Epoxy prepolymers cured with boron trifluoride-amine complexes, 3. Polymerization mechanism of a prepolymer containing epoxy groups

The reaction of a diepoxy prepolymer ( 2 ) initiated by the BF₃-complex of 4-chloroaniline ( 3 ), solubilized in poly(ethylene oxide) (PEO) of molecular weight 300, was studied. An initial induction period was observed, followed by a pseudo-stationary stage with decreasing reaction rate due to gelation. In the pseudo-stationary stage, the homopolymerization rate is given by the expression: R_p = k[ 2 ]¹ · [ 3 ]^·?0,55 · [PEO]^·0,57. In addition to 2 , we also used a Novolac prepolymer 4 . Different networks were prepared and analyzed by thermal and mechanical techniques, and their water-absorption rates were measured.     

Epoxy prepolymers cured with boron trifluoride-amine complexes, 1. Influence of the amine on the curing window

Different pure complexes were synthesized by an interchange reaction of BF₃-etherate with amines in dichloromethane. All of the resulting complexes are solid, but they can be dissolved in a low-molecular-weight poly(ethylene oxide) to favour their use as initiators. DSC analysis demonstrated strong interactions between the BF₃-complex and the polyether. This is probably caused by the formation of hydrogen bonding between the hydrogens of the amine and the ether groups. The reactivity of amine complexes was characterized by reaction with the epoxy prepolymer. The reactivity increases with decreasing basicity of the amine, showing a relationship between the initiation and propagation of the expoxy homopolymerization and the stability of the complex. A slight influence on the reaction temperatures by the amount of polyether is observed, but the glass transition temperature of the epoxy network decreases when a large excess is used.     

Polylactones 21. Polymerization of L,L-lactide and rac-D,L-lactide with hematin and related porphyrin complexes

The polymerization of L,L-lactide was conducted in bulk at 120, 150 and 180°C. Hematin ( 1a ), hemin ( 1b ), 5,10,15,20-tetraphenylporphiniron(III) chloride, 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphiniron(III) chloride ( 3 ) and chlorophyllin ( 4a ) were used as initiators. Moderate yields and moderate molecular weights up to 15000 were obtained. Gradual racemization was observed with increasing reaction time and temperature. The results obtained with chlorophyllin and benzylated chlorophyllin suggest an anionic polymerization mechanism for the former initiator. When rac-D ,L -lactide was polymerized with hematin in bulk at 120, 150 and 180°C, yields up to 74% and molecular weights up to 10⁴ were found. The polymerization of meso-D ,L -lactide gave lower yields and molecular weights. Both ¹H- and ¹³C NMR spectroscopy indicate the formation of nearly random stereosequences in all polymerization of rac- and meso-D ,L -lactide.     

Polymerization of 2-diethylamino-1,3,2-dioxaphosphorinane, 2. Kinetics

Kinetic studies of the anionic polymerization of 2-diethylamino-1,3,2-dioxaphosphorinane were performed in THF solution with (CH₃)₃SiO^?K⁺ as initiator at temperatures close to r.t. Initiation involves nucleophilic attack of the anion on P atom in the monomer molecule. Breaking of the P? O bond leads to an alcoholate anion as the growing species. Polymerization was shown to proceed via macroion-pairs and to be nearly living; e.g. at r.t. for every 250 propagations there is one termination. Rate constant of propagation k = 3,4 ± 0,31·mol^?1·s^?1 at 25°C, ΔH = 13,3 kcal·mol^?1 and ΔS = ?32,2 cal·mol^?1·K^?1. The ratio k/k was determined by solving a kinetic scheme involving propagation and termination. It was shown that termination consists in the alcoholate anion attack on P in either polymer or monomer molecule with expulsion of (C₂H₅)₂N^? anion and formation of a P? O bond. The dialkylamide anions cannot reinitiate polymerization. In solving the kinetic scheme it was assumed that termination involving both polymer and monomer proceeds with rate constants equal to each other.     

