Highly stereoelective polymerization of rac-(D,L)-lactide with a chiral schiff's base/aluminium alkoxide initiator

A chiral Schiff's base/aluminium alkoxide initiator bearing a ligand derived from R-(+)-1,1′-binaphthyl-2,2′-diamine was synthesized and used for the stereoelective polymerization of rac-(D ,L )-lactide. Rather high stereoelectivity is observed: a polymer with 88% enantiomeric enrichment in D units is obtained at 19% conversion. At high conversions a stereocomplex between D- and L-enriched stereocopolymers is formed. The polymerization reaction shows living type features, and narrow molecular weight distributions (M̄_w/M̄_n = 1,05–1,30) are obtained up to very high conversions. This indicates that transesterification reactions do not occur significantly with this sterically hindered initiator.     

Metal-containing initiator systems, 35. Synthesis of block copolymers by using the binary initiator system cobaltocene/bis(ethyl acetoacetato)copper(II)

Polymerization of methyl methacrylate (MMA) with the binary initiator system cobalt-ocene (Co(C₅H₅)₂)/bis(ethyl acetoacetato)copper(II)(Cu(eacac)₂), was studied kinetically at 25°C in acetonitrile. The relationship between yield and molecular weight of the resulting poly(MMA) was suggestive of a living polymerization character in this system. The synthesis of block copolymers of MMA with styrene (St) was attempted by adding St as the second monomer to the polymerization system of MMA. The block copolymer was formed in a yield of 20% when MMA was polymerized for 1 day, while the yield went up to 80% when the MMA polymerization was carried out for 3 days. The resulting block copolymer was characterized by IR spectroscopy and gel-permeation chromatography.     

Synthesis and enzymatic degradation of poly(ester-amide) stereocopolymers derived from alanine

Poly(ester-amide)s with L -alanine contents of 100 (LalaS-100), 90, 80, 70, 50, 30 and 0 were prepared by interfacial polycondensation of the mixture of 1,6-hexanediol diester of L - and D -alanine with sebacoyl chloride. The enzymatic degradation of the poly(ester-amide)s was followed by the weight loss in a buffer solution (pH 7.2) of proteolytic enzymes (proteinase-K, papain and α-chymotrypsin) and lipase enzymes (R. delemar, P. cepacia and C. rugosa) at 37°C. The former enzymes degrade LalaS-100 much faster than the latter. Above all proteinase-K degrades it most rapidly and the weight loss is about 78% after 8 h of incubation. It was found that the degradation with the proteolytic enzymes is not caused by hydrolysis of the semi-peptide linkage but of the ester linkage. The effect of the stereochemical composition on the enzymatic degradation was examined using proteinase-K and papain, which degrades LalaS-100 (100% L ) as opposed to LalaS-0 (100% D ). In contrast, the highest rate of degradation was observed for LalaS-90.     

On the kinetics and initiation mechanism of acrylamide polymerization using the ceric ion/acetoacetanilide system as initiator

It is found that acetoacetanilide (system. name: N-phenyl-3-oxobutanamide) possesses a very high reactivity towards ceric ions, Ce⁴⁺, in initiating the polymerization of vinyl monomers. In the presence of acetoacetanilide, the rate of the acrylamide polymerization is larger by a factor of 30 than the rate of initiating by ceric ions alone. The reaction orders were 0,58, 0,75 and 1,2 with respect to acetoacetanilide, ceric ion and acrylamide concentration, respectively. The activation energy of the acrylamide polymerization initiated by ceric ion/acetoacetanilide is only 12,9 kJ/mol. According to the IR spectrum, fragments of acetoacetanilide are present in the polymer produced by the ceric ion redox initiator comprising acetoacetanilide. The C-centered and N-centered primary radicals generated via redox reaction of ceric ion and acetoacetanilide were detected by spin trapping and electron paramagnetic resonance. An initiation mechanism of the ceric ion/acetoacetanilide initiated polymerization is proposed on the basis of experimental results.     

