Synthesis and properties of co-isotactic copolymers of functional vinyl aromatic monomers with chiral α-olefins

Optically active co-isotactic copolymers of α-olefins, like (R)-3,7-dimethyl-1-octene ( 1 ) or (S)-4-methyl-1-hexene ( 2 ), with functional para-substituted styrenes were obtained by both copolymerization in the presence of Ziegler-Natta catalysts and by chemical modification of the copolymers of styrene ( 5 ) with 1 or 2 . By the first mentioned route copolymers of 4-iodostyrene ( 3 ) and propyl 4-vinylbenzenesulfonate ( 4 ) were prepared in satisfying yields. Fractionation by solvent extraction as well as the physical properties of the polymeric products, guarantee for the formation of co-isotactic copolymer macromolecules. By the other route co-isotactic copolymers of 1 or 2 with several para-substituted styrenes (like 4-halo, 4-chloromethyl, 4-diphenylphosphino, 4-diphenylphosphinomethyl, and 4-alkylammoniomethyl chloride derivatives) were prepared under conditions which do not give appreciable racemization of the chiral centers present in the main chain as well as in the lateral chain of the starting copolymers of styrene with α-olefins. The reported data provide a useful way to attain stereoregular and chiral macromolecular materials, bearing either electron donor or chemically active functional groups, inserted in a highly homogeneous conformational environment.     

13C NMR sequence analysis, 15. Copolymerization of alanine-NCA with other α-amino acid NCAs

Alanine-N-carboxylic acid anhydride (Ala-NCA) was copolymerized with glycine-NCA, phenylalanine-NCA, valine-NCA, leucine-NCA, and sarcosine-NCA in various solvents and with various catalysts; 22,6 MHz ¹³C NMR spectra and 90 MHz ¹H NMR spectra of the resulting copolypeptides were measured in trifluoroacetic acid and from the signal intensities the rate of incorporation was estimated. The copolypeptides poly(Ala/Gly) and poly(Ala/Phe) contained both monomeric units in an almost equal concentration, while the low reactivity of Val-NCA and Leu-NCA resulted in a lower concentration of these residues compared with Ala in poly(Ala/Val) and poly(Ala/Leu). The ¹³C NMR spectra of poly(Ala/Gly) exhibit four CO-signals which could be assigned by comparison with the corresponding homopolypeptides and with the sequence polypeptides (Ala-Ala-Gly-Gly)_n, and (Ala-Gly-Gly)_n. From the intensities of these CO-signals the average length of the homogeneous blocks was calculated. Both block lengths and rate of incorporation proved to be almost independent of solvent, catalyst, and reaction temperature. Poly(Ala/Phe) and poly(Ala/Sar) show also four CO-signals, but poly(Ala/Val) only two and poly(Ala/Leu) one. The number and shape of the CO-signals allow one to decide whether random copolypeptides were obtained or not.     

Monomer-isomerization polymerization, 27. Monomer-isomerization in competition with polymerization of α-olefins with Ziegler-Natta catalysts

The monomer-isomerization was studied in the polymerization of α-olefins with the catalytic systems TiCl₃/(C₂H₅)₃Al and Ti(OC₄H₉)₄/(C₂H₅)₃Al. The isomerization to the corresponding olefins with internal double bond was found to occur during the polymerization. With the system TiCl₃/(C₂H₅)₃Al only a small amount of isomerized olefin was produced as result of the rapid polymerization of the α-olefins. Significant isomerization was observed to take place with a poor polymerization catalyst such as Ti(OC₄H₉)₄/(C₂H₅)₃Al. Isomerization and polymerization of α-olefins with Ziegler-Natta catalysts were found to depend on the oxidation state of titanium.     

13C NMR spectra of poly(alkyl α-chloroacrylate)s

¹³C NMR spectra of isotactic and syndiotactic poly(ethyl α-chloroacrylate)s and poly(isopropyl α-chloroacrylate)s were measured in toluene-d₈ at 25 MHz. The spectra were analyzed for triad, tetrad and pentad tacticities comparing the spectra with the results previously obtained by ¹H NMR spectroscopy. Tetrad tacticities of the polymers were measured from the resonance of the backbone methylene carbon and favorably compared with tetrad values obtained by ¹H NMR. The resonance of the carbonyl carbon showed the peaks corresponding to the tactic triad with a pentad splitting in mm resonance. The triad values obtained from the carbonyl carbon resonance were in good agreement with those calculated from the tetrad values as well as the triad tacticities obtained by ¹H NMR. The quaternary carbon reconances were very complicated because the chemical shift differences for the different tactic triads were of the comparable magnitude with those for the pentads, and a tentative peak assignment was made with the aid of the statistical calculation of pentad values assuming first-order Markovian statistics for the isotactic polymer and Bernoullian statistics for the syndiotactic polymer.     