Polymerization and light-induced heat of dental composites cured with LED and halogen technology

Most commercial light curing units (LCUs) for dental applications use conventional halogen bulbs. Commercial LCUs using light emitting diodes (LEDs) have recently become established on the market, even though some aspects of their performance have not been fully investigated. Temperature rise of dental composites during the light-induced polymerization is considered to be a potential hazard for the pulp of the tooth. This study, therefore, investigated the temperature rise in three different composites (Z100, Durafill, Solitaire2) in two shades (A2, A4) polymerized for 40s with two LED LCUs (Freelight, custom-made LED LCU prototype) and two halogen LCUs (Trilight, Translux). The Trilight was used in the standard and soft-start mode. The temperature rise within the composites were recorded for 60s with a thermocouple and also observed with a high-resolution infrared (HRIR) camera. The factors LCU (p < 0.0001), composite (p < 0.0001) and shade (p = 0.0014) had statistically significant influences on the temperature rise. All composites cured with the halogen LCUs reached at a depth of 2 mm, a statistically significant higher temperature (p < 0.0001) than those cured with the LED LCUs. Only one composite showed a statistically significant lower temperature rise for the halogen LCUs at the 95% confidence level, when the soft-start mode was used instead of the standard mode. In general, the composites with the lighter shade (A2) reached higher temperatures than the darker shade (A4), if the LED LCUs were used. When the halogen LCUs were used, the situation was reversed, the composites with the darker shade (A4) reaching higher temperatures than the lighter shade (A2). This study showed that a HRIR camera represents a powerful tool for the observation of temperature propagation on small samples. This study also showed that LED LCUs represent a viable alternative to halogen LCUs for the light polymerization of dental composites because of a generally lower temperature increase within the composite.     

Polymerization of lactams, 53. Anionic polymerization of 2-pyrrolidone accelerated with CO2, part 2

The anionic polymerization of 2-pyrrolidone ( 3 ) initiated with its potassium salt ( 1 ) (2-oxo-1-pyrrolidinylpotassium) and carbon dioxide was studied under optimal conditions. On the basis of a detailed experimental analysis, it is suggested to consider the investigated polymerization in the presence of gaseous CO₂ as an accelerated non-activated polymerization. Alkali 2-oxo-1-pyrrolidine carboxylate, which is formed in the system by reaction of CO₂ with 1 , probably does not take part directly in the polymerization process and excerts only a controlling role.     

Polymerization of tetrahydrofuran, 2. Equilibrium monomer concentration

The equilibrium polymerization of tetrahydrofuran was studied at relatively low initial monomer concentrations with 1-methyltetrahydrofurylium hexachloroantimonate as initiator. The equilibrium concentrations of monomer, [M]_e, and initiator, [I]_e, as well as the equilibrium number-average degree of polymerization, P _n, were determined by means of ¹H NMR spectroscopy, and found to follow simplified relations, which were derived from Tobolsky's equations.     

Polymerization of 1-alkynes initiated by transition metal carbene complexes, 2. Kinetics and mechanism of the initiation reaction

A kinetic study of the initiation reaction of alkyne polymerization by a transition metal intramolecular coordinated carbene complex, was carried out. It showed that there is a slow disappearance of the initial carbene and simultaneously appearance of both active species and organic side-products. Their structures were determined by GC-MS. The apparent rate constants of both initiation and side-reactions were measured and the corresponding thermodynamic parameters established. According to the experimental results, a mechanism of the initiation reaction is proposed. In the first step a replacement of the intramolecular double bond of the initiator by the triple bond of a monomer molecule takes place and in the second step the insertion of the monomer. The resulting carbenic active species may lead either to organic products via cyclopropanation reaction or to a high-molecular-weight polymer.     

Polymerization of lactams, 54. Anionic polymerization of 2-pyrrolidone accelerated with CO2, part 3

Intrinsic viscosities in m-cresol and weight average molecular weights, M?_w, were measured for samples of high molecular weight poly(2-pyrrolidone) (poly ( 1 )) prepared by anionic polymerization of 2-pyrrolidone ( 1 ) accelerated with CO₂. It was proved that the earlier found relationship [η] = 4 · 10^?2 · M^0,77 (in cm³ · g^?1) holds for M?M_w up to 8 · 10⁵ g. mol^?1. The probable reason for the formation of poly ( 1 ) with an exceptionally high molecular weight is discussed.     

