe)2Fe units are bound together by one Li2O species, which leads to the formation of an interesting {Fe3Li2O} core structure. Complex 1 can be used as a single-component initiator for the ring-opening polymerization of ε-caprolactone at room temperature, achieving a monomer conversion of 98% within 100 min, and a narrow molecular weight distribution (PDI = 1.28) of the resulting polymer.

New Racemic and Single Enantiopure Hybrid Scorpionate/Cyclopentadienyl Magnesium and Zinc Initiators for the Stereoselective ROP of Lactides

The preparation of the first racemic bis(pyrazol-1-yl)methane-based NNCp-donor hybrid scorpionate/cyclopentadienyl magnesium and zinc complexes [Mg(R)(κ2-η5-NNCp)] (1–6) [NNCp = bpztcp, R = Me (1), Et (2), CH2SiMe3 (3); bpzpcp, R = Me (4), Et (5), CH2SiMe3 (6)] and [Zn(R)(κ2-η1-NNCp)] (7–12) [NNCp = bpztcp, R = Me (7), Et (8), CH2SiMe3 (9); bpzpcp, R = Me (10), Et (11), CH2SiMe3 (12)] has been carried out by the reaction of the corresponding racemic NNCp-H-donor scorpionate pro-ligands with Grignard reagents RMgCl, after deprotonation with nBuLi, or directly with ZnR2. The resulting alkyl magnesium complexes (1–6) have been employed in a protonolysis reaction with 2,6-dimethylphenol to yield the aryloxide magnesium complexes [Mg(OAr)(κ2-η5-NNCp)] (13 and 14) [NNCp = bpztcp (13), bpzpcp (14)]. Subsequently, the enantiopure scorpionate pro-ligand (R,R)-bpmycpH (16a) {bpmycpH = 1-{2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-(2R)-2-[(1R)-6,6-[3.1.1]-2-hepten-2-yl]ethyl}cyclopentadiene} and (R,R)-bpmycp′H (16b) {bpmycp′H = 2-{2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-(2R)-2-[(1R)-6,6-[3.1.1]-2-hepten-2-yl]ethyl}cyclopentadiene} reacted with RMgCl (after addition of nBuLi) or with ZnR2 to give the single enantiopure complexes [Mg(R)(κ2-η5-R,S-bpmycp)] (17–19) (R = Me (17), Et (18), CH2SiMe3 (19)) {R,S-bpmycp = 2-{2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-(2S)-2-[(1R)-6,6-[3.1.1]-2-hepten-2-yl]ethyl}cyclopentadienyl} and [Zn(R)(κ2-η1-R,S-bpmycp)] (20–22) (R = Me (20), Et (21), CH2SiMe3 (22)). The structures of these complexes were elucidated by 1H and 13C{1H} NMR spectroscopy, and the X-ray crystal structures of 2, 3, 7 and 21 were also established. These racemic and single enantiopure complexes 1–3, 5–9, 11–14, 19, and 21 can act as single-component initiators for the ring-opening polymerization of rac-lactide under mild conditions. Microstructural analysis of poly(rac-lactide)s revealed that, whereas the myrtenal substituent on the single enantiomerically pure initiator 21 exerts an appreciable influence on the degree of stereoselectivity to produce isotactic-enriched poly(lactide)s with a Pi value up to 0.77, the racemic mixtures of initiators impart more discrete levels of heteroselectivity in the production of PLAs, with a Ps value of 0.72.

Aluminum, Indium, and Mixed Yttrium–Lithium Complexes Supported by a Chiral Binap-Based Fluorinated Dialkoxide: Structural Features and Heteroselective ROP of Lactide

The new chiral Binap-based fluorinated dialcohol proteo-ligand {ONNO}H2 has been prepared under its enantiomerically pure and racemic forms following reaction of (R)- or (rac)-1,1′-binaphthyl-2,2′-diamine with 5,5,5-trifluoro-4-hydroxy-4-(trifluoromethyl)pentan-2-one. The racemic proteo-ligand reacts with the trivalent metallic precursors AlEt2Cl and In(CH2SiMe3)3 to yield the complexes (rac)-{ONNO}AlCl (1) and (rac)-{ONNO}In(CH2SiMe3) (2), while the reaction of (R)-{ONNO}H2 with a 1:1 mixture of Y(N(SiMe3)2)3 and LiN(SiMe3)2 gives the enantiomerically pure heterobimetallic [((R)-{ONNO})2Y·Li] (3). Complex 3 acts as a single-component initiator for the near-perfect heteroselective ring-opening polymerization (ROP) of racemic lactide, yielding polymers with Pr up to 0.99 under mild conditions. It also produces syndiotactic-enriched polylactide (Pr = 0.80) by ROP of meso lactide.