1H and 13C NMR spectra of poly(propyl α-bromoacrylate) and poly(methyl α-bromoacrylate)

The ¹³C NMR spectra measured at 25 MHz of the methyl and propyl esters of isotactic and syndiotactic poly(α-bromoacrylate) were sufficiently resolved to be analysed for pentad tacticity sequences. The pentad tacticity of the syndiotactic polymers prepared with free radical initiators at ?40°C agreed with those calculated for Bernoullian sequence distributions based on P_r values of 0,83–0,87. The tacticities of the isotactic polymers prepared with heterogeneous catalysts were determined on the basis of these assignments. Good internal consistency was obtained between the calculated and observed pentad proportions from the quaternary and carbonyl carbon peaks in the spectra of these polymers. The order of chemical shifts for the meso and racemic dyads and tetrads in these polymers were opposite to those found in the equivalent methacrylate polymers, but as with the latter, the ¹³C-T₁ values were longer in the isotactic than in the syndiotactic polymers.     

Conformational analysis and reactivity in isotactic specific polymerization of C-3 branched α-olefins

Quantitative conformational analysis of 3-methyl-1-butene, 3-methyl-1-pentene and 3-ethyl-1-pentene seems to support the hypothesis that the conformation of these monomers in the active state of Ziegler-Natta isotactic polymerization is either H-skew + or H-skew ?. It seems, however, that the experimentally determined reactivities of the considered α-olefins in the initiation steps of the isotactic polymerization could be accounted for quantitatively only by assuming somewhat different activation energies for the attack of the different monomers.     

Polymerization behavior of α-olefins with rac- and meso-type ansa-metallocene catalysts: Effects of cocatalyst and metallocene ligand

Polymerizations of propene, 1-butene and 1-hexene were conducted with a mixture of rac- and meso-[dimethylsilylenebis(2-methylindenyl)]zirconium dichloride ( 1 ) combined with methylaluminoxane (MAO), triethylaluminium (AlEt₃)/triphenylcarbenium tetrakis(pentafluorophenyl)borate ( 2 ) or triisobutylaluminium (AliBu₃)/ 2 as a cocatalyst. The polymerization profiles of propene with rac- 1 and meso- 1 were determined from the rate of overall propene consumption and the fractions of isotactic and atactic polymers which were sampled during polymerization. An induction time to reach the maximum R_p (rate of polymerization) followed by gradual decay was observed in the case of using the systems rac-, meso- 1 –MAO and rac- 1 –AliBu₃/ 2 . Besides, a rapid drop of R_p from the initial value was found when using AlEt₃/ 2 . Molecular weights of the isotactic and atactic polymers sampled do not change during polymerization, and it is suggested that the change of [C*] (number of active centers) is reflected in the profiles of R_p. The rate ratio of rac- 1 to meso- 1 (R_p(rac)/R_p(meso)) in propene and 1-butene polymerizations decreases in the following order: AlEt₃/ 2 > AliBu₃/ 2 > MAO. In the case of 1-hexene polymerization, the highest R_p(rac)/R_p(meso) value was obtained. This result indicates that the coordination of 1-hexene to the sterically hindered site of meso- 1 is difficult compared with propene and 1-butene.     

Stereoselective copolymerization of racemic α-olefins with ethylene by isospecific Ziegler-Natta catalyst

Copolymers of ethylene with racemic and optically active 3,7-dimethyl-1-octene (3,7-DM-1-O) have been prepared in the presence of ZIEGLER-NATTA isospecific catalysts. The copolymers of racemic 3,7-DM-1-O have been separated in fractions having optical activity of opposite sign by chromatography on optically active poly[(S)-3-methyl-1-pentene]. The degree of separation obtained is very close to that found for the isotactic homopolymer of 3,7-DM-1-O. The fractional precipitation of both types of copolymers has been also carried out in order to obtain information on the distribution of the comonomers. The results exclude a mere steric control by the growing chain end. The stereoselectivity can be better explained on the basis of the presence of asymmetric catalytic complexes, which are stereospecific in the isotactic sense.     

