Retarded Anionic Polymerization, 5. Influence of the Structure of Dialkylmagnesium Additives on the Reactivity of Polystyryllithium Species

The influence of the structure of various dialkylmagnesium additives on the retardation of styrene polymerization, initiated by alkyllithium in cyclohexane, was investigated. The effect of n,sec‐dibutylmagnesium, di‐sec‐butylmagnesium, butyloctylmagnesium and dibenzylmagnesium, in ratios ranging from 0–20 with respect to alkyllithium, was studied in detail. The presence of the magnesium derivatives leads to a drastic reduction of the overall reactivity depending on the type, concentration and proportion of dialkylmagnesium used. This was related to the formation of mixed complexes as shown by UV‐Visible spectroscopy. The spectral changes were interpreted by the formation of several types of “ate” complexes of different stoichiometry and structure. Experimental polystyrene molar masses and MALDI‐TOF measurements show a different degree of participation of magnesium alkyl groups as chain initiator (or reversible transfer agent) of styrene polymerization: secondary alkyls are the most efficient ones, whereas primary alkyls (or aryl) yield only a slow initiation (or transfer) and can be considered as almost ineffective.     

Lithium alkylnickelate and alkylpalladate bimetallic “ate” complexes as initiators for anionic polymerization of methyl methacrylate

Anionic-coordinative polymerization of methyl methacrylate (MMA) was carried out in the presence of new bimetallic initiators based on lithium alkylnickelates and alkylpalladates in tetrahydrofuran, at ?78°C. Well-defined poly(methyl methacrylate), PMMA, with molecular weights increasing linearly with conversion and relatively narrow polydispersities (M?_w/M?_n = 1,26 to 1,4) were prepared by initiation with some of the “ate” complexes. In the presence of lithium “ate” Ni-based complexes, PMMA with enhanced syndiotacticity [(rr) = 73 to 75%] was formed.     

Effect of N,N,N′,N′-tetramethylethylenediamine on the reaction parameters of Ziegler-Natta macromolecular complexes in the polymerization of ethylene

Soluble complexes of ω-lithiumbutenylpoly(styrene-co-butadiene) (≈ 100: ≈ 3 DP) with TiCl₄ were used in conjunction with N,N,N′,N′-tetramethylethylenediamine (TMEDA), to initiate the polymerization of ethylene. The activity of such a system was measured as a function of the number of amino groups (N) per active center (A. C.) ([N]/[A. C.]). From measurements of the amount of polystyrene-block-polyethylene produced and from kinetic studies of the polymerization of ethylene, the influence of TMEDA on both the efficiency of the system and the reactivity of the active species was studied. The depressor effect of the diamine on the reaction parameters, controlling the polymerization of ethylene, was connected with the proposed structure of the active sites. The role of the Lewis base was discussed by assuming a reversible blockage of a vacant coordination site at the titanium throughout the complexation of the electrophilic “host sites” of the polymer [TiCl₂, TiCl₃, 3 LiCl] complexes.     

Mehrfache stationäre Zustände und periodische Teilchenbildung bei der Emulsionspolymerisation von Styrol im kontinuierlichen Rührkesselreaktor

It was found that at low residence time τ there is only one solution of the mass balance equation and only one steady state of the isothermal continuous stirred tank reactor applied in this work. Increasing τ leads to a region with three solutions corresponding to two stable and one unstable steady states. The “upper” stable steady state at high conversion becomes less sensitive against disturbances with increasing distance from the labile steady state. With the reactor in the “upper” steady state (conversion x ≈ 95%) periodic cycles of surface tension σ and particle number N are observed, polymerization rate R_P and x remaining constant. In periods without particle formation N decreases with time t according to N = N₀·e^?t/τ. x ≈ 95% causes diffusion control of the termination reaction, the number of radicals per latex particle being n? ≈ 4 even in small particles. Thus, the normal subdivision effect of emulsion polymerization is eliminated, kinetics correspond to homogeneous polymerization. Therefore, n? increases linear with particle volume v and R_P remains constant and independent of N. Very big latex particles with diameter D? = 1,6.10^?4 cm and n? = 35000 are observed shortly before particle formation starts again. The limit cycles found in this work have been explained on a qualitative basis, a quantitative treatment is still lacking.     

