Bestimmung der langkettenverzweigung von hochdruckpolyäthylen durch streulicht- und viskositätsmessungen

Linear and branched polyethylene were fractionated by solvent gradient chromatography. The fractions were characterized by light scattering and viscosity measurements. The relationship between the molecular weight and the limiting viscosity number and that between the molecular weight and the radius of gyration in α-chloronaphthaline at 127°C were determined. The exponent of the equation between the ratio 〈g_η〉Θ of the limiting viscosity numbers of branched and linear fractions of the same molecular weight M_w and the corresponding ratio 〈g_η〉_Θ,w of the radii of gyration for Θ-conditions was By this equation, the long chain branching of high pressure polyethylene can be determined from 〈g_η〉, the ratio of the limiting viscosity numbers in thermodynamically good solvents, which is within the experimental limits of error identical to 〈g_η〉_Θ in a Θ-solvent. Almost independent of the molecular weight, for the branched polyethylene fractions 1,4 long chain branching points per 1000 C-atoms were obtained.     

Degree of Branching of Highly Branched Polyurethanes Synthesized via the Oligomeric A2 Plus B3 Methodology

Summary: The oligomeric A₂ plus monomeric B₃ synthetic methodology provided highly branched, poly(ether urethane)s based on TMP (B₃) and isocyanate endcapped polyethers. ¹³C NMR spectroscopic assignments for the branched polyurethanes were verified using model urethane‐containing compounds based on TMP and a monofunctional isocyanate (either cyclohexyl or phenyl isocyanate (PI)). Derivatization of hydroxyl endgroups with trifluoroacetic anhydride enhanced the ¹³C NMR resolution in spectra for branched polyurethanes. The ¹³C NMR resonance for the linear unit exhibited a broad shoulder due to quaternary carbons that were attributed to cyclic species in the highly branched polyurethanes. The classical DB calculation revealed the efficiency of the B₃ monomer for branching; however, an equation that incorporated the linear contribution of the A₂ oligomer provided a more accurate DB for highly branched polyurethanes.

SEC chromatograms for increasing addition of A₂.     

Analysis of optical rotatory dispersion of poly- and oligo(L-lactate) by two-term DRUDE equation

Optical rotatory dispersion (ORD) and circular dichroism (CD) of poly(L -lactate) were investigated and were compared with those of optically active oligo(L -lactate), prepared by the stepwise method. Two-term DRUDE equation was applied to analyse the ORD curves using n-π^* and π-π^* transition bands as the critical wavelengths, Λ₀ and Λ₁. No drastic difference in ORD between poly(L -lactate) and oligo(L -lactate) was detected in chloroform and dioxane, which suggests that the polymer does not form any special conformation in these solvents. However, in acetonitrile it was found that the value of ?A₁/A₀ (A₁ and A₀ are constants characteristic of π-π^* and n-π^* transitions of ester chromophores, respectively) of the polymer differed from that of the oligomer (n = 3).     

Stereocomplex Crystallization Behavior and Physical Properties of Linear 1‐Arm, 2‐Arm,and Branched 4‐Arm Poly(L‐lactide)/Poly(D‐lactide) Blends: Effects of Chain Directional Change and Branching

For number‐average molecular weight (M _n) below 1 × 10⁴ g mol^?1, the comparison of cold crystallization temperature and spherulite growth rate and crystallinity of linear 1‐arm, 2‐arm, and branched 4‐arm poly(L ‐lactide)/poly(D ‐lactide) blends exhibits that the effects of chain directional change and branching significantly disturb stereocomplex crystallization. In contrast, the comparison of glass transition and melting temperatures of linear 1‐arm, 2‐arm, and branched 4‐arm poly(L ‐lactide)/poly(D ‐lactide) blends indicates that the effects of chain directional change and branching insignificantly alter and largely increase the segmental mobility of the blends, respectively, and the crystalline thickness of the blends is determined by M _n per one arm not by M _n and is not affected by the molecular architecture.

     

Zur Taktizität von Poly(α-methylstyrol), 5. Die Abhängigkeit der Taktizität von der Reaktionstemperatur und der Anfangsmonomerkonzentration

The tacticity of poly(α-methylstyrene) was measured by ¹H NMR spectroscopy. The polymerisation initiated in tetrahydrofuran by butyllithium gives a dependence on the initial monomer concentration and a S-shaped plot of log(m/r) vs. 1/T, which is not influenced by a stronger reaction medium like 1,2-dimethoxyethane or hexamethylphosphoric triamid or by addition of LiB(C₆H₅)₄. The activation parameters of the two straight lines A and B limiting the experimental curve are according to the equation The temperature dependence of the propagation rate constant k_w measured dilatometricly between ?11°C and ?75°C gives a linear Arrhenius plot. The rate constant of depolymerisation k_d was calculated using the equilibrium monomer concentration. The activation parameters are The influence of ionic dissociation on the tacticity is discussed. The results lead to the conclusion, that the contact ion pair, the solvent separated ion pair, and the free anion generate tacticities, which are undistinguishable by the applied experimental methods.     