Enantioselective hydro-oligomerization (protio- or deuterio-) of α-olefins

The enantioselective protio(deuterio)oligomerization of propene, 1-pentene and 4-methyl-1-pentene was carried out with the diastereomerically pure catalyst precursor (?)-(R)-ethylenebis [1–3a,7a-η-(4,5,6,7-tetrahydro-1-indenyl)]zirconium (S)-1,1′-bis-2-naphtholate ( 3 ) and/or with (?)-(R)-ethylene [1–3a,7a-η-bis(4,5,6,7-tetrahydro-1-indenyl]dimethylzirconium ( 1 ) in the presence of methylalumoxanes. The enantioselectivity of both reactions which take place (deuteration and oligomerization) is from fair to high and essentially independent of the catalyst precursor. In the hydrooligomerization the enantioface of the olefinic monomer which prevailingly reacts is from a geometric point of view the same for all three substrates. In the case of 1-pentene and 4-methyl-1-pentene the enantioface prevailing embedded in the oligomers is opposite to that prevailingly deuterated. The significance of the growing chain for enantioface selection during stereospecific polymerization is thus emphasized.     

Stereochemical Pseudohexad 13C NMR Resonances and Regioregular Propylene/Ethylene‐1‐13C Copolymers

This paper reports the assignment of the ¹³C NMR resonances of the S_ββ and S_αγ methylenes observed in the spectra of copolymers of propylene with trace amounts of enriched ethylene‐1‐¹³C. The assignment is achieved by comparing the chemical shifts calculated by an empirical method (at the CH₂ stereochemical pseudohexad level), with the experimental spectra of regioregular copolymers with different stereochemical structure. The implications of the observed S_ββ and S_αγ resonances on the stereochemical polymerization mechanisms are also discussed.

     

Reciprocal modulation of C/EBP‐α and C/EBP‐β by IL‐13 in activated microglia prevents neuronal death

In response to aggravation by activated microglia, IL‐13 can significantly enhance ER stress induction, apoptosis, and death via reciprocal signaling through CCAAT/enhancer‐binding protein alpha (C/EBP‐α) and C/EBP‐beta (C/EBP‐β). This reciprocal signaling promotes neuronal survival. Since the induction of cyclooxygenase‐2 (COX‐2) and peroxisome proliferator‐activated receptor gamma/heme oxygenase 1 (PPAR‐γ/HO‐1) by IL‐13 plays a crucial role in the promotion of and protection from activated microglia, respectively; here, we investigated the role of IL‐13 in regulating C/EBPs in activated microglia and determined its correlation with neuronal function. The results revealed that IL‐13 significantly enhanced C/EBP‐α/COX‐2 expression and PGE₂ production in LPS‐treated microglial cells. Paradoxically, IL‐13 abolished C/EBP‐β/PPAR‐γ/HO‐1 expression. IL‐13 also enhanced ER stress‐evoked calpain activation by promoting the association of C/EBP‐β and PPAR‐γ. SiRNA‐C/EBP‐α effectively reversed the combined LPS‐activated caspase‐12 activation and IL‐13‐induced apoptosis. In contrast, siRNA‐C/EBP‐β partially increased microglial cell apoptosis. By NeuN immunochemistry and CD11b staining, there was improvement in the loss of CA3 neuronal cells after intrahippocampal injection of IL‐13. This suggests that IL‐13‐enhanced PLA2 activity regulates COX‐2/PGE₂ expression through C/EBP‐α activation. In parallel, ER stress‐related calpain downregulates the PPAR‐γ/HO‐1 pathway via C/EBP‐β and leads to aggravated death of activated microglia via IL‐13, thereby preventing cerebral inflammation and neuronal injury.     

Optically active hydrocarbon polymers with aromatic side chains, 7. Synthesis and properties of copolymers of vinyl aromatic monomers with chiral α-olefins

Optically active alkyl-α-olefins such as (R)-3,7-dimethyl-1-octene ( 1 ), (S)-4-methyl-1-hexene ( 2 ), and (S)-5-methyl-1-heptene ( 3 ) were copolymerized with styrene ( 4 ), 1-vinylnaphthalene ( 5 ) or 2-methylstyrene ( 6 ) by Ziegler-Natta catalysts based on VCl₄, TiCl₄ or TiCl₃ “ARA” and Al[CH₂CH(CH₃)₂]₃ or Al(C₂H₅)₃. Fractionation by solvent extraction as well as optical rotation and differential dichroic absorption (Δε) demonstrate unequivocally the formation of copolymer macromolecules. The isotactic enchainment of the units is supported by IR and X-ray spectra. A remarkable contribution to optical rotation by aromatic units is observed in the copolymers of 1. In addition circular dichroism data relative to the lowest energy π→π^* electronic transition of aromatic nuclei show that aromatic units, when inserted in optically active copolymer macromolecules, assume a preferential chiral conformation.     

