This Perspective article summarizes efforts paid in our group to develop efficient metal-based catalysts for the immortal ring-opening polymerization (iROP) of cyclic esters in the presence of large amounts of alcohols (ROH) as chain transfer agents. The catalyst systems reviewed include discrete organometallic complexes based on rare earths, magnesium, calcium and more specifically zinc, as well as simple systems employing metal triflate salts, notably Al(OTf)(3), for the (stereo)controlled iROP of lactide (LA), beta-butyrolactone (BBL) and trimethylenecarbonate (TMC). Special emphasis is given to systems that allow the use of minute amounts of metal catalysts and large loadings of both monomer and alcohol for the rapid and productive formation of functional polyesters (H-Pol-OR) with controlled molecular features.
A series of methoxy-amino-bis(phenol)s (ONOO(R(1),R(2)))H(2) possessing on the phenol rings R(1) ortho substituents with variable steric and electronic properties (R(1)=CMe(2)Ph, 1; CMe(2)tBu, 3; CMe(2)(4-CF(3)C(6)H(4)), 5; CPh(3), 9; Cl, 10) has been synthesized and further reacted with [Y{N(SiHMe(2))(2)}(3)](THF)(2) to give cleanly the corresponding yttrium compounds [Y(ONOO(R(1),R(2))){N(SiHMe(2))(2)}(thf)(n)] (Y-x); the solid-state structures of Y-3 and Y-10 have been determined. These amido complexes have been used as initiators for the ring-opening polymerization (ROP) of rac-lactide (LA) and rac-β-butyrolactone (BBL) to provide heterotactically enriched poly(lactic acid)s (PLAs) and syndiotactically enriched poly(3-hydroxybutyrate)s (PHBs), respectively, by means of a chain-end control mechanism. Most of these polymerizations proceeded in a controlled fashion, giving polymers with narrow polydispersities and experimental molecular weights in good agreement with calculated values. The nature of the R(1) ortho substituents has a profound impact on the rates and, more spectacularly, on the stereocontrol of the polymerizations. The heterotactic stereocontrol in the ROP of rac-LA appears to be governed essentially by steric considerations; the larger the substituent, the higher the heterotacticity: R(1)=Cl (P(r)=0.56)≪CMe(3) (P(r)=0.80)≪CMe(2)Ph (P(r)=0.90)
The mechanism of the ring-opening polymerization of rac-β-butyrolactone using yttrium complexes supported by dianionic aminoalkoxybis(phenolate) ligands as initiators has been investigated by NMR and shown to occur via a coordination-insertion pathway. The microstructure of the resulting syndiotactic-enriched poly(3-hydroxybutyrate)s (PHBs, P
r up to 0.94) has been studied by 13C NMR spectroscopy, enabling a detailed assignment of resonances at the diad and triad levels. On this basis, a statistical Bernoullian analysis has been performed which evidenced that syndioselectivity originates from a chain-end control. Some thermal properties of these PHBs have been studied by WAXD and thermoanalytical techniques and shown to be markedly affected by the syndiotacticity degree. This is especially the case for the melting temperature which raises up to 183 °C for P
r = 0.94, a temperature higher than that of pure isotactic PHB (ca. 180 °C).
Rare-earth-metal borohydrides are known to be efficient catalysts for the polymerization of apolar and polar monomers. The bis-borohydrides [{CH(PPh2 NSiMe3)2}La(BH4)2(THF)] and [{CH(PPh2NSiMe3)2}Ln(BH4)2] (Ln = Y, Lu) have been synthesized by two different synthetic routes. The lanthanum and the lutetium complexes were prepared from [Ln(BH4)3(THF)3] and K{CH(PPh2NSiMe3)2}, whereas the yttrium analogue was obtained from in situ prepared [{CH(PPh2NSiMe3)2}YCl2]2 and NaBH4. All new compounds were characterized by standard analytical/spectroscopic techniques, and the solid-state structures were established by single-crystal X-ray diffraction. The ring-opening polymerization (ROP) of ε-caprolactone initiated by [{CH(PPh2NSiMe3)2}La(BH4)2(THF)] and [{CH(PPh2NSiMe3)2}Ln(BH4)2] (Ln = Y, Lu) was studied. At 0 °C the molar mass distributions determined were the narrowest values (M(w)/M(n) = 1.06-1.11) ever obtained for the ROP of ε-caprolactone initiated by rare-earth-metal borohydride species. DFT investigations of the reaction mechanism indicate that this type of complex reacts in an unprecedented manner with the first B-H activation being achieved within two steps. This particularity has been attributed to the metallic fragment based on the natural bond order analysis.