Ring opening in the polymerization of propylene oxide by the initiator systems aluminium isopropoxide or aluminium isopropoxide/zinc chloride (1:1) and the microstructure of the resulting polymers

2-Methyloxirane (propylene oxide) was polymerized by aluminium isopropoxide [Al(OiPr)₃] and by the initiator system Al(OiPr)₃/zinc chloride (1:1). The microstructure of the resulting polymers was investigated by ¹³C NMR spectroscopy. The polymer obtained with Al(OiPr)₃ alone was found to contain head-to-tail and head-to-head linkages in the chain, indicating an oxirane ring opening in both α- and β-positions. The polymerization by the aluminium isopropoxide/zinc chloride initiator system led to the formation of a crystalline, highly isotactic fraction and to an amorphous, almost atactic fraction of the polymer with head-to-tail linkages only.     

Block copolymerization of triphenylmethyl methacrylate with other methacrylates using (−)-sparteine/9-fluorenyllithium as an initiator

Diblock and triblock copolymerization of triphenylmethyl methacrylate (TrMA) with methyl methacrylate (MMA), butyl methacrylate (BuMA), benzyl methacrylate (BzMA) or diphenylmethyl methacrylate (DPMMA) in the presence of the chiral anionic complex initiator, (?)-sparteine/9-fluorenyllithium, were investigated. The resulting block copolymers show high specific rotation and were characterized by means of gel-permeation chromatography, scanning electron microscopy, nuclear magnetic resonance, circular dichroism spectra and thermal analysis.     

4-[Diphenyl(trimethylsilyl)methyl]benzophenone as initiator in the photopolymerization of methyl methacrylate and styrene

The polymerization of methyl methacrylate (MMA) and styrene (St) has been studied using 4-[diphenyl(trimethylsilyl)methyl]benzophenone 1 as photoinitiator. The polymerization follows a free radical mechanism; the polymerization rate increases linearly with the monomer concentration and was found to be proportional to the 0.33 and 1.40 power of the photoinitiator and the monomer (MMA) concentration, respectively. The overall activation energy in the case of MMA photopolymerization was calculated to be 25.0 kJ/mol. From ¹H NMR studies it is concluded that the obtained polymers contain two different trimethylsilyl moieties, one at the head and the other at the tail of the polymer chain, showing primary termination reactions even at low initiator concentrations. The p-benzoyltrityl radical 1^· is incorporated into the polymer chain to a very small extent, acting as a scavenger. This is also concluded by laser flash photolysis (LFP) and ESR spectroscopy measurements. A “living” character of the polymerization was observed only at very low initiator concentrations. The triplet state (³ 1 ^*) of the initiator was quenched by styrene, reducing its efficiency. The rate constant k_q of the quenching process of ³ 1 ^* was measured by LFP (k_q = 3.1 · 10⁹ M⁻¹ · S⁻¹). The triplet state and the photodissociation efficiency of the initiator is unaffected by MMA at various concentrations.     

The dibenzylmagnesium/tetrahydrofurfuryl alcohol system as an initiator of polymerizations of styrene and butadiene

The dibenzylmagnesium/tetrahydrofurfuryl alcohol system showed a very high reactivity as initiator for the anionic polymerization of styrene and butadiene, though the use of the magnesium alkyl alone could not induce the polymerization. A series of model reactions using 1,1-diphenylethylene gave information on the structure and reactivity of the dibenzylmagnesium complex as initiator. It was concluded from the model reactions, together with UV spectroscopic data, that the initiating species, the benzyl anion, should be deliberated from the coordination-agent-separated ion pairs of the benzylmagnesium complex.     