Polymerization of lactams, 52. Anionic polymerization of 2-pyrrolidone accelerated with CO2, part 1

The polymerization of 2-pyrrolidone ( 1 ) initiated with its potassium salt (2-oxo-1-pyrrolidinylpotassium) ( 2 ) and accelerated by the presence of CO₂, was investigated. For the optimal concentration of 2 (≈5 mol-%) and 0,5 mol-% CO₂, the most suitable polymerization temperature was found to the range between 45 and 55°C. The variation of CO₂ concentration, 0,5 < [CO₂] < 3 mol-%, does not affect the polymerization yield under the optimal conditions. The similarity of the course of polymerization in the presence of CO₂ directly introduced into the reaction mixture with that in the presence of isolated carboxylate of 1 , is considered as a proof that CO₂ is transformed into the active potassium carboxylate of 1 . The order of polymerization reaction with respect to initiator was found to be ≈0,8 indicating the complexity of the reaction mechanism.     

Polymerization by [3 + 2]-cycloaddition, 5. Telechelics with isocyanate end-groups

The “criss-cross” cycloaddition reaction between derivatives of benzaldehyde azine and several diisocyanates is used for the synthesis of polymers with an uncommon structure element: the 1,3,5,7-tetrazabicyclo[3.3.0]octane-2,6-dione. The nature of the diisocyanate determines the reactivity in this cycloaddition polycondensation and the solubility of the new polymers. A degree of polymerization up to P _n = 72 could be achieved. Due to the nature of the polycondensation reaction, the polymers have two isocyanate end-groups, independent of the stoichiometry of the monomer mixture at the beginning of the reaction. Thus, a prepolymer was synthesized and the chain extension by reaction of its isocyanate end-groups with diamines was studied in some detail.     

Living Anionic Polymerization of Functionalized Monomers, 2. Anionic Polymerization of p‐Alkenylstyrene Derivatives

A study has been carried out on the anionic polymerization of eight p‐alkenylstyrene derivatives. The monomers used are p‐(2‐propenyl)styrene ( 1 ), p‐(2‐propenyl)‐α‐methylstyrene ( 2 ), p‐(2‐butenyl)styrene ( 3 ), p‐(2‐butenyl)‐α‐methylstyrene ( 4 ), p‐(2‐methyl‐2‐propenyl)styrene ( 5 ), p‐(2‐methyl‐2‐propenyl)‐α‐methylstyrene ( 6 ), 8‐(p‐vinylphenyl)‐1‐octene ( 7 ), and 2‐methyl‐7‐(p‐vinylphenyl)‐1‐heptene ( 8 ). The anionic polymerizations of 1 – 4 were accompanied by competitive side reactions presumably due to the proton abstraction from their allyl groups. Furthermore, significant degrees of the propagating chain‐end carbanions derived from 1 , 3 , and 4 were deactivated in THF at –78°C after 1–2 h. In contrast, the polymerizations of 5 – 8 proceed in a living manner in THF at –78°C to afford polymers with predictable molecular weights and narrow molecular weight distributions. Novel well‐defined block copolymers of 5 – 8 with styrene or 2‐hydroxyethyl methacrylate were successfully prepared.     

Polymerization of lactams, 80. Anionic polymerization of lactams accelerated with N-iminolactams, 1 2-pyrrolidone

The anionic polymerization of 2-pyrrolidone (PD) was initiated with the potassium salt of PD at 30–50°C and accelerated with the following N-iminolactams (NIL): 1-(1-pyrrolin-2-yl)-2-pyrrolidone (1) , 1-(1-azacyclohept-1-en-2-yl)-2-pyrrolidone (2) , 1-(1-azacyclonon-1-en-2-yl)-2-pyrrolidone (4) , 1-(1-azacyclohept-1-en-2-yl)-1- azacycloheptan-2-one (3) and 1-(phenyliminomethyl)-2-pyrrolidone (5) . The polymerizations exhibit formal kinetics of zero order with respect to the monomer with substantially differing apparent rate constants depending on the type of NIL used. The same limit conversions are attained in all cases. The possible role of NIL in the polymerization system is discussed.     

Polymerization of lactams, 23. Non-activated polymerization of 2-pyrrolidone initiated with potassium tert-butoxide

The non-activated polymerization of 2-pyrrolidone initiated with potassium tert-butoxide (conc. range 0,24–4,97 mole-%) was studied at 40°C. In accordance with theory, the conversion increases linearly with polymerization time and with initiator concentration. The linear dependence on conversion of the content of basic groups in the polymer and, above all, of the number of polymer molecules, and its independence on the initiator concentration up to 30% conversion indicate that a stationary state between formation and destruction of growth centres is very rapidly reached.     