Effects of branching on some properties of ethylene/α-olefin copolymers

Copolymers of ethylene with relatively small amount of nine species of α-olefins, i.e., propylene, butene-1, pentene-1, hexene-1, octene-1, dodecene-1, octadecene-1, 4-methylpentene-1, and 4.6-dimethylheptene-1 were prepared with the use of ZIEGLER-NATTA catalyst and their melt properties, crystalline behaviors, and various physical properties were investigated in relation to the side-chain length and structure. The melt index-intrinsic viscosity (MI-[η]) correlation, melt tension, and melt extensibility were found to be hardly affected by the branches. On the crystalline behavior and physical properties, the effects of branching generally became more pronounced when the branch sizes became larger. It was noticeable, however, that ethyl and propyl branches exert strong effects in comparison with their relatively small sizes. It was also observed that the ethylene/octadecene-1 copolymers have lower β-peak heights and higher β-peak temperatures in E″-temperature curves than expected from their branch size, and this was considered to be due to the crystallization of the side-chains. The properties of high pressure polyethylenes coincided, in many respects, with those of the copolymers having bulky branches.     

Stereospecific polymerization of higher α-olefins with the solvay type catalyst TiCl3/Cp2Ti(CH3)2

Polymerizations of higher α-olefins (C₁₀–C₂₀) were carried out at 40°C in heptane, using the following three catalysts which differ in the isospecificity for propene polymerization: Solvay type TiCl₃/Cp₂TiMe₂((A) highly isospecific), Solvay type TiCl₃/AlEt₃((B) isospecific) and TiCl₃.3Py/MgCl₂/AlEt₃((C) aspecific). The isospecificity of the catalysts was found to decrease in the following order: (A) ? (B) ? (C), which agrees well with the results obtained in propene polymerization. The crystallinity of these polymers is discussed in brief.     

Polymerization of α-olefins in the presence of δ-TiCl3/Al(CH3)3: Chain propagation rate constants for ethylene and propene

The chain propagation rate constants for the polymerization of ethylene and propene in the presence of δ-TiCl₃/Al(CH₃)₃ at 22 °C are determined by means of a ¹³C NMR analysis of suitable block copolymers. The numerical values of the rate constants are compared with those previously reported.     

Stereoregularity of polystyrene derivatives, 4. Poly(α-methylstyrene) obtained by anionic catalysts

The stereoregularity of poly(α-methylstyrene)s prepared by anionic initiators was determined from the peak area of α-methyl proton absorptions and aromatic C¹ carbon absorptions. Highly syndiotactic polymers were obtained at 0°C in both toluene and tetrahydrofuran (THF). Sodium, potassium, and cesium naphthalenes gave polymers with the same syndiotacticity in THF. The syndiotacticity of polymers prepared by sodium and potassium naphthalenes as catalyst in both toluene and THF decreased at low temperatures. This behavior is similar to that of o-methylstyrene and m-methylstyrene in THF. The stereoregulation mechanism in the anionic polymerization of α-methylstyrene is discussed and compared with that of polystyrene and ring substituted polystyrenes.     

Methine and α-methyl proton resonance in methyl acrylate/methacrylonitrile copolymers

Methyl acrylate (A)/methacrylonitrile (M) copolymers, prepared by emulsion and solution polymerization at 50°C, were characterized by 220 MHz NMR spectroscopy. The methine and α-methyl proton resonances of such copolymers consisted of three peak patterns, individual resonance areas being assignable to protons centered in various A or M centered triads. Triad fractions determined from methine and α-methyl resonance patterns were in excellent agreement with calculated values based on reactivity ratios of r_A = 0.39 ± 0.06 and r_M = 3.56 ± 0.50 for copolymers prepared in emulsion and on reactivity ratios of r_A = 0.24 ± 0.02 and r_M = 3.21 ± 0.11 for copolymers prepared in solution.     

Calculation of solid‐state 13C NMR spectra of cellulose I α, I β and II using a semi‐empirical approach and molecular dynamics

Using the semi‐empirical bond polarization theory (BPT) it is possible for the first time to calculate the complete solid‐state ¹³C NMR spectra of crystalline cellulose polymorphs I α, I β, and II and to model the effect of molecular motions on the NMR spectra by molecular dynamics simulations. Crystal dynamic simulations at room temperature over a time span of 20 ps were performed with a quantum‐mechanical molecular‐mechanical force field (QM‐MM) using fluctuating atomic charges to account for mutual polarizations. Calculations of isotropic ¹³C chemical shifts as well as complete chemical shift tensors were performed. Calculated powder spectra for the individual carbon atoms show significant differences between the static and dynamic model of the cellulose polymorphs due to restricted molecular motions within the crystal. The NMR line splittings of the C1 and C6 sites could clearly be assigned to the different cellulose chains in the I β and II polymorphs. The results are discussed and compared to experimental cellulose spectra.