The effect of branching on the Huggins coefficient of statistical styrene/butadiene copolymers

The Huggins coefficient k_H of solutions of statistical styrene/butadiene copolymers in cyclohexane (at 25 °C) and 2-pentanone (theta temperature 21 °C) is discussed. With an increase in branching the values of (i. e. k_H at the theta temperature) increase from 0,5 to 0,73. Lower values in cyclohexane (good solvent) correlate with the viscosity expansion factor α_η³, and the dependence is well fitted by the equation where is a function of the degree of branching, whereas C″₀ only slightly exceeds the value found for linear polymers.     

Long chain branching and solution properties of low density polyethylene

The intrinsic viscosity, osmotic pressure and light scattering of solutions of fractionated samples of low density polyethylene (LDPE) were measured to study its long chain branching. The following relation was obtained between the two branching indices: g_η is the ratio of the intrinsic viscosities of the branched and linear polymers with the same molecular weight, and g_s is the corresponding ratio of the radii of gyration. The average number of long branches per molecule, n_w, was calculated according to the theory of ZIMM and STOCKMAYER derived for random branching, and it was found that the dependence of n_w on the molecular weight is expressed by with α = 0.85. This relation indicates that the long branching frequency or density decreases gradually with molecular weight for LDPE.     

Selective Dimerization of γ‐Branched α‐Olefins in the Presence of C2v Group‐4 Metallocene‐Based Catalysts

Summary: The oligomerization of γ‐branched α‐olefins in the presence of catalytic systems based on group‐4 metallocenes with C_2v symmetry has been investigated. The highest reactivity was obtained by using dimethylsilyl‐bis(cyclopentadienyl)zirconium‐dichloride activated by methylaluminoxane. Highly regioregular dimers were selectively obtained for hindered γ‐branched monomers, while the less hindered ones produced higher molecular weight oligomers. A molecular modeling approach was used to rationalize the experimental results. In fact, a decrease in the β‐hydrogen elimination barrier and an increase in the insertion barrier with the monomer bulkiness were calculated.

General structure of the obtained dimers.     

Atom Transfer Radical Polymerization Using Multidentate Amine Ligands Supported on Soluble Hyperbranched Polyglycidol

Summary: Controlled polymerization of styrene in toluene was achieved by atom transfer radical polymerization (ATRP) using hyperbranched polyglycidol‐supported multidentate amine ligands/Cu^IBr catalyst systems. These catalyst systems with nanoscopic dimensions were more active than the corresponding low‐molecular‐weight ligands. The controlled/living nature of the polymerization is supported by linear first order plots (ln[M]_o/[M] versus time) and a linear increase of versus conversion as well as low polydispersity. Similar controlled polymerization of methyl methacrylate was possible in acetonitrile. Up to 97% of total copper used for polymerization could be removed from the polymer by simple precipitation in methanol and filtration.

Molecular weight characteristics for styrene polymerization using PG‐triamine as a ligand.     

Oligourethan dimers having asymmetric carbon atoms in the main chain. Oligourethans. V1

A series of oligourethan dimers having asymmetric carbon atoms in the main chain was prepared by the reaction between 1-alkyl-2-acetoxyethyl isocyanates and methyl l-alkyl-2-hydroxyethylcarbamates; their melting points, rotatory powers and R_F-values were determined. The dimers prepared are represented by a general formula, where R and R' are H, methyl (L and DL-forms), i-propyl (L and DL), i-butyl (L and DL) or benzyl (L) groups. Certain relationships have been found between the structure and rotatory power resp. R_F-value.     

Novel Cyclopentadithiophene‐Based D–A Copolymers for Organic Photovoltaic Cell Applications

Here, the synthesis, characterization, and photovoltaic properties of four new donor–acceptor copolymers are reported. These copolymers are based on 4,4‐difluoro‐cyclopenta[2,1‐b:3,4‐b′] dithiophene as an acceptor unit and various donor moieties: 4,4‐dialkyl derivatives of 4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene and its silicon analog, dithieno[3,2‐b:2′,3′‐d]‐silol. These copolymers have an almost identical bandgap of 1.7 eV and have a HOMO energy level that varies from ?5.34 to ?5.73 eV. DSC and X‐ray diffraction (XRD) investigations reveal that linear octyl substituents promote the formation of ordered layered structures, while branched 2‐ethylhexyl substituents lead to amorphous materials. Polymer solar cells based on these copolymers as donor and PC₆₁BM as acceptor components yield a power conversion efficiency of 2.4%.

