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Star-shaped polymers, bearing a strictly defined number of linear arms, provide useful model structures for studies of branched macromolecular structures. In the dynamically growing area of biocompatible and biodegradable polymers, the synthesis and characterization of starshaped aliphatic polyesters are particularly advanced. Among others, some of our recent articles report the synthesis and characterization of 1-, 2-, 3-, 4-, 5-, 6-, 8, 13-, and 32-arm star-shaped poly(L-lactide)s by size exclusion chromatography (SEC), liquid chromatography under critical conditions (LC–CC), two-dimensional LC–CC/SEC, NMR, and fluorescence spectroscopy. The method perhaps most commonly used for starshaped polymer synthesis is based on the core-first approach, in which a multifunctional reagent (the core) plays the role of the (co)initiator and/or transfer agent. Another possibility in this area, originally developed by Rempp et al. for a styrene/divinylbenzene (DVB) system, was followed more recently by Lapienis and Penczek for an epoxide/diepoxide system. The principle of this approach is based on a two-step synthesis with the one-pot, arms-first coremethod. In the first step, a linear living polymer (the precursor) is prepared (e.g., polystyrene or polyepoxide). Then, in the second step, the resulting precursor initiates the polymerization of a tetrafunctional monomer (e.g., DVB or diepoxide), giving a highly branched macromolecular structure (the core). This communication reports the successful synthesis of highly branched, star-shaped aliphatic polyesters from various lactone and lactide/bislactone systems—e-caprolactone (CL)/5,50-bis(oxepan-2-one) (bis-CL), L,L-lactide (LA)/ bis-CL, CL/1,6-dioxaspiro[4,4]nonane-2,7-dione) (bis-BL), and LA/bis-BL—with aluminum isopropoxide [Al(OPr)3] as the initiator. Experimental evidence of the formation of starlike structures is provided by SEC and matrixassisted laser desorption/ionization time-of-flight (MALDITOF) analysis.

Linear telechelic, α,ω-ditelechelic, and star-shaped tri-, tetra-, penta-, and hexa-arm poly(L-lactide)s (PLAs) fitted at every arm with pyrene end group have been prepared. Internal dynamics and mobility of the PLA chains in tetrahydrofuran solution at 25 °C, with regard to the number of PLA arms in one macromolecule and the individual arm average degree of polymerization, was followed by fluorescence spectroscopy. Analysis of both static and time-resolved spectra of the star-shaped polymers revealed dynamic segmental motion resulting in end-to-end cyclization, accompanied by an excimer formation. Probability and rate of the latter reaction increased with increasing number of arms and with decreasing their polymerization degree. Moreover, time-resolved measurements revealed that for macromolecules containing few arms (2 or 3) the pyrene moieties are located in the interior of the star-shaped PLAs, whereas in the instance of the higher number of arms (4–6) they are located at the periphery of the star-shaped PLAs. Thus, increasing the number of arms leads to their stretching away from the center of the star-shaped PLA macromolecule. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4586–4599, 2005

Star-shaped poly(L-lactide)s (PLAs) bearing variable numbers of secondary hydroxyl groups at linear arms chain-ends and primary hydroxyl groups directly attached to dipentaerithritol core (DPE) ((HO)6−xDPE(PLA-OH)x, where x = 1–6) were prepared and then analyzed by means of size exclusion chromatography (SEC), 1H NMR spectroscopy, MALDI-TOF mass spectrometry, and eventually by Liquid Chromatography at Critical Conditions (LC-CC). First, starting from DPE(OH)6 a series of polyols with various number of hydroxyl groups has been obtained ((BnO)6−xDPE(OH)x, where Bn denotes benzyl moiety and x = 1–6). The living ring-opening polymerization of L-lactide (LA) with (BnO)6−xDPE(OH)x/tin(II) octoate mixtures as initiating and catalytic system led to star-shaped (BnO)6−xDPE(PLA-OH)x polymers with molar masses (Mn) controlled by LA and DPE concentrations ratio in the feed. Finally, deprotection (via hydrogenation) gave a series of (HO)6−xDPE(PLA-OH)x PLA's. SEC (with Multiangle Laser Light Scattering Detector (MALLS)), NMR, and MALDI-TOF analyses confirmed the assumed structures and Mn's of the prepared (BnO)6−xDPE(PLA-OH)x and (HO)6−xDPE(PLA-OH)x PLA's. LC-CC measurements revealed that for (BnO)6−xDPE (PLA-OH)x series the elution volumes increase monotonically with the increasing number of –PLA-OH arms in one macromolecule and are independent on the given PLA molar mass because of the critical conditions. Contrary to the polymers having the protected core hydroxyl groups, the elution volume for (HO)6−xDPE(PLA-OH)x series decreases with the increasing number of -PLA-OH arms reaching a minimum value for 4-arm PLA and then slightly increases for 5- and 6-arm PLA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6116–6133, 2005

Let us consider an attempt to determine the actual structure and size of star-shaped macromolecules that are assumed, on the basis of the known chemistry, to have n arms. Such assumptions are usually made but seldom proved. Indeed, it is difficult if not impossible on the basis of the published methods to find out whether within starlike macromolecules prepared with the intention of having n arms there are macromolecules with n x (where x 1, 2, 3, . . . , n 1) arms. Methods such as size exclusion chromatography (SEC) would merely show a broader “molar masses” distribution for such structural imperfections. In certain specific instances, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry could help and shows the corresponding populations. Another approach is possible when arms can be detached from the core. Then measurements of the absolute molar masses of the original starshaped polymer and arms, after their detachment, could provide unambiguous information. This method was used, for instance, to analyze star-shaped polytetrahydrofuran, attached with easily hydrolyzable ester bonds to the aromatic core. Star-shaped polymers having a different number of arms also have a different number of end groups. Because liquid chromatography at the critical point of adsorption (LC-CC) separates macromolecules according to the structure of the end groups, we decided to use two-dimensional (2D) chromatography with LC-CC as the first dimension. We expected to suppress the influence of the differences in molar masses and to separate macromolecules according to the number of end groups. In the second dimension, SEC provides information on molar masses. The method applying critical conditions is based on the use of solvent (usually a mixture of solvents) at a specific temperature when enthalpic and entropic factors of macromolecules stationary-phase solvent interactions cancel each other. 4 Macromolecules at these conditions are separated exclusively with regard to the end groups and independently of the molar mass. Interaction of the end groups with the column packing is responsible for discrimination between macromolecules bearing different end groups. This method was confirmed to be successful for a number of systems and allowed, for instance, the determination of 13 different combinations of end groups in the bisphenol A epoxy resins. In a short overview on 2D chromatography one can find a monograph published by one of us. Correspondence to: S. Penczek (E-mail: spenczek@bilbo. cbmm.lodz.pl)

The microstructure of polyphosphites and polyphosphates obtained by the ring-opening polymerization of asymmetrically substituted 5-membered cyclic phosphites was studied. It has been established, using 31P NMR, that 4-substituted 2-hydro-2-oxo-1,3,2-dioxaphospholanes underwent polymerization giving polymers with head-to-tail dyads as well as head-to-head and tail-to-tail structures. Analyses of 31P NMR spectra of racemic and optically active poly(2-hydro-4-methyl-2-oxo-1,3,2-dioxaphospholane) and model compounds estimated the statistical mode of ring scission of cyclic phosphites. Similar results were obtained for the polymerization of 4-acetoxymethyl-2-hydro-2-oxo-1,3,2-dioxaphospholane, which provided the simplest model of teichoic acid, namely poly(1,2-glycerol phosphate).