Polymerization of β-propiolactones by nickel carboxylate/tributylphosphine complexes

The kinetics of polymerization of β-lactones — β-propiolactone, α-ethyl-α-phenyl- and α-benzyl-α-ethyl-β-propiolactone — was investigated using nickel carboxylate/phosphine complexes as initiators. The apparent rate constants of initiation and propagation, k_i and k_p, were calculated assuming first order kinetics. The variations of k_i values with monomer concentration suggest that the mechanism is more complex, with a reaction order which is not as admitted for calculation. The infrared study has shown that the polymerization initiated by the complex is of zwitterionic type, and the kinetic curves indicate that the nickel carboxylate participates in the reaction.     

Polymerization by [3 + 2] cycloaddition, 3. Polyimides

The reaction between hexafluoroacetone azine and bis(bicyclo[2.2.1]hept-5-en-2,3-dicarboximide)s ( 5 and 6 ) (dinadimides) (mole ratio 1:1) yields polymers with number-average molar masses between M?_n = 1700 and M?_n = 5700 depending on the reactants. The possible isomeric structures of the polymers are discussed in detail. From steric considerations which are supported by ¹³C and ¹⁹F NMR spectral data the number of realized isomers is reduced from sixteen to only three ( 12, 13 and 14 ). In contrast to these results, hexafluoroacetone azine and dimaleimides do not react to form polymers. This is explained on the basis of a spontaneous rearrangement of the primarily formed 1:1 adduct to 1 H-pyrazolines.     

Polymerization of 1-alkynes initiated by μ-alkylidene complexes of tungsten, 2. Kinetics and mechanism of the initiation reaction

The existence of two successive and quite different stages in the polymerization of monosubstituted 1-alkynes (1-heptyne), initiated by a homo dinuclear μ-alkylidene derivative of tungsten, is demonstrated. The first step leads to a mixture of species, the stability of which increases with the number of inserted monomeric units until inactivation. In order to explain the reactivation of these species in the second step, the loss of one carbonyl ligand is postulated, although no experimental evidence of that exists. The second step leads to high molecular weight polymer.     

Vinyl polymers with aromatic substituents, 2. Polymerization of 9-allylanthracene and structures of the polymers

Radical, cationic, and Ziegler-Natta polymerizations of 9-allylanthracene ( 1 ) were carried out. The radical polymerization was found to be an inefficient method leading to oligomers with complex structure. Reaction of initiator radicals and growing macroradicals with the anthryl group or with the allylic hydrogen leads to resonance stabilized radicals which stop the polymerization. The cationic and Ziegler-Natta polymerizations gave a copolymer with 9-anthrylmethylethylene units ( 8 ) (from normal 1,2 addition) and 9,10-anthrylenepropylene units ( 11 ) (from the electrophilic attack of carbocations at the aromatic nucleus). Low polymerization temperatures lead to a preference for the alkylation reaction.     

Vinyl polymerization, 307. Polymerization of acrylonitrile initiated by tetramethyl-2-tetrazene

The structure of the aggregates found in dilute solutions of PVC with tacticity 0,53 in THF has been investigated. It is shown that a model of an aggregate describing it as a composition of 10 to 15 single molecules bound together by a crystalline nucleus, is in accordance with the experimental evidence from light-scattering and GPC analysis.     

Polymerization of cyclic monomers, 2. Synthesis and radical polymerization of vinylcyclopropanes

New 1,1-disubstituted 2-vinylcyclopropanes were synthesized by reaction of the corresponding malonyl diesters with trans-1,4-dibromo-2-butene and of sodium hydride. The structure of the 2-vinylcyclopropanes could be confirmed by elemental analysis, IR, ¹H NMR and ¹³C NMR spectroscopy. Radical polymerization of the 2-vinylcyclopropanes in bulk with 2,2′-azoisobutyronitrile (AIBN) and di-tert-butyl peroxide (DtBPO), respectively, gives transparent polymers. In case of the liquid monomers, the polymerization is accompanied by negative volume changes. Spectroscopic investigations show that 2-vinylcyclopropanes primarily undergo opening of the cyclopropane ring forming an 1,5-addition polymer.