Positive two-stage carcinogenesis in female sprague-dawley rats using 7,12-dimethylbenz(a)anthracene (DMBA) as initiator and 12-o-tetradecanoylphorbol-13-acetate (TPA) as promotor

Summary In contrast to a previous report by Shubik, the validity of the 2-stage skin carcinogenesis experiment was demonstrated in the rat. The modified experiment was carried out in female Sprague-Dawley rats using intragastrically administered DMBA as a carcinogen and the topically applied phorbol ester TPA as a promoter.Seven groups of animals were used. Two groups were treated with TPA only, two groups were initiated only with DMBA, two further groups were both initiated and promoted, and one group served as a control. Each of the initiated/promoted groups or only initiated or promoted groups contained one sub-group in which the animals had been bilaterally ovarectomized prior to the experiment.Hyperplasia of the dorsal epidermis occurred only in the promoted and in the initiated/promoted groups. Tumors of the back skin were observed exclusively after initiation/promotion. Ovarectomy — leading to a prolonged survival time of the animals — seems to be crucial for the manifestation of malignant skin tumors. Initiation/promotion also gives rise to tumors of the forestomach, the small intestine, the liver and the colon. Tumors in other organs (especially in the mammary gland and the Zymbal gland) were also be observed after initiation alone.Dedicated to Professor Wilhelm Doerr on the occasion of his 65th anniversary     

13C and 1H NMR study of polymers obtained by polymerization of styrene oxide using diethylzinc/water system as initiator

Styrene oxide was polymerized by the initiator ZnEt₂/H₂O and the structure of the resulting polymers was determined by means of ¹H and ¹³C NMR spectroscopy. Both the crystalline and the amorphous fraction have regular head-to-tail structure indicating that the oxirane ring opens exclusively at the β position. The stereochemistry of the growth of the crystalline, highly isotactic fraction follows first order Markov statistics, while that of the amorphous fraction follows Bernoulli statistics.     

Studies on the kinetics and initiation mechanism of acrylamide polymerization using persulfate/aliphatic diamine systems as initiator

The effects of various aliphatic diamines on vinyl polymerization using ammonium persulfate as initiator were studied. It was found that diamines with two amino groups separated by an ethylene unit possess higher promoting activities than those with two amino groups linked by other α,ω-alkylene units such as methylene or propylene. The promoting activities of diamines on vinyl polymerization are in the order: tertiary diamine > secondary diamine > primary diamine. The methyl group is a preferred substituent at the amino groups of the diamine, being faster oxidized by persulfate (to produce the dialkylaminomethyl free radical) than the α-methylene groups. The primary free radicals produced through redox-reaction of persulfate and aliphatic diamines were studied using the spin trapping technique and ESR spectroscopy. An initiation mechanism of persulfate/aliphatic diamine systems is proposed.     

Synthesis and properties of star oligo/poly(trimethylene carbonate)s with cholic acid moieties as cores

Star oligo/poly(trimethylene carbonate)s with cholic acid moieties as cores were synthesized by ring-opening polymerization of trimethylene carbonate (TMC) initiated by cholic acid with hydroxyl groups. The molecular weights of the star oligomers/polymers were controlled by adjusting the feed ratio of the initiator cholic acid to the monomer TMC. The star oligo/poly(trimethylene carbonate)s were characterized by Fourier transform infrared spectroscopy (FT-IR), (1)H nuclear magnetic resonance spectroscopy ((1)H-NMR) and combined size-exclusion chromatography and multiangle laser light scattering (SEC-MALLS) analysis. The water contact angles of the star oligo/poly(trimethylene carbonate)s were measured. Using 3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay, the star oligo/poly(trimethylene carbonate) was proved to have a low cytotoxicity. A microsphere drug-delivery system based on a star polymer was fabricated and its in vitro drug release property was studied.     

Differential effects of (D)- and (L)-homocysteic acid on the membrane potential of cat caudate neurons in situ

The enantiomers of homocysteic acid have been applied by microiontophoresis to neurons of the cat caudate nucleus in situ. The (L)-enantiomer elicited a bursty firing pattern similar to the one caused by N-methyl-D-aspartate, but differing from the N-methyl-D-aspartate pattern inasmuch as (L)-homocysteate induced depolarization shifts were shorter and had a smaller amplitude. (L)-Homocysteate induced excitations could be strongly inhibited by the selective N-methyl-D-aspartate antagonist 2-amino-7-phosphonoheptanoic acid but they were less sensitive to this antagonist than N-methyl-D-aspartate itself. (D)-Homocysteate elicited a more regular firing pattern similar to the one caused by non-N-methyl-D-aspartate excitatory amino acids such as quisqualate. These excitations were only rarely inhibited by 2-amino-7-phosphonoheptanoic acid. Our results suggest that (L)-homocysteate, a transmitter candidate at central mammalian synapses, is a mixed excitatory amino acid agonist with a strong preference for N-methyl-D-aspartate receptors in the cat caudate nucleus, while (D)-homocysteate has a predominant action at non-N-methyl-D-aspartate excitatory amino acid receptors.     

