A Highly Active and Selective Zirconium-Based Catalyst System for the Industrial Production of Poly(lactic acid).

The biodegradable, aliphatic polyester poly(lactic acid), PLA, is a leading bio-based alternative to petrochemical-derived plastic materials across a range of applications. Widely reported in the available literature as a benchmark for PLA production via the bulk ring-opening polymerization of lactides is the use of divalent tin catalysts, and particularly tin(II) bis(2-ethylhexanoate). We present an alternative zirconium-based system that combines an inexpensive Group IV metal with the robustness, high activity, control, and designed compatibility with existing facilities and processes, that are required for industrial use. We have carried out a comprehensive kinetic study and applied a combined experimental and theoretical approach to understanding the mechanism by which the polymerization of lactide proceeds in the presence of this system. In the laboratory-scale (20 g) polymerization of recrystallized racemic d,l-lactide (rac-lactide), we have measured catalyst turnover frequencies up to at least 56,000 h-1, and confirmed the reported protocols' resistance toward undesirable epimerization, transesterification, and chain scission processes, deleterious to the properties of the polymer product. Further optimization and scale-up under industrial conditions have confirmed the relevance of the catalytic protocol to the commercial production of melt-polymerized PLA. We were able to undertake the efficient preparation of high-molecular-weight PLA on the 500-2000 g scale, via the selective and well-controlled polymerization of commercial polymer-grade l-lactide under challenging, industrially relevant conditions, and at metal concentrations as low as 8-12 ppm Zr by weight ([Zr] = 1.3 × 10-3 to 1.9 × 10-3 mol %). Under those conditions, a catalyst turnover number of at least 60,000 was attained, and the activity of the catalyst was comparable to that of tin(II) bis(2-ethylhexanoate).

Ring-Opening Polymerisation of Low-Strain Nickelocenophanes: Synthesis and Magnetic Properties of Polynickelocenes with Carbon and Silicon Main Chain Spacers.

Polymetallocenes based on ferrocene, and to a lesser extent cobaltocene, have been well-studied, whereas analogous systems based on nickelocene are virtually unexplored. It has been previously shown that poly(nickelocenylpropylene) [Ni(η5 -C5 H4 )2 (CH2 )3 ]n is formed as a mixture of cyclic (6x ) and linear (7) components by the reversible ring-opening polymerisation (ROP) of tricarba[3]nickelocenophane [Ni(η5 -C5 H4 )2 (CH2 )3 ] (5). Herein the generality of this approach to main-chain polynickelocenes is demonstrated and the ROP of tetracarba[4]nickelocenophane [Ni(η5 -C5 H4 )2 (CH2 )4 ] (8), and disila[2]nickelocenophane [Ni(η5 -C5 H4 )2 (SiMe2 )2 ] (12) is described, to yield predominantly insoluble homopolymers poly(nickelocenylbutylene) [Ni(η5 -C5 H4 )2 (CH2 )4 ]n (13) and poly(tetramethyldisilylnickelocene) [Ni(η5 -C5 H4 )2 (SiMe2 )2 ]n (14), respectively. The ROP of 8 and 12 was also found to be reversible at elevated temperature. To access soluble high molar mass materials, copolymerisations of 5, 8, and 12 were performed. Superconducting quantum interference device (SQUID) magnetometry measurements of 13 and 14 indicated that these homopolymers behave as simple paramagnets at temperatures greater than 50 K, with significant antiferromagnetic coupling that is notably larger in carbon-bridged 6x /7 and 13 compared to the disilyl-bridged 14. However, the behaviour of these polynickelocenes deviates substantially from the Curie-Weiss law at low temperatures due to considerable zero-field splitting.

New reactivity at the silicon bridge in sila[1]ferrocenophanes.

We describe two new types of reactivity involving silicon-bridged [1]ferrocenophanes. In an attempt to form a [1]ferrocenophane with a bridging silyl cation, the reaction of sila[1]ferrocenophane [Fe(η-C5H4)2Si(H)TMP] (12) (TMP = 2,2,6,6-tetramethylpiperidyl) towards the hydride-abstraction reagent trityl tetrakis(pentafluorophenyl)borate ([CPh3][B(C6F5)4]) was explored. This yielded the unusual dinuclear species [Fe(η-C5H4)2Si(TMP·H)(η-C5H3)Fe(η-C5H4)Si(H)TMP][B(C6F5)4] [13][B(C6F5)4] in low yield. The formation of [13]+ is proposed to involve abstraction of hydride from the silicon bridge in 12 with subsequent C-H bond cleavage of a cyclopentadienyl group by the resulting electrophilic transient silyl cation intermediate. We also explored the reaction of dimethylsila[1]ferrocenophane [Fe(η-C5H4)2SiMe2] (1) with the Au(i) species AuCl(PMe3). This was found to result in addition of the Au-Cl bond across the Cpipso-Si bond to yield the ring-opened species [1'-(chlorodimethylsilyl)-ferrocenyl](trimethylphosphine)gold(i), [Fe(C5H4SiMe2Cl){C5H4Au(PMe3)}] (14). This represents the first example of ring-opening addition of a metallocenophane with a reagent possessing a transition metal-halogen bond.

Influence of cyclopentadienyl ring-tilt on electron-transfer reactions: redox-induced reactivity of strained [2] and [3]ruthenocenophanes.