Synthesis of alkoxysilyl-terminated polyisoprenes by means of “living” anionic polymerization, 1. Modeling of the termination step by studying the reaction of butyllithium with various alkoxysilane reagents

In order to optimize the preparation of trimethoxysilyl- or triethoxysilyl-terminated 1,4-polyisoprene and to facilitate their characterization, the termination reaction of the anionic polymerization of isoprene was investigated by means of a model reaction. Butyllithium (n-Buli) was used as a model molecule of the “living” ω-carbanionic polymer chain, and its reaction with various alkoxysilyl reagents (3-chloropropyltrimethoxysilane, tetramethoxysilane and tetraethoxysilane) was investigated. Studies were carried out by varying the molar ratio r = [n-BuLi]/[alkoxysilane] (0.5 ≤ r ≤ 3). The corresponding rate of substitution depends on r and on the nature of the coupling reagent. It was shown that alkoxy groups at the silicon center are easily n-Bu-substituted. Whatever, r, higher n-Bu-substituted derivatives are always simultaneously formed with the n-Bu-monosubstituted compound. With tetraalkoxysilane reagents, the formation of butyltrialkoxysilane is always favoured, compared to that of the di- and trisubstituted homologs. This was interpreted in terms of reactivity differences existing between the reacting alkoxysilanes present in the mixture. Attempts realized with 3-chloropropyltrimethoxysilane in order to obtain selective formation of alkyltrimethoxysilane derivative showed that chlorine substitution was impossible because nucleophilic attack of butyl carbanion occurs exclusively on the silicon atom.     

Synthesis of End‐Functionalized Star‐Shaped Poly(methylthiirane)s

Star‐shaped poly(methylthiirane)s were prepared by ring‐opening polymerization of methylthiirane with thiolate ions as initiators. As expected, the “arm first method” using 1,2,4,5‐tetrakis(bromomethyl) benzene as a linking agent gave mixtures of tetra‐, (tri) and di‐armed polymers. The “core first method” with tri‐ and tetra‐thiols as core components seemed to be a better method and gave polymers of different molar masses, varying from 800 to 60 000 g·mol^–1 with narrow polymolecularity indices; but the presence of macromolecular disulfides – formed by oxidation of the thiolate ions – broaden the molar distribution. These disulfides could be reduced with lithium aluminium hydride. The star polymers could be end‐capped with various functional groups through reaction of the thiolate end groups, either at the end of the polymerization or after acid regeneration from the thiol polymers.     

Solution properties of poly(octadecyl methacrylate) in butyl acetate, 2. Semidilute solution

Semidilute solutions of a fraction of poly(octadecyl methacrylate) (PODMA) obtained by radical-initiated polymerization were investigated in the theta-solvent butyl acetate (BuAc) with static and dynamic light scattering (SLS, DLS, resp.) at four different temperatures between 13°C (near theta-temperature Θ) and 70°C. The molar mass of the investigated sample (PODMA 4) was 3,2 · 10⁶ g · mol^?1. A concentration region from dilute to 8 c^* was covered, where c^* = 1/[η] was taken as the coil overlap concentration, [η] denoting intrinsic viscosity. Above c = 3c^* the DLS correlation function exhibited bimodal behaviour at all temperatures. At these concentrations one finds two “apparent” molar masses, radii of gyration and diffusion coefficients for each concentration. The smaller apparent molar masses, smaller radii of gyration and larger diffusion coefficients correspond to characteristics of the transient network. The larger apparent molar masses, larger radii of gyration and smaller diffusion coefficients may be attributed to the formation of inhomogeneities in the semidilute solution.     