Enantiopure N,N,O-scorpionate zinc amide and chloride complexes as efficient initiators for the heteroselective ROP of cyclic esters.

The reaction of bpzbeH, bpzteH (racemic mixture) or (R,R)-bpzmmH (enantiopure) with the amide complexes Zn{N(SiMe3)2}2 or Zn{N(SiHMe2)2}2 in 1 : 1 molar ratio in toluene afforded the mononuclear amide zinc complexes [Zn(NR2)(κ(3)-NNO)] (1-6) [κ(3)-NNO = bpzbe, R = SiMe3 1, SiHMe2 2; bpzte, R = SiMe3 3, SiHMe2 4; (R,R)-bpzmm, SiMe3 5, SiHMe2 6]. These complexes were employed in a protonolysis reaction with HCl-Et2O in 2 : 1 molar ratio to yield the dinuclear amide/chloride zinc complexes [Zn(κ(2)-NN-μ-O)2{ZnCl(NR2)}] (7-12) [κ(2)-NN-μ-O = bpzbe, R = SiMe3 7, SiHMe2 8; bpzte, R = SiMe3 9, SiHMe2 10; (R,R)-bpzmm, SiMe3 11, SiHMe2 12]. The mononuclear complexes 5 and 6 and dinuclear complexes 11 and 12 are the first enantiopure-scorpionate zinc amide complexes to be synthesized. The single-crystal X-ray structure analysis of derivatives 1 and 3 confirmed a monomeric 4-coordinative structure in which the heteroscorpionate ligands are in a κ(3) coordination mode, while 8 had a dimeric molecular disposition with two μ-bridging alkoxides of the heteroscorpionate ligands between the two six- and four-coordinate Zn(II) centers. Interestingly, the chiral amide-containing zinc complexes 1-5 and 11 can act as single-component initiators for the ring-opening polymerization of ε-caprolactone and lactides under mild conditions, affording, in a few hours, medium/low molecular weight polymers with low polydispersity indices. MALDI-ToF mass spectra confirmed that the initiation occurred through a nucleophilic attack by the amide on the lactide monomer, and inspection of the kinetic parameters showed that propagations present the usual pseudo-first order dependence on monomer and catalyst concentrations. In addition, microstructural analysis of poly(rac-lactide)s revealed that the myrtenal substituent on the alkoxide fragment has a significant influence on the degree of stereoselectivity, producing enriched-heterotactic PLAs with a P(s) value of up to 0.79 under mild conditions.

Organometallic mediated radical polymerization of vinyl acetate with Fe(acac)2

The radical polymerization of vinyl acetate (VAc) is moderated by iron(II) acetylacetonate (Fe(acac)2) by the organometallic route (OMRP), as well as by degenerative transfer polymerization (DTP) when in the presence of excess radicals, through the formation of thermally labile organometallic FeIII dormant species. The poly(vinyl acetate) (PVAc)-FeIII(acac)2 dormant species has been isolated in the form of an oligomer and characterized by 1H NMR, EPR, and IR methods, and then used as a single-component initiator for the OMRP of VAc. The degree of polymerization of this isolated oligomeric species demonstrates the limited ability of Fe(acac)2, relative to the Co(acac)2 congener, to rapidly trap the growing PVAc radical chain. Control under OMRP conditions is improved by the presence of Lewis bases, especially PMe2Ph. On the other hand, iron(II) phthalocyanine inhibits the radical polymerization of VAc completely. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3494–3504

Elaboration and thermal analysis of nanocomposites based on poly(methyl methacrylate-co-4-vinylpyridine) and Maghnia bentonite

Nanocomposites based on an organically modified bentonite, from Maghnia Algeria (OBT) and a copolymer of methyl methacrylate with 4-vinylpyridine (PMM4VP) synthesized in dioxan at room temperature using a neutral Ni(II)a-benzoinoxime com- plex as a single component initiator, were elaborated via solution intercalation method and characterized by several techniques. X-ray diffraction and transmission electron microscopy investigations indicate that mainly exfoliated and intercalated PMM4VP/OBT nano- composites were elaborated and that the degree of exfoliation decreases with an increase of the OBT loading. Thermal analyses of these nanocomposites compared with their virgin copolymer confirmed a significant improvement of their thermal stability as evi- denced by an increase of 28 � C in their onset degradation temperatures. In addition, differential scanning calorimetry displayed an increase in the range of 12-18 � C in their glass transition temperatures relative to their virgin copolymer. V C 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 000: 000-000, 2013

Chiral Indium Alkoxide Complexes as Initiators for the Stereoselective Ring-Opening Polymerization of rac-Lactide