In contrast to ruthenocene [Ru(η(5) -C5 H5 )2 ] and dimethylruthenocene [Ru(η(5) -C5 H4 Me)2 ] (7), chemical oxidation of highly strained, ring-tilted [2]ruthenocenophane [Ru(η(5) -C5 H4 )2 (CH2 )2 ] (5) and slightly strained [3]ruthenocenophane [Ru(η(5) -C5 H4 )2 (CH2 )3 ] (6) with cationic oxidants containing the non-coordinating [B(C6 F5 )4 ](-) anion was found to afford stable and isolable metalmetal bonded dicationic dimer salts [Ru(η(5) -C5 H4 )2 (CH2 )2 ]2 [B(C6 F5 )4 ]2 (8) and [Ru(η(5) -C5 H4 )2 (CH2 )3 ]2 [B(C6 F5 )4 ]2 (17), respectively. Cyclic voltammetry and DFT studies indicated that the oxidation potential, propensity for dimerization, and strength of the resulting RuRu bond is strongly dependent on the degree of tilt present in 5 and 6 and thereby degree of exposure of the Ru center. Cleavage of the RuRu bond in 8 was achieved through reaction with the radical source [(CH3 )2 NC(S)SSC(S)N(CH3 )2 ] (thiram), affording unusual dimer [(CH3 )2 NCS2 Ru(η(5) -C5 H4 )(η(3) -C5 H4 )C2 H4 ]2 [B(C6 F5 )4 ]2 (9) through a haptotropic η(5) -η(3) ring-slippage followed by an apparent [2+2] cyclodimerization of the cyclopentadienyl ligand. Analogs of possible intermediates in the reaction pathway [C6 H5 ERu(η(5) -C5 H4 )2 C2 H4 ][B(C6 F5 )4 ] [E=S (15) or Se (16)] were synthesized through reaction of 8 with C6 H5 EEC6 H5 (E=S or Se).

Ring-opening polymerization of a strained [3]nickelocenophane: a route to polynickelocenes, a class of S = 1 metallopolymers.

We report the synthesis, reactivity studies, and ring-opening polymerization of a tricarba[3]nickelocenophane. The resulting green polynickelocene (5) possesses a -(CH2)3- spacer between the nickelocene units and is shown to be of high molecular weight. SQUID magnetometry measurements indicate that 5 is a macromolecular material with an S = 1 repeat unit.

Comparative studies of thermally induced homolytic carbon-carbon bond cleavage reactions of strained dicarba[2]ferrocenophanes and their ring-opened oligomers and polymers.

Reactivity studies of dicarba[2]ferrocenophanes and also their corresponding ring-opened oligomers and polymers have been conducted in order to provide mechanistic insight into the processes that occur under the conditions of their thermal ring-opening polymerisation (ROP) (300 °C). Thermolysis of dicarba[2]ferrocenophane rac-[Fe(η(5)-C5H4)2(CHPh)2] (rac-14; 300 °C, 1 h) does not lead to thermal ROP. To investigate this system further, rac-14 was heated in the presence of an excess of cyclopentadienyl anion, to mimic the postulated propagating sites for thermally polymerisable analogues. This afforded acyclic [(η(5)-C5H5)Fe(η(5)-C5H4)-CH2Ph] (17) through cleavage of both a Fe-Cp bond and also the C-C bond derived from the dicarba bridge. Evidence supporting a potential homolytic C-C bond cleavage pathway that occurs in the absence of ring-strain was provided through thermolysis of an acyclic analogue of rac-14, namely [(η(5)-C5H5)Fe(η(5)-C5H4)(CHPh)2-C5H5] (15; 300 °C, 1 h), which also afforded ferrocene derivative 17. This reactivity pathway appears general for post-ROP species bearing phenyl substituents on adjacent carbons, and consequently was also observed during the thermolysis of linear polyferrocenylethylene [Fe(η(5)-C5H4)2(CHPh)2]n (16; 300 °C, 1 h), which was prepared by photocontrolled ROP of rac-14 at 5 °C. This afforded ferrocene derivative [Fe(η(5)-C5H4CH2Ph)2] (23) through selective cleavage of the -H(Ph)C-C(Ph)H- bonds in the dicarba linkers. These processes appear to be facilitated by the presence of bulky, radical-stabilising phenyl substituents on each carbon of the linker, as demonstrated through the contrasting thermal properties of unsubstituted linear trimer [(η(5)-C5H5)Fe(η(5)-C5H4)(CH2)2(η(5)-C5H4)Fe(η(5)-C5H4)(CH2)2(η(5)-C5H4)Fe(η(5)-C5H5)] (29) with a -H2 C-CH2- spacer, which proved significantly more stable under analogous conditions. Evidence for the radical intermediates formed through C-C bond cleavage was detected through high-resolution mass spectrometric analysis of co-thermolysis reactions involving rac-14 and 15 (300 °C, 1 h), which indicated the presence of higher molecular weight species, postulated to be formed through cross-coupling of these intermediates.

Metal-metal bond formation between [n]metallocenophanes: synthesis and characterisation of a dicarba[2]ruthenocenophanium dimer.

Feeling the strain: The first example of metal-metal bonding between strained [n]metallocenophanes is reported. A dicarba[2]ruthenocenophanium dimer has been synthesised through the oxidation of a dicarba[2]ruthenocenophane (see figure). The structural and electrochemical characterisation of the dimer is also discussed.