Controlled Synthesis of “Reverse Pluronic”‐Type Block Copolyethers with High Molar Masses for the Preparation of Hydrogels with Improved Mechanical Properties

Hydrogels based on Pluronics (EO_n/2‐PO_m‐EO_n/2, EO = ethylene oxide, PO = propylene oxide) have been frequently investigated, yet key limitations still remain, including a propensity for quick erosion and insufficient mechanical robustness. This issue can be alleviated by creating “reverse Pluronics” (PO_n/2‐EO_m‐PO_n/2), which is proposed to enable the formation of physical cross‐links via a micellar network. Until recently, however, efforts in this direction were aggravated by synthetic difficulties, specifically prohibiting the realization of poly(propylene oxide) (PPO)‐moieties with a high DP. In this study, an organocatalytic polymerization method is employed to synthesize “reverse Pluronics,” resulting in highly defined polymers (Ð_M ≤ 1.02–1.07, M_n up to 35 000 g mol^?1) with exceptionally long PPO blocks. The higher molar mass and the reverse constitution of the polyether combine to enable the generation of thermoresponsive hydrogels with a storage modulus that is increased tenfold relative to reference samples. Gelation temperature and maximum storage modulus (G′_max) are readily influenced by the choice of the polyether (down to 5 wt%). The improved mechanical properties are accompanied by an increased resistance toward erosion in water. Isotactic enrichment is presented as an additional tuning parameter for hydrogel properties.     

Der einfluß aromatischer azoverbindungen auf die polymerisation von vinylacetat,vinylchlorid und methylmethacrylat

The effect of aromatic azo compounds on the polymerization of vinyl acetate (VA), vinyl chloride (VC), and methyl methacrylate (MMA) was investigated with respect to initiation by di-(3-benzeneazobenzoyl)-peroxide (mABPO) as well as the effect of added aromatic azo compounds on the polymerization initiated by dibenzoylperoxide (BPO). With VA polymer formation occurred in neither case, so that aromatic azo compounds are considered to be inhibitors in the polymerization of this monomer. With VC and mABPO only negligible polymer yields were obtained in comparison to corresponding concentrations of BPO; addition of azobenzene to the BPO-initiated polymerization led to an extensive suppression of polymerization. With MMA only a weak retardation was found in similar experiments and the rates of polymerization obtained with mABPO were of the same order of magnitude as those with BPO-initiation. Owing to the low extent of side reactions occurring with aromatic azogroups during the polymerization of MMA the basic requirements for a “colorimetric” molecular weight determination are met to a high degree; the complicated character of chain termination, however, also in this case prevents a simple “integer number” relationship between degree of polymerization and the number of azo units incorporated in mABPO-prepared polymethylmethacrylates. Nevertheless the determination of azo endgroups in these polymers might be a valuable tool for the evaluation of further kinetic data.     

On the reactivity of styryl-terminated polystyrene macromonomers in anionic copolymerization with butadiene

Styryl-terminated polystyrene macromonomers (MA) with molar masses ranging from M?_n = 1 600 to 38 000 g/mol were copolymerized with butadiene (B) by application of the initiator system butyllithium/sodium tert-butoxide in cyclohexane. The temporal insertion of both monomers into the growing graft polymer chain was observed by means of size-exclusion chromatography and gravimetrical analysis. The values of 1/r_B = k_BMA/k_BB (k_BMA, k_BB: rate constants of MA and B insertion, respectively) show that the reactivity of the macromonomers is clearly influenced by their molar mass, the conversion of butadiene, and thus by the obtained molar mass of the growing graft polymer as well as by the mole ratio of the macromonomer and comonomer in the reaction mixture. An almost “ideal” insertion was observed for macromonomers with molar masses of up to about M?_n = 5000 g/mol and a conversion of about 50%. An increase in the values of molar mass, conversion and concentration of the macromonomers has the effect of lowering 1/r_B.     