The indium complex InL(2)N'' has been prepared from the reaction of 2 equiv of ((t)Bu)(2)P(O)CH(2)CH((t)Bu)OH (HL) with InN''(3) (N'' = N(SiMe(3))(2)). This complex reacts with a further equivalent of 2,6-di-tert-butylphenol or HL to afford the adducts InL(2)(OAr) and InL(3), respectively. Confirmation that the anion L(-) exhibits "ligand self-recognition" in the formation of predominantly homochiral complexes RR-InL(2)N'' and SS-InL(2)N'' is obtained from (1)H and (31)P NMR spectroscopic data. However, the self-recognition is less effective at the indium cation, and mixtures of InL(3) complexes with different configurations are observed. Single-crystal X-ray diffraction data confirm the five-coordinate, distorted bipyramidal In center in InL(2)N'' and InL(2)(OAr) as anticipated. Selected crystals of InL(3) show two of the possible configurations: one is the fac-RRR-InL(3) complex, analogous to the lanthanide complexes LnL(3) reported previously (Ln = Y, Eu, Er, Yb); another is the alternative, homochiral mer form RRR'-InL(3). All three complexes are efficient single-component initiators for the ring-opening polymerization of rac-lactide over a wide range of temperatures and monomer-to-initiator ratios, exhibiting reasonable control over the synthesis of isotactic polylactide. Despite its poorly defined structure, InL(3) is the fastest initiator among the three complexes for the polymerization of rac-lactide, and shows the best tacticity control. The polylactide samples have high molecular weights M(n,exp) (between 44 000 and 270 000 g/mol at completion) and narrow polydispersities (as low as 1.25 at completion).

Synthesis, structure and catalytic activity of complexes [Ln(EDBP)2(DME)Na(DME)3](Ln=Er, Yb, Sm) for ring-opening polymerization of ɛ-caprolactone

Discrete ion-pair complexes [Ln(EDBP)2(DME)Na(DME)3] [Ln=Er (1), Yb (2), Sm (3)] have been synthesized by the reaction between sodium salt of 2,2′-ethylidene-bis(4,6-di-tert-butylphenol)(EDBPH2) and Ln(BH4)3·3THF (Ln=Er, Yb, Sm) followed by centrifugation and recrystalization. The complexes were characterized by elemental analysis and FT-IR, and the bonding model of these compounds was confirmed by X-ray single crystal diffraction for complex 1. It was found that four O atoms in two biphenol ligands as well as two O atoms in one ethylene glycol dimethyl ether (DME) molecule connect to the center rare earth metal atom, while sodium exists as counterpart cation to balance the charge. Complexes 1–3 can all be used as single component initiators for the ring-opening polymerization of ɛ-caprolactone.

Controlled Synthesis of Polylactides Using Biogenic Creatinine Carboxylate Initiators

Two biogenetic guanidine carboxylates, i.e., creatinine acetate (CRA) and creatinine glycolate (CRG) based on creatinine (CR) were synthesized and structurally characterized by (1)H NMR, (13)C NMR and X-ray diffraction (XRD) analysis of the single crystals. The characterization demonstrated that CRA and CRG are CR-guanidinium carboxylates in nature, and the carboxylates are associated with the guanidinium via hydrogen bonds. CRA and CRG were successfully used as single-component initiators for ring-opening polymerizations (ROPs) of l-lactide (LLA), dl-lactide (DLLA) producing polymers with controlled molecular weights and narrow polydispersities (PDI = 1.16-1.26). The investigation on the kinetics of the ROPs indicated the typical characteristics of living polymerization as evidenced by the linear relationship of ln [M](0)/[M](t) versus time and M(n) (number-average molecular weight of formed polymer) versus monomer conversion. A study of the terminator-adding effect on the growing polymers revealed that the polymerization follows the coordination propagation mode. The structures of the living oligomers and product polymers were characterized with (1)H NMR spectroscopy. The mechanism of the polymerization was proposed on the basis of the experimental investigation.

Synthesis, structures and ring-opening polymerization studies of new zinc chloride and amide complexes supported by amidinate heteroscorpionate ligands