     

Novel polymeric surfactants: Synthesis of semi‐branched,non‐ionic triblock copolymers using ATRP

This article reports the synthesis of novel amphiphilic triblock copolymers with a semi‐branched PLURONIC^®7R structure by atom transfer radical polymerization (ATRP) in aqueous media. Poly(ethylene oxide)s (PEOs) with molecular weights 10 000 and 16 000 were end‐functionalized and used as bifunctional macroinitiators for the polymerization of oligo(propylene oxide) monomethacrylate by ATRP in a 1/3 v/v water/methanol mixture and in a 1/1 v/v water/1‐propanol mixture. Deviations from first‐order kinetics with respect to the monomer concentration were observed indicating that termination reactions were taking place. However, linear plots were obtained, when ln[M]₀/[M] was plotted against time^2/3 as suggested by Fischer. The effect on the control of the polymerization by adding Cu(II)Br₂ to the polymerization medium has been investigated. When 10 mol‐% of Cu(II)Br₂ was substituted for Cu(I)Br, normal first‐order kinetics were observed. A large reduction in the rate of polymerization was observed for the polymerization initiated by bifunctional PEO10 000 initiator, but almost no reduction in the rate of polymerization was observed, when the bifunctional PEO16 000 initiator was used. When the polymerizations were conducted in 1/1 v/v water/1‐propanol, unexpectedly high rates of polymerization were observed.

Synthesis of amphiphilic block copolymers with a semi‐branched PLURONIC^®R architecture using ATRP.     

Evaluation of the ratio of mean square inertia radii of branched and linear macromolecules from intrinsic viscosity measurements

A method for the evaluation of the ratio of the mean square inertia radii \documentclass{article}\pagestyle{empty}\begin{document}$ (\overline {R^2 } )^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern‐\nulldelimiterspace} 2}} $\end{document} of branched (b) and linear (1) molecules \documentclass{article}\pagestyle{empty}\begin{document}$ g[\eta ] = {{(\overline {R_{\rm b}^2 } )_{[\eta ]} } \mathord{\left/ {\vphantom {{(\overline {R_{\rm b}^2 } )_{[\eta ]} } {(\overline {R_{\rm l}^{\rm 2} } }}} \right. \kern‐\nulldelimiterspace} {(\overline {R_{\rm l}^{\rm 2} } }})_{[\eta ]} $\end{document} from intrinsic viscosity measurements is proposed. The evaluation is based on the solution of the equation: where erf \documentclass{article}\pagestyle{empty}\begin{document}$ x = \frac{2}{{\sqrt \pi }}\int\limits_0^{\rm x} {{\rm e}^{ ‐ t^2 } } {\rm d}t $\end{document}; a is the exponent in the Mark-Kuhn-Houwink relation [η₁] = KM^a, ε = (2a?1)/3 and ?(a) is the shielding function, tabulated by Debye and Bueche. The method is used for the evaluation of g[η] in Θ- and in good solvents from experimental data published in the literature. The results show that it is possible to find the dimensions of branched molecules from intrinsic viscosity measurements in good solvents using the above formula. These results are obtained by the analysis of changes of the macromolecular dimensions in different solvents and in part by the results of second virial coefficient measurements of different polymers.     

Etude par 1H NMR des groupements chlorométhyles du poly(chlorure de vinyle), 2. Etude par écho de spin et comparaison avec des modèles moléculaires

In order to assign all ? CH₂Cl resonances observed by ¹H NMR in poly(vinyl chloride), (PVC), a preliminary study of the diastereoisomers of molecular models for the groups CH₂Cl? CHCl? [II] and is used, in conjunction with spin echo experiments. The complex region [3,5–4,0 ppm] which corresponds to saturated ? CH₂Cl chloromethyl protons is completely assigned. A separate evaluation of ? CHCl? CH₂Cl [II], ? CH₂? CH₂Cl [III] and is proposed and tested on raw PVC samples. The main interest of this ¹H NMR determination is the fact that raw PVC can be used and that it gives the ratio [II]/[III] which is not accessible by ¹³C NMR on reduced samples.     