Mechanisms and kinetics of the anionic polymerization of acrylates, 1. Oligomerization of tert-butyl acrylate and characterization of products

The anionic polymerization of tert-butyl acylate (tBuA) initiated by tert-butyl α-lithioisobutyrate was investigated in THF at 25°C. The individual oligomers were isolated and characterized by UV, IR and NMR spectroscopy. The distribution of products was determined in the range of reaction times 0,02 s ≤ t ≤ 1800 s. The oligomerization is extremely fast. Even after 0,02 s at -30°C, no residual monomer could be detected. After short reaction times the reaction mixture almost exclusively consists of linear oligomers of tBuA. This indicates that there is no termination reaction by Claisen condensation during polymerization. Nevertheless, the molecular weight distribution is rather broad (M?_w/M?_n = 2,1₅). The very large amount of dimer observed is an indication of its low reactivity. A two-state mechanism is proposed to account for the high polydispersity. Only after longer reaction times side products are formed by Claisen condensation (“back-bitting”). The termination product of the trimer is an enolized cyclic β-ketoester.     

How to Fight Kinetics in Complex Radical Polymerization Processes: Theoretical Case Study of Poly(divinyl ether‐alt‐maleic anhydride)

Products of free‐radical polymerization (FRP) are usually not regulated on the molecular scale, consisting of blocks obtained through the fastest kinetic scheme pathways. The side or kinetically restricted products can be a source of impurities in a complex FRP case, or possess new properties if isolated solely. FRP synthesis of poly(divinyl ether‐alt‐maleic anhydride), known as “DIVEMA”, serves as a polymerization example with such kinetic and thermodynamic complexities. Uncertainty in factors regulating polymer structure is a challenge in advancement “DIVEMA” derivatives toward medical practice. In‐depth investigation via quantum‐chemical and molecular mechanics methods unveils mechanistic aspects of polymer stereoisomerism and confirms possible isolation of thermodynamically or kinetically controlled products on a large data set. Strategies toward regulation of 5‐exo/6‐endo cycloisomerism are theorized and then studied via microkinetic modeling. Thermodynamically controlled products can be isolated utilizing lower monomer concentrations, in range of 10^?3 to 10^?1 m , and/or application of a complexing agent that is better to realize via solvents, capable of formation π‐ and σ‐radical complexes. Change of electrophilic monomer is proposed as an approach for designing more molecularscale adjustable copolymerization processes. Methodology, obtained results, and conclusions for “DIVEMA” can be valuable to control other FRP processes on the molecular scale, unlocking polymers with improved or new functionalities.     

Kinetic studies of the anionic polymerization of methyl methacrylate in tetrahydrofuran with Na+ as counter ion,using monofunctional initiators

A newly designed automatically controlled stirred reactor suitable for kinetic measurements of reactions with half lives ≥2s has been applied to follow the anionic polymerization of methyl methacrylate in THF with Na⁺ as a counter ion in the presence of an excess of NaB(C₆H₅)₄. As initiators were used: benzylsodium reacted with α-methylstyrene (I), fluorenylsodium (II), and 9-methylfluorenylsodium (III). With I the initiation is fast as compared with the polymerization reaction which is first order in monomer concentration. Within the range of ?50°C to ?100°C an almost unperturbed “living” polymerization is observed. The Arrhenius plot of the rate constants results in a straight line with activation energy E_a = 4,4kcal·mol^?1 (= 18kJ·mol^?1) and frequency exponent A = 7,0.II and III are slow initiators, II giving rise to side reactions because of the “acidic” proton in 9-position after initiation, III exhibiting a rate constant of initiation k_i = 1l·mol^?1·s^?1 at ?72°C. The termination reaction is becoming increasingly important with increasing temperature and seems to be a unimolecular reaction with E_a,t = 11,5kcal·mol^?1 (= 48 kJ·mol^?1) and A_t = 10. Since the basic feature of the reactor is the possibility of drawing samples, polymers from each state of the reaction were available to be investigated also with respect to their tacticity. The monomer addition was shown to follow Bernoullian statistics. A structure of the “living” end being in harmony with the results observed is discussed.     

