Mechanism of anionic polymerization of (meth)acrylates in the presence of aluminium alkyls, 2. Kinetic investigations with methyl methacrylate in toluene

The kinetics of the polymerization of methyl methacrylate initiated by lithium alkyls (tert-butyllithium or ethyl α-lithiobutyrate) was investigated in the presence of aluminium alkyls (triethylaluminium or triisobutylaluminium) in toluene at −78°C. The rate of polymerization decreases considerably once the living dimer is formed. This suggests that the aluminate end-group coordinates with the penultimate ester group of the polymer chain, thus decreasing reactivity. The results are at variance with an activated monomer mechanism.     

Novel initiating systems for the living polymerization of acrylates and methacrylates

The polymerization of methyl methacrylate with lithiated initiators in the presence of aluminium alkyls in toluene has living character but it deviates from conventional first-order kinetics and the polymers have fairly broad molecular weight distributions. This results from the formation and precipitation of a coordinative polymer network in which the lithium ions of the living chain ends are coordinated to the in-chain ester carbonyl groups. Thus, the network formation can be prevented by adding Lewis bases like methyl pivalate which coordinate to the living chain ends instead of the polymer. Alternatively, one can introduce tetraalkylammonium salts aiming at an exchange of the lithium ion by a less electropositive cation. Both approaches lead to linear first-order time conversion plots for the polymerization of methacrylates and acrylates and to narrow molecular weight distributions, i.e., to a living and controlled polymerization. With these new initiating systems, various block and graft copolymers can be synthesized.     

Thermolytic behaviour of polybutadienyllithium and polystyryllithium in ethylbenzene at high concentrations

The polymerization of butadiene and styrene with lithium and lithium alkyls has been extensively studied. The kinetics and mechanism of the pyrolysis of alkyllithium compounds has also been investigated thoroughly. The mechanism of these elimination reactions is conveniently elucidated by manometric methods, and mechanistic interpretations have been proposed. However, there are less published accounts of the thermal stability of polymer-lithium systems. There is a need to have kinetic data describing the thermal termination of “living” polymers at lithium concentrations approaching those of commercial polymerization systems. This paper will discuss the thermal stabilities of concentrated solutions in ethylbenzene of polystyryllithium and polybutadienyllithium.     

(Chiral) lithium–(magnesium–)zinc and lithium–cobalt combinations as dual reagents for aromatic deproto-metalation and aryl transfer to aldehydes

The deprotonating ability of mixed lithium–zinc or lithium–magnesium–zinc combinations containing amido and alkyl ligands in tetrahydrofuran were compared using anisole as substrate and iodine to quantitatively trap the formed arylmetal species. The results showed that the deprotonating ability is hampered if a Grignard reagent is employed to introduce the alkyl ligand, and is reduced when 2,2,6,6-tetramethylpiperidino ligands are replaced by less hindered/basic chiral amido or alkyls. Concerning the interception of the generated lithium–zinc aryl species by aldehydes, the presence of amido ligands leads to side reactions/lower yields, and no clear improvement was observed if lithium–magnesium–zinc aryl species are used. Racemic mixtures to very low enantioselectivities were noted when chiral amido ligands were incorporated in the composition of the bases.Still with enantioselective aryl transfer to aldehyde as purpose, the deprotonating ability of mixed lithium–cobalt combinations containing amido and alkyl ligands were compared using anisole as substrate and anisaldehyde to trap the formed arylmetal species. As before, the deprotonating ability is reduced when 2,2,6,6-tetramethylpiperidino ligands are replaced by less hindered/basic alkyls or chiral amido. The trapping step using aldehydes being in this case more efficient, even in the presence of amido ligands, the alcohols were obtained in higher yields. With recourse to a lower interception temperature, and using only bis[(R)-1-phenylethyl]amino as ligands, 32 and 22% yield, and 69 and 65% ee were obtained using, respectively, anisaldehyde and 3,4,5-trimethoxybenzaldehyde to intercept the metalated anisole.     

Polymerisation of methyl methacrylate with tert-butyllithium and triisobutylaluminium in toluene: mechanism of initiation and termination by matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI TOF MS)

The polymerisation of methyl methacrylate (MMA) using triisobutylaluminium/tert-butyllithium in hydrocarbon at 0°C and −78°C has been investigated by matrixassisted laser desorption ionisation time-of-flight mass spectrometry (MALDI TOF MS). Although the system gives extremely good control over molecular weight and narrow molecular weight distribution, initiation of polymerization can be complicated by formation of small amounts of tert-butyl isoprenyl ketone ( 1 ), and termination of polymerization can occur by end cyclisation of the end unit of 3 . Both complications in initiation and termination can be avoided by suitable reaction conditions. Incorporation of 1 is suppressed by increasing the alkylaluminium content and by higher temperature. Termination by cyclisation has been found to occur over much longer time scales than propagation and can therefore be avoided by controlled termination after the appropriate reaction time.     