Two-Phase(water/organic) free radical polymerization of water-soluble vinyl monomers using lipophilic acylated β-cyclodextrins as initiator carrier

The acylated β-cyclodextrins, heptakis(2,3,6-tri-O-acetyl)-β-cyclodextrin, heptakis(2,3,6-tri-O-obutyryl)-β-cyclodextrin, heptakis(2,3,6-tri-O-butyryl)-β-cyclodextrin, and heptakis(2,3,6-tri-O-pivaloyl)-β-cyclodextrin, cause a marked increase in the rate of polymerization of water- soluble vinyl monomers using water insoluble initiators in the two-phase system consisting of water and an organic solvent (chloroform/ligroin, volume ratio 1 : 4). A plausible mechanism for this polymerization is proposed: The lipophilic acylated cyclodextrin acts as an initiator carrrier from the organic to the aqueous phase where polymerization takes place.     

Synthesis and properties of poly(dimethyldiphenylsilylenemethylene)

The novel polycarbosilane poly(dimethyldiphenylsilylenemethylene) (PDMDPSM) ( 2 ) was prepared by H₂PtCl₆-catalyzed ring-opening polymerization (ROP) of 1,1-dimethyl-3,3-diphenyl-1,3-disilacyclobutane ( 1 ). Polymerization in bulk at elevated temperature afforded high molecular weight PDMDPSM (M _w = 577000) with monomodal molecular weight distribution and regularly alternating SiR₂/CH₂ backbone structure. NMR spectroscopy revealed that a minor fraction of the monomer units is incorporated irregularly during polymerization in bulk, leading to structural units of the type  SiPh₂ CH₂ SiPh₂ and  SiMe₂ CH₂ SiMe₂ . The amount of regularly alternating dimethyl- and diphenylsilylenemethylene units increases when the polymerization is performed in solution at lower temperatures. PDMDPSM shows a glass transition temperature (T_g) at 50°C, which lies between the T_gS of poly(methylphenylsilylenemethylene) (PMPSM) and poly(diphenylsilylenemethylene) (PDPSM), demonstrating the stiffening effect of the SiPh₂ groups. Thermogravimetric analysis (TGA) evidenced nearly identical thermal stability of PDMDPSM and PMPSM.     

纳米磷灰石/聚D,L-乳酸生物复合材料制备与力学性能

目的观察溶液共混热压制备法能否提高纳米磷灰石(HA)/聚D,L-乳酸(PolyD,L杔actide,PDLLA)复合生物材料的力学性能。方法将HA/PDLLA分散至N,N-二甲基甲酰胺(DMF)有机溶剂中,在45℃下不断搅拌缓慢去除有机分散剂,干燥后打碎,放入模具中经100℃热压,获得纳米HA/PDLLA复合材料薄膜。结果透射电子显微镜(TEM)对不同含量的HA复合材料观察发现,纳米HA能被分散在PDLLA基材中。随着HA含量在复合材料中增加,HA出现了明显团聚现象,但总体上呈均匀的分布状态,说明热压前溶液共混处理有利于HA在复合材料中的分散。力学性能测试结果表明,上述溶液共混热压制备法能够增加HA与PDLLA的界面结合力,在合适的HA含量下,提高了材料的弹性模量、断裂应力及拉伸应力。结论溶液共混热压制备法能改善HA/PDLLA复合材料的力学性能。     

Metal-containing initiator systems, 34. Polymerization of vinyl monomers initiated by the binary system cobaltocene/bis(ethyl acetoacetato)copper (II)