Aluminum Initiators Supported by Asymmetric [ONNO′]‐Type Salan Ligands for the Ring‐Opening Polymerization of rac‐Lactide

A novel series of aluminum complexes supported by asymmetric [ONNO′]‐type Salan ligands ( 1–4 ) is successfully synthesized. Polylactides (PLAs) with molecular weights close to the theoretical values and narrow polydispersity indices (PDIs) can be obtained. Kinetic studies reveal first‐order polymerization kinetics and a linear relationship between the molecular weight and the percentage conversion. Complexes 1–4 display heteroselective polymerization to give heterotactic‐enriched PLAs with P_r values ranging from 0.64 to 0.74. The character of the ligand substituents from the two different phenolic rings and the complex fluxionality arising from the ligand interconversion are suggested to be responsible for the different levels of stereoselectivity and activity.

     

Highly Branched Poly(arylene ether)s via Oligomeric A2 + B3 Strategies

Summary: Branched poly(arylene ether)s were prepared in an oligomeric A₂ + B₃ polymerization of phenol endcapped telechelic poly(arylene ether sulfone) oligomers as A₂ and TFPPO as trifunctional monomer B₃. The molar mass of the A₂ oligomer significantly influenced the onset of gelation and the DB. A high level of cyclization during polymerization of low molar mass A₂ oligomers (U₃ = 660 and U₆ = 1 200 g · mol^?1) led to a high conversion of functional groups in the absence of gelation, and the level of cyclization reactions in the polymerization decreased as the molar mass of the A₂ oligomer was increased. The pronounced steric effect in the polymerization of higher molar mass A₂ oligomers (U₈ = 1 800 and U₁₆ = 3 400 g · mol^?1) resulted in low reactivity of the third aryl fluoride in the B₃ monomer. As a result, only slightly branched (U₈ = 1 800 g · mol^?1) or nearly linear (U₁₆ = 3 400 g · mol^?1) high molar mass products were obtained with higher molar mass A₂ oligomers. The branched polymers exhibited lower Mark‐Houwink exponents and [η] relative to linear analogs, and differences between the branched polymers and linear analogs were less significant as the molar mass of the A₂ oligomers was increased due to a decrease in the overall DB.

     

Formation of the intermediate cyclic six-membered oxonium ion in the cationic polymerization of β-propiolactone initiated with CH3CO⊕SbF

Analysis of the ¹H NMR spectra of \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} \mathop {\rm C}\limits^ \oplus {\rm OSbF}_{\rm 6}^ \ominus $\end{document} /β-propiolactone and of the model systems \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} \mathop {\rm C}\limits^ \oplus {\rm OSbF}_{\rm 6}^ \ominus $\end{document}/(CH₃)₂O and \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} \mathop {\rm C}\limits^ \oplus {\rm OSbF}_{\rm 6}^ \ominus $\end{document}/CH₃COOCH₃ in liquid SO₂ in the temperature region ?70 to ?20°C revealed that the reaction of the acetylium cation with β-propiolactone leads to the cyclic six-membered oxonium ion 4 , participating as an intermediate in the initial stage of polymerization.     

AB2 Functional Polyesters via Ring Opening Polymerization: Synthesis and Characterization

Aliphatic AB₂ functional polyesters were conveniently prepared by the ring opening polymerization of ε‐caprolactone and L ‐lactide in the presence of the AB₂ functional initiator 2,2‐bis(hydroxymethyl)propionic acid (bis‐MPA) and Sn(Oct)₂ as the catalyst. In L ‐lactide polymerization, both bis‐MPA hydroxyl groups initiated the polymerization reaction, but for ε‐caprolactone polymerization this depended on the monomer to initiator to catalyst ratio. Initiation at two hydroxyl groups occurred at high monomer to initiator ([M]/[I]) ratio and at high Sn(Oct)₂ to monomer ratio. The melting temperatures of the AB₂‐functional PLLA and PCL polymers were comparable to linear polymers with a equal to the per arm in the polymer.

     

Aryltriazene Photopolymers for UV‐Laser Applications: Improved Synthesis and Photodecomposition Study

An improved synthesis of photosensitive homopolymers containing aryltriazene chromophores covalently incorporated into the polymer backbone is reported. Such photopolymers proved to have promising properties for novel UV‐laser applications. A homologous series of new aryltriazene polymers with increasingly branched side chains (R = Me, Et, iPr, tBu) was synthesized and characterized. Homogeneous thin films with thicknesses from ≈15 to >150 nm were prepared by spin‐coating. Photodecomposition was studied in solution and on thin films. Polymers with increasingly branched and bulky substituents showed decreasing photodissociation rates. NMR studies suggested an enhanced hindrance of the N(2)–N(3) bond rotation in the aryltriazene moiety with increasing steric demand of the substituents.