The reaction of the heteroscorpionate lithium salts [Li(pbpamd)(THF)] [pbpamd = N,N'-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and [Li(tbpamd)(THF)] [tbpamd = N-ethyl-N'-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with 1 equivalent of ZnCl2 in THF affords very high yields of the neutral heteroscorpionate chloride zinc complexes [ZnCl(NNN)] (NNN = pbpamd 1 and tbpamd 2). Compound 1 was used as a convenient starting material for the synthesis of aromatic amide zinc compounds [Zn(NHAr)(pbpamd)], where NHAr = 4-methylphenylamide (NH-4-MeC6H4) 3, 2,4,6-trimethylphenylamide (NH-2,4,6-Me3C6H2) 4 and 2,6-diethylphenylamide (NH-2,6-Et2C6H3) 5, by the reaction of the corresponding aromatic primary amide lithium salts. Alternatively, aliphatic amide derivatives [Zn(NR2)(pbpamd)] (R = SiMe3 6, SiHMe2 7 and iPr 8) were cleanly prepared by reacting the amidine-heteroscorpionate compound Hpbpamd with the corresponding bis(amide) zinc complexes [Zn(NR2)2] (R = SiMe3, SiHMe2 and iPr). The single-crystal X-ray structures of complexes 2, 3 and 6 confirm a 4-coordinate arrangement in all cases, with the zinc metal surrounded in a distorted tetrahedral geometry and the heteroscorpionate ligands arranged in a kappa3 coordination mode. Whereas aliphatic amide heteroscorpionates 6-8 can act as efficient single-component initiators for the ring-opening polymerization of epsilon-caprolactone at room temperature, aromatic amide derivatives were not capable of yielding polymers even at high temperature. Epsilon-caprolactone is polymerized within minutes to give medium-high molecular weight polymers under mild conditions and with narrow polydispersities (M(w)/M(n) = 1.26). Polymer end group analysis shows that the polymerization mediated by aliphatic amide zinc complexes is initiated by amide transfer to the monomer.

Catalytic polymerization of alkynes with the quadruply bonded octachloroditungsten anion

In this work are described the reactions of the quadruply bonded ditungsten complexes Na 4[W 2Cl 8(THF) x ] ( 1) and [W 2Cl 4(PMe 3) 4] ( 2), which contain the W 2 4+ core and have the σ 2π 4δ 2 electronic configuration, as well as of the mononuclear [WCl 4(THF) 2] ( 3) with alkynes R 1C CH, R 2C CR 3 (R 1 = Ph ( 4), n Bu, t Bu, Me 3Si, C 10H 7 (naphthyl), C 12H 9 (biphenyl); R 2 R 3 Me, R 2 Me, R 3 n Pr; R 2 R 3 Et; R 2 Ph, R 3 Me; R 2 Ph, R 3 Et; R 2 R 3 Ph; R 2 R 3 AcO). Complex 1 acts as an efficient single-component initiator for the homogeneous or heterogeneous high-yield polymerization of monosubstituted alkynes. The reaction is very fast in CH 2Cl 2 or heterogeneously and slow in THF. The yield is not significantly affected by the bulk of the alkyl substituents. The reaction is not so efficient with internal alkynes, and is limited to those bearing the less sterically demanding substituents. Polymerization of 4 in THF shows that the reaction comprises three distinct steps. The first is very fast ( t = 3 min), with consumption of nearly half of 4 ( ca. 42%), producing Poly( 4). The polymer formed is degraded equally fast regenerating 4 ( ca. 23%) and oligomeric species ( M n < 1000). Subsequent polymerization is slower and complete within 8 h. Direct evidence for the metathetical mechanism of the polymerization was obtained by the in situ examination of the above reaction by 1H NMR at various temperatures. In contrast, complex 2 alone or activated with EtAlCl 2 is unreactive towards 4, whereas 3 produces Poly( 4) in small yield (20%) and very broad polydispersity.

SINGLE LANTHANUM TRIS(N-PHENYLETHYL-3,5-DI-t-BUTYLSALICYLALDIMINATO)S INITIATOR FOR RING-OPENING POLYMERIZATION OF 1,4-DIOXAN-2-ONE IN BULK

A rare earth Schiff base complex: lanthanum tris(N-phenylethyl-3,5-di-t-butylsalicylaldiminato)s [La(OPEBS)3] was successfully applied as single component initiator for the ring-opening polymerization of 1,4-dioxan-2-one (PDO) in bulk. The influence of reaction conditions, such as polymerization temperature, the molar ratio of the monomer to initiator (M/I) on the monomer conversion and molecular weight of the polymer has been investigated. Poly(1,4-dioxan-2-one) with a viscosity-average molecular weight (Mv) of 8.29 × 104 and yield of 91.5% was prepared.

Kinetics and mechanism studies on scandium calix[6]arene complex initiating ring-opening polymerization of 2,2-dimethyltrimethylene carbonate

Scandium p-tert-butylcalix[6]arene complex has been synthesized from scandium isopropoxide and p-tert-butylcalix[6]arene and used as a single component initiator for the first time. The polymerization of 2,2-dimethyltrimethylene carbonate (DTC) using this complex can proceed under mild conditions. Poly (2,2-dimethyltrimethylene carbonate) (PolyDTC) with weight-average molecular weight of 33700 and molecular weight distribution of 1.21 can be prepared. Kinetics study indicates that the polymerization rate is first order with respect to both monomer and initiator concentrations, and the apparent activation energy of the polymerization is 22.7 kJ/mol. 1H NMR spectrum of the polymer reveals that the monomer ring opens via acyl-oxygen bond cleavage leading to an active center of Sc-O.