Initiation of polymerization with substituted ethanes, 13. Free radical polymerization of methyl methacrylate and styrene with substituted succinonitriles

Tetraarylsuccinonitriles 2 were synthesized via oxidative dimerization of diarylacetonitriles 1 in basic media. The thermal decomposition of 2 results in two identical diarylcyanomethyl semi-radicals 3 . The dissociation energy (E_a) measured using the thiophenol radical scavenging technique was found to amount to 94,3 ± 9,2 kJ/mol in the case of tetraphenylsuccinonitrile 2a . Tetraarylsuccinonitriles 2 show an atypical “stepwise” initiation mechanism in free-radical polymerization, particularly in the case of methyl methacrylate (MMA) as monomer. In the initial phase of the polymerization reaction a very high concentration of diarylcyanomethyl primary radicals 3 leads to the formation of short-chain telechelics with both end groups originating from the initiator. In the further course of the polymerization these MMA telechelics are able to re-form radicals by the scission of thermolabile carbon-carbon bonds and by the release of initiator end groups, and so the MMA telechelics ‘re-initiate’ the free-radical polymerization (“resuscitable free-radical polymerization”). In styrene polymerization, tetraarylsuccinonitriles 2 indeed cause a pronounced primary radical termination effect, but the styrene telechelics formed in contrast to MMA telechelics are thermostable, and therefore no “re-initiation” effect occurs. The X-ray structure determination of the styrene telechelic 2,2,3,5,5-pentaphenyladiponitrile 4a exhibits a bond length of the thermolabile carbon-carbon bond of 158,7 pm as compared to 162,8 pm of the corresponding MMA telechelic 2,2,5,5-tetraphenyl-3-methoxycarbonyl-3-methyladiponitrile and emphasizes the higher thermal stability of 4 .     

Molecular weight distribution studies, 2. Radiation-induced polymerization of solid α-methylstyrene

The radiation-induced post - and “in-source” solid state polymerizations (the polymerizations occurred after and during the γ-irradiation, resp.) of α-methylstyrene were studied. Post-polymerizations studied at ?30°C, occurred only in pure and “dry” monomer. At a constant initial radiation dose of 2,5 Mrad (2,5.10⁴J/kg), the rate of polymerization increased, although slightly, with increasing conversion. With increasing initial radiation dose of up to 7,4 Mrad (7,4.10⁴J/kg) and post-polymerized for 8 h, resp., the percentage of polymer yield per unit radiation dose decreased with increasing conversion. The GPC chromatograms of the polymer samples obtained showed a peak at 27,4 elution counts resulting from ionic polymerization and a shoulder at 29,0 counts from free radical polymerization. In the case of the “in-source” polymerization, studied at ?30 and ?40°C, the “wet” monomer yielded rates of polymerization lower than those from “dry” monomer. GPC analysis of the polymer samples obtained at ?40°C from both “dry” and “wet” monomer showed bimodal distributions, the higher molecular weight peak resulting from ionic polymerization. At increased conversions, the positions of both GPC peaks remained unchanged, whereas the heights and hence the ionic and free radical polymerization contribution changed, the ionic contribution decreasing. At ?30°C polymerization temperature only one peak at 27,3 elution counts resulting from ionic polymerization was obtained.     

Saccharide polymers, 2. Synthesis and polymerization of exo-glucal derivatives

Unsaturated glucopyranose derivatives such as 1,2,3,4-tetra-O-acetyl-6-desoxy-β-D -xylo-hex-5-enopyranose (3) and 1,2,3,4-tetra-O-benzoyl-6-desoxy-β-D -xylo-hex-5-enopyranose (6) , briefly called “Ac-exo-glucal (3) ” and “Bz-exo-glucal (6) ”, were synthesized. These exo-cyclic sugar vinyl ethers were investigated in polymerization reactions. The corresponding “saccharide polymers”, homo- and copolymers, were synthesized under free radical conditions. The structures and the compositions of the soluble “saccharide polymers” were established by elemental analysis, ¹H and ¹³C NMR, and FTIR spectroscopy. Characterization of the polymers, like molecular weights and optical rotations are reported. Saccharide polymers with different sugar content and low as well as high molecular weights were obtained.     