Study of Ziegler-Natta catalysts. Part II. The liquid phase and polymerization activity of the catalyst

The polymerization activity of the Ziegler-Natta catalysts is significantly affected by the aluminium alkyls in the liquid phase. These may be studied provided the solid phase of the catalyst remains unchanged. Part of these aluminum alkyls is adsorbed on the surface of the solid phase. It is possible to displace one type of alkyls by another one or to elute it from the adsorbed layer. This last affects the polymerization rate.     

Living Free Radical Polymerisation Under a Constant Source of Gamma Radiation – An Example of Reversible Addition‐Fragmentation Chain Transfer or Reversible Termination?

The primary mechanism for living polymerisation under a source of gamma radiation at low dose rates is shown to be reversible addition‐fragmentation chain transfer. This was demonstrated by showing that the initial transfer step determines the success of the polymerisation. When an inappropriate leaving group is chosen for the RAFT agent, the polymerisation is non‐living. Under a reversible termination mechanism the ‘living’‐ness should be independent of this initial transfer step.     

Ziegler-Natta 型催化剂NdCl3·3I-PtOH-AlEt3 两组分间的反应及其反应产物对双稀烃聚合的催化活性

The reactions between the two components of the rare-earth coordination catalyst, NdCl3.3i-PrOH and AlEt3, were studied. It was found that the activated solid product was a mixture containing alkylated neodymium compounds, but no alkyl chloride was formed during the reaction. The alkyls bounded to neodymium or to the bridge between neodymium and aluminium atoms are thermally less stable than those bonded to the aluminium atom. Two possible structures of alkylated neodymium compounds were postulated. The catalytic activity in the isoprene polymerization increases with the increase of degree of alkylation and decreases in the presence of alkyl aluminium chloride.     

Reductive cleavage of axially disymmetric tertiary amines and quaternary ammonium salts by lithium aluminium hydride. Synthesis of new 1,1′-binaphthyl substituted amines

Axially disymmetric tertiary amines or quaternary ammonium salts $\text{A}$ are synthesized by double alkylation of primary or secondary amines with racemic or optically pure 2,2′-bis (bromomethyl)-1,1′-binaphthyl. Their reductive cleavage by lithium aluminium hydride in refluxing THF leads to chiral secondary or tertiary amines $\text{B}$, substituted by a binaphthyl unit, with high yields and absence of racemization.     

Mechanism of stereospecific polymerization of propylene with titanium trichloride–aluminum alkyl catalysts

The effect of various aluminum alkyls at varying concentrations on the rate and stereospecificity of propylene polymerizations with titanium trichloride was examined. It was concluded that dialkylaluminum halides were merely chemisorbed on the surface of the titanium trichloride while the trialkylaluminums reacted more extensively with the surface. In the case of diethylaluminum chloride standard chemisorption kinetics were observed. The rate of polymerization was also found to be a function of solvent, with certain aromatic solvents causing significant rate increases. With diethylaluminum chloride essentially no termination occurs; the polymers are “living polymers.”     

Well-controlled reversible addition-fragmentation chain transfer radical polymerisation under ultraviolet radiation at ambient temperature

Controlled reversible addition-fragmentation chain transfer radical polymerisation of methyl acrylate was carried out under long-wave (lambda > or = 365 nm) ultraviolet radiation using an acylphosphine oxide as a photoinitiator at ambient temperature; the polymerisation shows a "living" character at high conversions of polymerisation and leads to well-defined polymers with narrow polydispersities (Mw/Mn < 1.1).     

Poly(vinyl chloride) stabilisation with organo-tin compounds: Part V—Model studies of epoxy—Tin synergism

Organo-tin chlorides are able to catalyse the reaction of epoxy compounds with allylic chlorine atoms. Three main reactions are observed: nucleophilic substitution of the epoxy compound which leads to the formation of a β-chloroether, elimination which gives a diene compound, and cationic polymerisation of the epoxy. Increasing the Lewis acidity of the tin chlorides strongly favours the substitution reaction which is actually a stabilisation reaction for the PVC. On the other hand, the most basic epoxy compounds are very prone to the polymerisation. The details of the kinetics can be explained in terms of a concerted mechanism inside the co-ordination sphere of the tin atom, after initial coordination of the epoxy compound for which there is NMR spectroscopic evidence. Two other side reactions are observed: consumption of the ionic chlorides through the termination reaction of the polymerisation and acidolysis of the ether by excess HCl, but that reaction may take place only when there is an excess of allylic chloride with respect to epoxy, or upon addition of HCl.     

Calorimetric study of photopolymerisation of divinyl monomers

Isothermal differential scanning calorimetry (photocalorimetry) enables real‐time measurements of rates of polymerisation as functions of irradiation time to be made. Since it is possible to stop initiation at any point in a reaction (by cutting off the light) and to monitor the dark reaction, this method has been widely used for determinations of the (apparent) rate constants of polymerisation of multifunctional monomers at various degrees of double bond conversion. The work reported here presents the results of photocalorimetric studies of polymerisation kinetics of six related acrylates and methacrylates. Two main topics are discussed: the effects of sulphide and ether groups present in the monomers on network formation, and the determination of polymerisation rate constants according to three termination models (monomolecular, bimolecular and mixed).     

