Andrzej Duda

Kinetics and mechanism of cyclic esters polymerization initiated with Tin(II) octoate. 3. Polymerization of L, L-dilactide

Following our previous papers on the mechanism of cyclic esters polymerization induced by tin(II) octoate (Sn(Oct) 2 ) and particularly papers on ∈-caprolactone (CL), the present work shows that L,L-dilactide/Sn(Oct)2 does not differ mechanistically from the CL/Sn(Oct) 2 system. Sn atoms bonded through alkoxide groups to macromolecules were also observed by MALDI-TOF mass spectrometry. Formation of the actual initiator from Sn(Oct) 2 and a hydroxy group-containing compound (ROH) was envisaged by kinetic arguments. The appropriate experiments were carried out to show that some mechanisms put forward during the past few decades by several research groups were not sufficiently substantiated. Eventually, we conclude that L,L-dilactide/Sn(Oct)2 polymerization proceeds by simple monomer insertion into the ...-Sn-OR bond, reversibly formed in the reaction ...-SnOct + ROH ⇄ ...-Sn-OR + OctH, where ROH is either the low molar mass co-initiator (an alcohol, hydroxy acid, or H 2 O) or a macromolecule fitted with a hydroxy end group. These interconversions take place throughout the whole polymerization process. Sn(Oct) 2 itself does not play an active role in the polymerization.

The effects of checkpointing on program execution time

A program may not terminate its execution because of failures which can cause the program to have to be restarted. The model of a computing system considering such performance measures as the apparent capacity and the expected elapsed time required to execute a given program correctly was first proposed by Castillo and Siewiorek [ 11. The analyzed scheme of program execution necessitates starting a fresh run after the fatal error. We shall analyze a program checkpointing as a method for reducing the overhead caused by restarts. Checkpointing allows the program to be resumed from the earlier point in its execution and not from the beginning. Brock [2] has analyzed and estimated the expected execution time of programs with and without checkpointing. In the present paper we derive the distribution and the expectation of the elapsed time assuming program checkpointing. The optimum interval between checkpoints is considered. Finally a comparison between the performance of the program with and without checkpointing is presented.

Block and random copolymers of ε-caprolactone

Abstract Conditions of the living homopolymerization of e-caprolactone (CL), lactides (LA), and of the homo-oligomerization of γ-butyrolactone (BL) are briefly described. Then block and random copolymerizations of CL with LA are shortly reviewed. The microstructure of the resulting copolyesters in relation to some peculiarities of these processes is discussed in more detail. It is also shown that the otherwise ‘non-polymerizable’ BL does form high molecular weight copolymers with CL, containing up to 50 mol% repeating units derived from BL. Their molecular weight is controlled by the concentrations of the consumed comonomers and the starting concentration of the initiator. NMR and DSC data indicate the random structure of copolymers. TGA traces of the BL/CL copolymers show that the presence of the γ-oxybutyryl repeating units randomly distributed within the poly(CL) chains improves the thermal stability of the latter.

On the mechanism of polymerization of cyclic esters induced by tin(II) octoate

Mechanism of initiation and propagation in polymerization of ϵ-caprolactone and L,L-dilactide induced with tin(II) octoate (Sn(Oct)2) and Sn(Oct)2/n-butyl alcohol system is presented. Tin(II) alkoxide bond formation is required in reaction of Sn(Oct)2 with hydroxyl group containing compound to form a true initiator. Then tin(II) alkoxide end group is an active centre in the further propagation.

Kinetics and mechanism of cyclic esters polymerization initiated with covalent metal carboxylates, 5. End-group studies in the model ε-caprolactone and L, L-dilactide/tin(II) and zinc octoate/butyl alcohol systems

Ring-opening polymerizations of e-caprolactone (CL) and L,L-dilactide (LA) initiated by tin(II) octoate (Sn(Oct)2) and zinc octoate (Zn(Oct)2) and co-initiated with butyl alcohol (BuOH) carried out in tetrahydrofuran as a solvent at 80 °C were studied. By means of MALDI-TOF mass spectrometry, the formation of several populations of polyester macromolecules bearing various end-groups was revealed, namely for poly(e-caprolactone) (PCL): BuO(O)CPCLOH (A), BuO(O)CPCLOct (B), HO(O)CPCLOH (C), HO(O)CPCLOct (D), and PCL cyclics (E), and for poly(L-lactide) (PLA): BuO(O)CPLAOH (A′), BuO(O)CPLAOct (B′), HO(O)CPLAOH (C′), and HO(O)CPLAOct (D′) (where Bu = C4H9 and Oct = O(O)CCH(C2H5)C4H9). In these polymerizations the end-groups in the originally formed macromolecules change slowly with time. In the LA/Sn(Oct)2/BuOH system at the beginning of polymerization almost exclusively macromolecules of the structure A′ are formed and then structures B′, C′, and D′ start to appear, however, after a period more than 300 times (at 80 °C) longer than that required for full monomer conversion, these macromolecules give exclusively esterified B′ and D′ chains. With Zn(Oct)2/BuOH all of these processes are much slower and less selective. Dependencies of mass fraction of macromolecules with various end-groups on time in L,L-dilactide polymerization initiated with Sn(Oct)2/BuOH system. Symbols ▵, ▴, ○, • correspond to PLA populations A′, B′, C′, and D′.