Influence of various proton traps on the bifunctional cationic polymerization of chloroethyl vinyl ether mediated by α-iodo ether/zinc dichloride

The synthesis of relatively high molar mass telechelic poly(chloroethyl vinyl ether)s (PCEVE) via bifunctional initiation of the “living” cationic polymerization of CEVE was examined in the absence and in the presence of various proton traps. In the absence of a proton trap, a monofunctional initiation involving hydrogen iodide (HI), formed by partial hydrolysis of trimethylsilyl iodide (TMSI) by adventitious water, occurred concurrently to the main bifunctional process. The addition of sterically hindered aromatic amines leads to complete inhibition of the polymerization due to the formation of a complex between the aromatic amines and ZnCl₂. In the presence of trialkylaluminium as HI trapping agent, monofunctional initiation is totally suppressed and a clean bifunctional polymerization takes place. Telechelic PCEVE of DP _n up to 300 with narrow molar mass distribution were synthesized. However, in the presence of these additives, a slow ligand exchange between AIR₃ and ZnCl₂ takes place, decreasing the rate of polymerization. For polymers with high DP _n (>300), the increasing amount of trialkylaluminium required to totally suppress the formation of monofunctional polymer results in a chain coupling process during the deactivation step. This reaction limits the clean synthesis of high molar mass bifunctional PCEVE.     

Free radical polymerization of a sugar residue-carrying styryl monomer with a lipophilic alkoxyamine initiator: synthesis of a well-defined novel glycolipid

Controlled free radical polymerization of N-p-vinylbenzyl-2,3,5,6-tetra-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-β-D -galactopyranosyl)-D -gluconamide (Ac-VLA) was achieved by the nitroxide-mediated free radical polymerization with a lipophilic alkoxyamine “initiator” with a dioctadecyl group in 1,2-dichloroethane at 90°C. The polymerization proceeds in a “living” fashion, providing Ac-VLA polymers with low polydispersity. The hydrolysis of the polymers results in well-defined glycopolymer-carrying amphiphiles, viz., artificial “glycolipids”.     

Nitroxide-controlled free radical polymerization of p-tert-butoxystyrene. Kinetics and applications

The free radical polymerization of p-tert-butoxystyrene (BOS) in the bulk at 125°C “initiated” with the adduct of 2-benzoyloxy-1-phenylethyl and TEMPO (BS-TEMPO) was found to proceed in a “living” fashion, providing low-polydispersity PBOS and block copolymers of the type PBOS-b-PS, where TEMPO is 2,2,6,6-tetramethylpiperidin-1-oxyl and PS is polystyrene. The acidolysis of the polymers gave well-defined poly(p-vinylphenol) (PVP) polymers. The PVP-b-PS block copolymers were observed to exhibit lamellar and cylindrical microdomain morphologies depending on the copolymer composition. The rate of BOS polymerization is independent of the “initiator” (BS-TEMPO) concentration, being essentially equal to the rate of thermal polymerization. The polydispersity of PBOS depends on polymerization time but not on the degree of polymerization, under the studied experimental conditions.     

Kinetics of irreversible,stoichiometric polycondensations

Equations are derived for the time dependence of the extent of functional group reaction, p_A, for catalyzed and uncatalyzed, irreversible, stoichiometric polycondensations of the AB and AA/BB type following pseudo-second-order and third-order chemical kinetics, resp., based on amount-of-substance concentrations. The resulting functions 1/(1 – p_A) - f(t) for pseudo-second-order polycondensations and 1/(1 – p_A)² - f(t) for third-order polycondensations are predicted to be non-linear for low extents of reaction and practically linear for higher ones which agree with previous, hitherto unexplained experimental observations. The slope of the linear part does not give the true rate constant, contrary to traditional assumptions. The extent of non-linearity at small conversions and the deviation of the apparent rate constant from the true one depends on the initial amount of A groups, the molar mass of the leaving molecules, and the initial mass of the reaction mixture, if the molar concentrations are based on masses (“molalities”). The density of the leaving molecules enters, in addition, if the molar concentrations are volume related (“molarities”). The equations predict, for a given system, that the range of the non-linear region increases with decreasing rate constants, i.e., with decreasing temperature, in accordance with experimental observations.