A series of new yttrium complexes supported by the bulky enediamido dianionic ligand DAB(2-) (DAB(2-) = (2,6-C(6)H(3)iPr(2))NC(Me)=C(Me)N(2,6-C(6)H(3)iPr(2))(2-)), that is, {DAB}Y(THF)(2)(mu-Cl)(2)Li(THF)(2) (1), {DAB}Y(OtBu)(THF)(DME) (2), and {{DAB}Y(BH(4))(2)}{Li(DME)(3)} (3), was synthesized by salt metathesis. The complexes were isolated after recrystallization in 73, 66, and 52% yield, respectively, and characterized in solution by NMR and in the solid state by single-crystal X-ray diffraction studies. In complex 1, the DAB(2-) ligand is bonded to the metal center via two covalent YN bonds, while in complexes 2 and 3 additional eta(2)-coordination of the C=C bond to the metal atom is observed, both in solution and in the solid state. The tert-butoxide and borohydride complexes 2 and 3 act as monoinitiators for the room temperature ring-opening polymerization of racemic lactide and beta-butyrolactone, providing atactic polymers with controlled molecular weights and relatively narrow polydispersities (M(w)/M(n) = 1.15-1.82). Effectively immortal ROP of lactide with as many as 50 equiv of isopropanol per metal center was performed using complex 2 as the catalyst.
A series of new bis(guanidinate) alkoxide Group 3 metal complexes [Ln((Me3Si)2NC(NiPr)2)2(OR)] (R=OtBu, Ln=Y, Nd, Sm, Lu; R=OiPr, Ln=Y, Nd, Lu) has been synthesized. X-ray structural determinations revealed that bis(guanidinate) tert-butoxides are monomeric complexes. The isopropoxide complex [Y((Me3Si)2NC(NiPr)2)2(OiPr)] undergoes slow decomposition in solution, to afford the unusual dimeric amido complex [(Y((Me3Si)2NC(NiPr)2)2(mu-N(iPr)C triple chemical bond N))2]. Complexes [Ln((Me3Si)2NC(NiPr)2)2(OR)] (R=OtBu, Ln=Y, Nd, Sm, Lu; R=OiPr, Ln=Y, Nd, Lu) are active catalysts/initiators for the ROP of rac-lactide and rac-beta-butyrolactone under mild conditions. Most of those polymerizations proceed with a significant degree of control. Bis(guanidinate) alkoxides appear to be well suited for achieving immortal polymerization of lactide, through the introduction of large amounts of isopropanol as a chain-transfer agent. The synthesized complexes are able to promote the stereoselective ROP of rac-beta-butyrolactone to afford syndiotactic poly(hydrobutyrate) through a chain-end control mechanism, while they are surprisingly non-stereoselective for the ROP of lactide under strictly similar conditions.
The kinetics of polymerization of ε-caprolactone
(CL) initiated
by aluminum-alkoxide complexes supported by the dianionic forms of N,N-bis[methyl-(2-hydroxy-3-tert-butyl-5-R-phenyl)]-N,N-dimethylethylenediamines,
(LR)Al(Oi-Pr) (R = OMe, Br, NO2) were studied. The ligands are sterically similar but have variable
electron donating characteristics due to the differing remote (para) ligand substituents R. Saturation kinetics were observed
using [CL]0 = 2–2.6 M and [complex]0 =
7 mM, enabling independent determination of the substrate coordination
(K
eq) and insertion (k
2) events in the ring-opening polymerization process.
Analysis of the effects of the substituent R as a function of temperature
on both K
eq and k
2 yielded thermodynamic parameters for these steps. The rate
constant k
2, related to alkoxide nucleophilicity,
was strongly enhanced by electron-donating R substituents, but the
binding parameter K
eq is invariant as
a function of ligand electronic properties. Density functional calculations
provide atomic-level detail for the structures of key reaction intermediates
and their associated thermochemistries.
A series of new pentacoordinated nickel complexes NiCl 2 (NZN) based on nitrogen-, oxygen-, or sulfurbridged bis(pyrazolyl) ligands (NZN) were synthesized and characterized by elemental analysis and an X-ray diffraction study for NiCl 2 {bis[2-(3,5-dimethylpyrazolyl)ethyl)]amine} (1). Upon activation with methylaluminoxane (MAO), these complexes show high activity in ethylene oligomerization (TOF ) 7-80 × 10 3 h -1 ), which varies according to the ligand environment. Up to 92% selectivity for 1-butene, combined with a TOF of 60 × 10 3 h -1 , has been obtained under moderate conditions (30 °C, 40 bar, MAO-to-Ni ) 250) using NiCl 2 {bis[2-(5-phenylpyrazolyl)ethyl)]ether} (3) as the catalyst precursor.
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