Ring‐opening polymerization of ε‐caprolactone catalyzed by Yttrium trisphenolate in the presence of 1,2‐propanediol. Do both primary and secondary hydroxyl groups initiate polymerization?

Although the ring‐opening polymerization (ROP) of ε‐caprolactone (CL) in toluene at 100 °C can be initiated by yttrium trisphenolate (Y(OC₆H₅)₃), in the presence of 1,2‐propanediol (PD) the ROP gives much better, that is, controlled polymerizations. In this case, the molecular weights (MWs) are controlled by the CL/PD molar ratios with primary and secondary hydroxyl groups both initiating the ROP and the MW distributions are narrow. The chain transfers between the active yttrium alkoxides and the residual hydroxyl groups on the PD and/or the chain ends appear to be much faster than chain propagation, consistent with the living character of the ROP. Computational studies support these facile reactions with estimated activation free energies in the 3.0–4.5 kcal/mol range compared with about 25–30 kcal/mol for the polymerization. Intramolecular transfer within the PD is predicted to be negligible having a calculated activation energy of 19 kcal /mol. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and thermal and spectroscopic properties of macrocyclic vinyl aromatic polymers

Well‐defined and narrow molecular weight distribution macrocyclic poly(2‐vinylnaphthalene) (P2VN) and poly(2‐vinyl‐9,9‐dimethylfluorene) (PDMVF) containing a single 1,4‐benzylidene or 9,10‐anthracenylidene unit have been synthesized via the potassium naphthalide initiated polymerization of the monomers followed by the end‐to‐end coupling of the resulting P2VN dianions under high‐dilution conditions with 1,4‐bis(bromomethyl)benzene or 9,10‐bis(chloromethyl)anthracene. Molecular characterization has been carried out by size exclusion chromatography, nuclear magnetic resonance, differential scanning calorimetry, ultraviolet–visible spectroscopy, and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry. The thermal properties show distinct differences between the cyclic and matching linear polymers, with the macrocycles showing much higher glass‐transition and decomposition temperatures. The absorption bands are both hyperchromic and hypochromic with respect to the model aromatic compounds, and this is consistent with intensity borrowing. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5488–5503, 2004     

Organocuprates as initiators for the polymerization of methyl methacrylate in THF

Several types of lithium organocuprates (I) e.g. R₂CuLi, RR'CuLi, R₃CuLi₂, R₂CuLi₂X (X- CN, SCN), were prepared and used as polymerization initiators for methyl methacrylate in THF at temperatures ranging from −30 to +20 ^*C. Polymerization initiated by a mixed homocuprate, e.g. lithium n-butyl (thien-2-yl)cuprate was studied in more detail. Kinetic measurements indicated an overall large (−4 to −14 kcal) apparent negative activation energy of polymerization in the −30 + +20 ^*C temperature range. The reaction was shown to have a kinetic order of about 1.5 with respect to initiator concentration. Conductance measurements indicate that the equivalent conductance increases with initiator concentration in the 10⁻² + 10⁻¹ M range indicating the formation of multiple ions formed by intermolecular association. The data are consistent with a polymerization involving aggregated ion pairs of as yet undetermined composition. The negative activation energy is tentatively interpreted as due to the exothermic formation of aggregates existing as highly solvated ion pairs.     

Oligomerization of vinyl monomers 18. Stereochemistry of anionic oligomerization of 4-vinylpyridine

Oligomers of 4-vinylpyridine were prepared by reaction in vacuo of 4-vinylpyridine with lithio-4-ethylpyridine at ?78°C, followed by reaction with CH₃I(¹³CH₃I) or CH₃OH. The stereochemistry was studied by NMR and GC. Methylation and the addition of 4-vinylpyridine occurred in a stereochemically nonselective manner. The data also indicated that in contrast to the oligomerization of 2-vinylpyridine the stereoisomeric meso and racemic 4-pyridyl carbanions propagated with equal reaction rates. The stereochemistry was readily explained by an absence of coordination of the Li ion by the nitrogen of the penultimate pyridine ring observed in the corresponding oligomerization of 2-vinylpyridine.     

Block copolymer-like self-assembly of fluorocarbon end-functionalized polystyrene and polybutylmethacrylate

We demonstrate the self-assembly through fluorophilic interactions of a blend of perfluorocarbon (RF) end-functionalized polystyrene and the corresponding RF-polybutylmethacrylate into optically transparent materials that retain domain characteristics typical of the component polymers but show well-defined lamellar nanostructured morphologies that qualitatively resemble that of the corresponding block copolymers.     

Intramolecular chelation as a factor in the stereochemistry of anionic vinyl polymerization

The methylation (CH₃I) of 1,3-bis(2-pyridyl)butyllithium in THF at −78°C is highly meso-selective (>98%) but the selectivity decreases with increasing cation-size or -coordination. The reaction of 1,3-bis(2-pyridyl)butyllithium with other electrophiles such as i-C₃H₇Br, PhCH₂Cl, Me₃SiCl (CD₃)₂CO and 4-vinylpyridine is also stereoselective under these conditions giving meso-like products. On the other hand, addition of 2-vinylpyridine is only slightly selective (64%) and this is consistent with the 65% meso content of P2VP formed by polymerization in the presence of Li ion. The chemistry of the above reactions is rationalized by intramolecular coordination of Li or other cations by the penultimate 2-pyridyl group and this is supported by equilibria involving proton abstraction of 1,3-bis(2-pyridyl)butane and similar compounds by bases having Li, Na or K counterions. It is shown that the tendency for intramolecular chelation is highest for Li and lowest for K ion. Temperature dependence of the above equilibrium shows that intramolecular chelation of Li and Na ions is exothermic (−1.4 and −1.3 kcal respectively) whereas the AH for K ion is very small (−0.5 kcal). Entropies of chelation are slightly positive for Li (0.8 e.u.) and negative for Na and K ions (-2.60 and −0.40 e.u. respectively). The lack of stereoregular polymerization of 2-VP in the presence of Li ion is most likely due to the requirement that the Li ion of the newly formed 2-pyridyl anion is coordinated with the 2-pyridyl group of the previous asymmetric center. Thus it would appear that intramolecular coordination of metal ion by penultimate 2-pyridine does not necessarily lead to isotactic-polymerization.     

Synthesis and characterization of topologically interesting vinyl polymers

A series of macrocyclic poly(2-vinylpyridine)&s were synthesized by high dilution (∼10⁻⁵ M) coupling of the two-ended living precursor dianions with 1, 2- and 1, 4-bis-(bromomethyl)benzene (1, 2- or 1, 4-DBX). SEC measurements indicate that the macrocycles contain less than 5% linear precursor and that the hydrodynamic size of the macrocycles is substantially (∼30%) less than that of the linear precursor. At very low MW, the sizes of macrocyclic and linear polymers differ less, particularly for the case of the 1, 2-DBX product. Viscometry measurements in THF of linear and macrocyclic polymers also indicated substantial size differences of linear and macrocyclic P2VP. The values of the glass transition temperature (T_g) for both the 1, 2- and 1, 4-DBX macrocycles were found to increase with decreasing MW as qualitatively predicted by entropy calculations. Thus, as the degree of polymerization decreases, the macrocyclic chains become conformationally stiffer and this is reflected by higher T_g values. The differences in T_g between linear and macrocyclic P2VP are quite large (∼40K) around a DP of 40 and decrease to values of a few degrees at a DP of around 200. These differences are diagnostically useful in estimating the linear content of the macrocycles below a DP of 100.