The effect of some metallocenes such as ferrocene (Fe(C₅H₅)₂), nickelocene (Ni(C₅H₅)₂), and cobaltocene (Co(C₅H₅)₂), on the vinyl polymerization initiated with bis(ethyl acetoacetato)-copper(II) (Cu(eacac)₂) was investigated. Co(C₅H₅)₂ was found to exert a markedly accelerating effect on the polymerization of methyl methacrylate (MMA) with Cu(eacac)₂. The polymerization of MMA with the system Co(C₅H₅)₂/Cu(eacac)₂ at 50°C was found to be fairly affected by the solvent used. The results of copolymerization of MMA with styrene (St) and the effect of hydroquinone (HQ) on the polymerization of MMA with Co(C₅H₅)₂/Cu(eacac)₂ showed that the polymerization proceeds via a radical mechanism. The polymerization of MMA with Co(C₅H₅)₂/Cu(eacac)₂ was studied kinetically in acetone. The overall activation energy of the polymerization was calculated to be 86,3 kJ/mol (20,6 kcal/mol). This value was somewhat higher than that (17,6 kcal/mol) obtained for the polymerization of MMA with Cu(eacac)₂ alone. The polymerization rate (R_p) is represented by the following equation: R_p = k[Co(C₅H₅)₂]^0,5 [Cu(eacac)₂]^0,2 [MMA]^1,3. The high order in monomer concentration suggests a participation of the monomer in the initiation process of this polymerization. This is supported by the examination of the ESR spectrum of the system Co(C₅H₅)₂/Cu(eacac)₂/MMA/acetone, where reduction of Cu(II) to Cu(I) occurs. To elucidate the initiation mechanism, the spin trapping technique was applied to the system Co(C₅H₅)₂/Cu(eacac)₂/methyl acrylate. From these results, an initiation mechanism for the binary initiator system Co(C₅H₅)₂/Cu(eacac)₂ is proposed and discussed.     

Isolation of rat/rat T cell hybrids using W/Fu C58(NT)D as fusion partner

Functional rat/rat T cell hybrids were isolated for the first time by the fusion of spleen cells from rats tolerized to the hapten TNP to a HAT-sensitive rat thymoma (C58(NT)D). 11 fusions using different protocols were attempted to assess the optimal conditions for high hybridization frequency of the desired specificity. Interestingly, the cell density requirement of the non-transformed fusion partner took precedence over that of the C58 cell line, i.e., rat cells needed to be at high density after fusion, but others have reported that mouse cells prefer a much lower density even when the same line (C58) is used. Six fusions yielded hybridomas with between 3% and 70% of wells containing hybrids after three weeks of culture, depending on the protocol used. Phenotypically, all of the hybrids and clones tested expressed the W3/25 (rat CD4) antigen, but no OX-8 (rat CD8) or immunoglobulin molecules. A minority of hybrids were found to secrete factors able to suppress (a) proliferation, (b) antibody production, and (c) cells bearing IL-2 receptors, but none appeared to suppress the production of IL-2 itself. In contrast to non-transformed rat T cell lines, the T hybrids isolated were easy to grow to high densities, clone and freeze without the need to add exogenous antigen or lymphokines to the cultures at any stage.     

Synthesis and characterization of copolymers of ethylene and 1-octadecene using the rac-Et(Ind)2ZrCl2/MAO catalyst system

The copolymerization of ethylene and 1-octadecene using a bridged metallocene was studied in order to observe the effect of the comonomer on the catalytic activity. A noticeable increase in activity is seen as the concentration of 1-octadecene in the reaction medium increases. ¹³C NMR analysis shows 6.4 mol-% incorporation of comonomer at the highest 1-octadecene concentration in the feed used here. The molecular weight of the copolymers shows a drastic decrease that may be attributed to chain termination by transfer or β-elimination of the comonomer. As to the molecular weight distribution, it remains within a narrow range, as expected with metallocene catalysts. The melting temperature and the enthalpy of melting of the copolymers show a decrease with increasing comonomer content. As usual for ethylene copolymers, the X-ray crystallinities are higher than those determined from the enthalpy of melting.