On Ethylene Polymerisation with Aluminium and Scandium Complexes: DFT and Post‐HF Calculations

Summary: The performance of Density Functional Theory (DFT) methods in predicting ethylene polymerisation and/or oligomerisation activity in selected aluminium and scandium based complexes was studied using both DFT and post‐Hartree‐Fock CCSD(T) calculations. Whereas previous reports have drawn attention to the underestimation of the barrier for the β‐hydrogen termination process for a few aluminium based species, we found that the same holds for the corresponding scandium complexes. New, however, is the observation that apart from underestimating the energy barrier connected to β‐hydrogen termination, the insertion of ethylene is also severely underestimated by the DFT methods applied compared to post‐Hartree‐Fock calculations up to the CCSD(T) level.

Structure of the diketiminate complex referred to in the text.     

Poly(ethylene glycol) as solvent for transition metal mediated living radical polymerisation

The use of poly(ethylene glycol)(PEG) with low molecular weights as a novel solvent for the transition metal mediated radical polymerisation of methyl methacrylate and styrene is reported. The utilisation of PEG leads to polymerisation kinetics that differ from those observed with more traditional organic solvents. Moreover, the amount of residual copper catalyst in the product is greatly reduced by precipitation of the polymer in ethanol.     

Atomistic-scale simulations of the initial chemical events in the thermal initiation of triacetonetriperoxide

To study the initial chemical events related to the detonation of triacetonetriperoxide (TATP), we have performed a series of molecular dynamics (MD) simulations. In these simulations we used the ReaxFF reactive force field, which we have extended to reproduce the quantum mechanics (QM)-derived relative energies of the reactants, products, intermediates, and transition states related to the TATP unimolecular decomposition. We find excellent agreement between the QM-predicted reaction products and those observed from 100 independent ReaxFF unimolecular MD cookoff simulations. Furthermore, the primary reaction products and average initiation temperature observed in these 100 independent unimolecular cookoff simulations match closely with those observed from a TATP condensed-phase cookoff simulation, indicating that unimolecular decomposition dominates the thermal initiation of the TATP condensed phase. Our simulations demonstrate that thermal initiation of condensed-phase TATP is entropy-driven (rather than enthalpy-driven), since the initial reaction (which mainly leads to the formation of acetone, O(2), and several unstable C(3)H(6)O(2) isomers) is almost energy-neutral. The O(2) generated in the initiation steps is subsequently utilized in exothermic secondary reactions, leading finally to formation of water and a wide range of small hydrocarbons, acids, aldehydes, ketones, ethers, and alcohols.     

Micellar polymerization: Computer simulations by dissipative particle dynamics

Nowadays, micellar polymerization is widely used in different fields of industry and research, including modern living polymerization technique. However, this process has many variables and there is no comprehensive model to describe all features. This research presents simulation methodology which describes key properties of such reactions to take a guide through a variety of their modifications. Dissipative particle dynamics is used in addition to Monte Carlo scheme to simulate initiation, propagation, and termination events. Influence of initiation probability and different termination processes on final conversion and molecular‐weight distribution are presented. We demonstrate that prolonged initiation leads to increasing in polymer average molecular weight, and surface termination events play major role in conversion limitation, in comparison with recombination. © 2018 Wiley Periodicals, Inc.     

The mechanism of polymerisation of butyl cyanoacrylate in aqueous dispersions

Butyl cyanoacrylate has been polymerised by dispersion in acidified water to yield nanoparticles. Reactions have been carried out at various temperatures and the molecular weight of the growing polymer measured. In all cases polymerisation proceeds by generation of discrete populations of oligomers, which are then re‐initiated and undergo further growth. The peak molecular weight is around 1300 g/mol and it is considered that a stepwise initiation, termination and re‐initiation process continues until diffusionally limited by the internal viscosity within the nanoparticle.     

Electropolymerisation of methyl methacrylate on carbon fibre surfaces

The electroinitiated polymerisation of methyl methacrylate containing sulphuric acid was found to be localised at the surface of a cathode of carbon fibre. The influences of various reaction parameters on the rate of polymerisation and the molecular weight have been recorded but do not agree totally with classical free radical electropolymerisations. The behaviour of molecular weight and evidence of post-polymerisation suggest the existence of living polymer radicals. It is argued that atomic hydrogen is responsible for initiation of the polymerisation.     

Comparison of oil-soluble and water-soluble initiation of styrene polymerization in a three-component microemulsion

The polymerization of styrene in three-component oil-in-water microemulsions made with the cationic surfactant dodecyltrimethylammonium bromide is studied by dilatometry and quasielastic light scattering as a function of type and concentration of initiator. Fast polymerization rates, high conversions, and high molecular weight polymers are achieved with both oil-soluble (AIBN) and water-soluble (potassium persulfate) initiators. The rate of polymerization shows initiation and termination intervals, but no constant-rate interval is observed. Stable monodisperse microlatexes are obtained with both types of initiators. For both AIBN and potassium persulfate, polystyrene molecular weight is proportional to initiator concentration [I]^–0.4 and particle radii decrease as [I]^–0.2. Polymerization initiation occurs in or at the microemulsion droplets, and polymer particles grow by recruiting monomer and surfactant from uninitiated swollen micelles.