Polymerization of l-lactide in dichloromethane at 25 °C using an initiating system comprising tris(2,6-di-tert-butylphenoxy)yttrium and 2-propanol was first order in monomer and initiator. A propagation rate constant of 1.92 × 103 L mol-1 min-1 was determined using in situ UV spectroscopy. In the first step the large 2,6-di-tert-butylphenoxy ligands are exchanged for the sterically less demanding 2-propoxide ligands. In the second step the alkoxide is able to attack the carbonyl group with formation of the ring-opened product. After the first two initiating steps the polymerization can proceed until the equilibrium conversion of monomer is reached. Evidence for these three steps is provided by 1H and 13C NMR spectroscopy of the reaction products of each step. The kinetic behavior of commercially available yttrium isopropoxide was more complicated.

ε-Caprolactone polymerization using an initiating system comprising tris(2,6-di-tert-butylphenoxy)yttrium and 2-propanol was first-order in monomer and initiator. A propagation rate constant of 1.65 L·mol-1·s-1 was determined in dichloromethane. The reaction proceeds via a three-step mechanism. In the first step the large 2,6-di-tert-butylphenoxy ligands are exchanged for the smaller 2-propanol. In the second step the polar alkoxide is able to attack the carbonyl group and form the thermodynamically favored ring-opened product. After the first two initiating steps, the polymerization can proceed until complete conversion of monomer is reached. Evidence for these three steps is provided by 1H and 13C NMR spectroscopy of the reaction products of each single step. The kinetic behavior of commercially available yttrium isopropoxide was more complicated. 2-Propanol could be used as an effective chain transfer agent for this initiator.

Insertion Chemistry of Cp*2Y(2-pyridyl) and Molecular Structure of the Unexpected CO Insertion Product (Cp*2Y)2(μ-η2 Deelman, Berth-Jan; Stevels, Willem M.; Teuben, Jan H.; Lakin, Miles T.; Spek, Anthony L. IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document VersionPublisher's PDF, also known as Version of record Publication date : 1994 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Deelman, B-J., Stevels, W. M., Teuben, J. H., Lakin, M. T., & Spek, A. L. (1994). Insertion Chemistry of Cp*2Y(2-pyridyl) and Molecular Structure of the Unexpected CO Insertion Product (Cp*2Y)2(μ-η2: η2-OC(NC5H4)2). Organometallics, 13(10). https://doi.org/10.1021/om00022a025 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Pyridine is metalated selectively at the 2-position by (Cp*zYH)z to yield Cp*zY(2-pyridyl) (1). Compound 1 reacts with HZ to give the hydride addition product Cp*zY(NC5Hs) (2). With THF and pyridine the adducts Cp*~Y(~,P-2-pyridyl)(THF) (3) and Cp*2Y(q1-2-pyridyl)-(py) (4) are formed. The pyridine complex 4 is not stable at higher temperatures, and after organic work up a stoichiometric amount of the C-C coupling product 2,2'-bipyridine is obtained. Ethylene and propylene react with 1 to give the monoinsertion products Cp*zYCHzCHz(2-NC5H4) (5) and C~* Z Y C H Z C H M~(~-N C~H~) (6). With alkynes HCCR, C-H activation to form the acetylides Cp*zY(CCR)(py) (7, R = H; 8, R = Me) is the dominant reaction. Also with 2-butyne C-H activation is observed yielding the propargylic metalation product Cp*zYCHzCCMe (9). 2-Pentyne gives a mixture of insertion products Cp*zY-(CEtCMe(2-NC5H4)) (10) and Cp*zY(CMeCEt(2-NCsH4)) (11). metathesis with pyridine to form 1 and 2-ethylpyridine. By using 1 as a catalyst, alkylation of pyridine to 2-ethylpyridine is possible. Also minor amounts of 2-n-butylpyridine, 2-nhexylpyridine, and polyethylene were formed in this catalytic process. Compound 5 is not thermally stable and decomposes to the isomers C~* Z Y (~-N C~H~(~-E~) ) (13) and Cp*zYCHMe-(2-NCsH4) (14) at 80 "C (60:40). Attempts to convert 5 to 1 and 2-ethylpyridine by hydrogenolysis of the Y-C bond also resulted in the formation of a mixture of 13 and 14 (5545). In this reaction the formation of a n intermediate hydride complex seems likely because (Cp*zYH>z and 2...

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SUMMARYDiblock copolymers composed of poly(e-caprolactone) and poly(L4actide) were synthesized via sequential living ring-opening polymerization of &-caprolactone and L-lactide in dichloromethane initiated by Y 5(p-0)(OiPr),, (yttrium isopropoxide). A series of diblock copolymers was synthesized with e-caprolactone blocks of constant length (DP = 70) and lactide blocks of varying length (DP = 5-80), with polydispersities of 1,18-1,27. Molecular weights were close to theoretical values. I3C NMR spectroscopy shows the absence of transesterification reactions and racemization. The thermal properties of the block copolymers were studied using differential scanning calorimetry. The block copolymers show a certain degree of phase mixing depending on the length of the poly(L4actide) blocks. Crystallization of L-lactide sequences did not occur when the block length consisted of less than 40 L-lactic acid residues.