Living polymerization with reversible chain transfer and reversible deactivation: The case of cyclic esters

Polymerization of cyclic esters at the properly chosen conditions can be treated as living polymerization, in agreement with the tentative definition of the Nomenclature Commission of IUPAC (Macromolecular Division) requiring that no irreversible transfer or irreversible termination take place. For these processes the most kinetic or structural (end group) studies do not reveal any deviation. However, since in these polymerizations reversible transfer to backbones of macromolecules and/or reversible deactivation take place, the molar mass distribution can be Poissonian only at certain conditions. These processes have been studied quantitatively and the corresponding rate constants were determined. Thus, the importance of these processes could be established by comparing the rate constants of transfer and/or deactivation with rate constants of propagation. In this way, polymerizations of cyclic esters were used to illustrate the meaning and scope of the definition of “living polymerization”, a process from which irreversible transfer and deactivation are absent and in which living polymers are formed, i. e. composed of macromolecules that do not irreversibly loose their ability to grow.

Complementarity of solvent-free MALDI TOF and solid-state NMR spectroscopy in spectral analysis of polylactides.

We report systematic studies of solvent-free modification of matrix-assisted laser desorption/ionization time-of-flight (SF MALDI-TOF) mass spectrometry in analysis of synthetic polymers employing solid-state NMR spectroscopy as a supporting technique. In the present work oligomeric (M(n) = 4000 g mol(-1)) poly(L-lactide) (PLLA) was employed as a reference sample. The analyte was embedded into four matrixes commonly used in MALDI-TOF analysis of polymers: 1,8-dihydroxy-9-anthracenone (DT), 2,5-dihydroxybenzoic acid (DHB), 2-(4 hydroxyphenylazo)-benzoic acid (HABA), and trans-3-indoleacrylic acid (IAA). Solid-state NMR measurements clearly showed that the initial crystallinity of PLLA had no influence on quality of SF MALDI-TOF spectra since the crystalline structure of the analyte was not preserved during analyte/matrix grinding. Interestingly, the matrix remained crystalline during the samples preparation. It was also found that, on the contrary to the dried droplet (DD) method, the SF approach leads to highly resolved mass spectra for a large variety of matrixes. Finally, problems of polymorphism and mechanochemical processes that can occur during the analyte/matrix grinding are briefly discussed.

Application of ionic liquid matrices in spectral analysis of poly(lactide) - solid state NMR spectroscopy versus matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry

The work presented here shows the complementarity of Solid State NMR (SS NMR) spectroscopy and Matrix-Assisted Laser Desorption/Ionization-time-of-flight (MALDI-TOF) mass spectrometry in spectral analysis of poly(L-lactide) (PLLA) using second-generation ionic liquid matrices (ILM II) prepared from N,N-diisopropylethylamine (DEA) and DHB (2,5-dihydroxybenzoic acid), IAA (3-indoleacrylic acid), and HABA (2-(4-hydroxyphenylazo) benzoic acid). The 13C cross-polarization (CP) magic angle spinning (CP/MAS) SS NMR technique was used to study the structure of ionic liquid matrices, their thermal stability, and the influence of ILM on the morphology of polymer. A comparison of MALDI-TOF spectra for samples prepared employing the dried droplet (DD) and the solvent free (SF) mode showed that the former approach gave better results (signal to noise ratio) very likely due to intimate contact between analyte and matrix domains. This hypothesis was verified by analysis of CP build-up curves for DHB–DEA–PLLA samples prepared employing both methods. We also showed that an alternative method of sample preparation based on the melting of ILM II together with a suspended polymer in the liquid matrix is unsatisfactory, particularly for those matrices which can undergo isomerization at higher temperatures (e.g., HABA–DEA and IAA–DEA).

Ring-opening polymerization of lactides initiated with yttrium Tris-isopropoxyethoxide

The polymerization of (D,L)-lactide was carried out at room temperature in dichloromethane using [Y(OCH 2 CH 2 OiPr) 3 ] 2 as an initiator. The polymerization is much faster than with the yttrium (μ-oxo)isopropoxide initiator system. Narrow molecular weight distributions (MWD) are observed up to high conversion. However, prolongation of the polymerization after the complete monomer consumption leads to broadening of MWD due to the occurrence of transesterification reactions. The polymerization proceeds through an acyl-oxygen bond cleavage and with yttrium Tris(macroalkoxide)s as active species.

Anionic Copolymerization of Elemental Sulfur with Propylene Sulfide. Equilibrium Sulfur Concentration

Abstract In the anionic copolymerization of elemental sulfur (S8) with propylene sulfide (P) below the floor temperature of elemental sulfur homopolymerization (Tf = 159°C), there is a certain concentration of elemental sulfur left when copolymerization is completed ([S8]eq). The dependence of [S8]eq on the feed ratio [P]0/[S8]0 and temperature was determined by using laser Raman spectroscopy, and this enabled us to distinguish between the S-S bonds in elemental sulfur and in the linear polysulfide. [S8]eq was found to decrease with increasing temperature and with an increasing [P0]/[S8]0 ratio. The experimental dependence of the average enthalpy and entropy of polymerization (δ[Hbar][Xbar], and δ [Sbar][Xbar]) on [Xbar], as described by the equilibrium …-CH2CH(CH3)SX − + S8 = …-CH2CH(CH3)SX+8 −, has shown that at [Xbar] ⇋; 9 the experimental δ[Hbar][Xbar] and δ [Sbar][Xbar] approach values determined earlier for the free radical homopolymerization of elemental sulfur …-Sn • + S8 ≤ …-S• n+8& The correspon...