Bifunctional initiation

Frequently, initiators bearing simultaneously azo and peroxy groups were used as bifunctional initiators. 

As mentioned in Section II. D., azoperoxy bifunctional initiators play a significant role regarding block copoly-... 

Of recent interest is the living cation of tetrahydrofuran. The oxonium ion with suitable counter ions is stable and exists in equilibrium with the monomer47. Bifunctional initiators were found to produce the polytetrahydrofuran dication. 

Bifunctional initiators are sometimes needed e.g., for the synthesis of triblock copolymers, networks, or a,co-difunctional macromolecules. Whenever it is possible to carry out the polymerization in a polar solvent no major difficulty is involved, as bifunctional initiators covering a wide range of nucleophilicities are available (Table 2). 

In nonpolar solvents, in spite of many attempts31), the problem of preparing efficient bifunctional initiators has not been entirely solved. Association between metal-organic sites causes insolubility (or even instability) of many bifunctional initiators. Satisfactory results 32,33) have been obtained with the diadducts of sec-butyllithium and diisopropenylbenzene (DIB). These adducts are made at a temperature chosen... 

Bifunctional Initiator Arising from Butyllithium and Diunsaturated Monomers... 

Functionalization yields have to be checked carefully by comparing the effective number average molecular weight Mn with its value calculated from the functional content of the sample, under the assumption that each macromolecule carries one function (two if a bifunctional initiator is used). Low molecular weight samples are preferred, as the functional analysis would otherwise lack accuracy. In all examples quoted high functionalization yields have been attained. 

On each of the curves, the points at lowest X represent swelling in cyclohexane, the next in tetrahydrofuran and the last in benzene. In all cases, the samples were swollen in the pure solvent. The curves are reproduced from Figure 13 of Reference 19. The networks were made from anionically polymerized polyr-styrene using a bifunctional initiator crosslinked subsequently by divinyl benzene. The curves correspond to different ratios of divinyl benzene (DVB) per polystyrene living end (LE),... 

Sodium 4-oxy-2,2,6,6-tetramethyl-l-piperidinyloxy, TEMPONa, was used as a bifunctional initiator for the synthesis of PEO-fc-PS block copolymers [133]. Initially the ROP of EO was performed in THF at 60 °C to provide narrow molecular weight distribution chains with terminal TEMPO moieties. Using these functionalized PEO chains the polymerization of styrene was... 

The bifunctional initiator 4-hydroxy-bulyl-2-bromoisobulyralc, HBBIB, promoted the ATRP of styrene as well as the cationic ring opening polymerization of THF [134], In the presence of Cu/CuBr2/PMDETA styrene was polymerized through the bromoisobutyrate function of HBBIB, to give PS chains end-functionalized with hydroxyl groups, PS-OH. The in situ... 

Kubica et al45 also investigated the effect of platinum-modified zeolites on the decalin reaction. They found that the addition of Pt enhances the catalyst activity. The initial isomerization was increased 3 times, which can be interpreted in terms of a change in the reaction initiation. In addition to initiation by a PC step over Bronsted acid sites, as proposed for H-form zeolites, a bifunctional initiation path... 

Group transfer polymerization allows the synthesis of block copolymers of different methacrylate or acrylate monomers, such as methyl methacrylate and allyl methacrylate [Hertler, 1996 Webster and Sogah, 1989]. The synthesis of mixed methacrylate-acrylate block copolymers requires that the less reactive monomer (methacrylate) be polymerized first. The silyl dialkylketene acetal propagating center from methacrylate polymerization is more reactive for initiation of acrylate polymerization than the silyl monoalkylketene acetal propagating center from acrylate polymerization is for initiation of methacrylate polymerization. Bifunctional initiators such as l,4-bis(methoxytri methyl si loxymethylene)cyclohexane (XXXIII) are useful for synthesizing ABA block copolymers where the middle block is methacrylate [Steinbrecht and Bandermann, 1989 Yu et al., 1988]. 

The only report on chemoenzymatic synthesis of branched polymers is from Peelers et al. [58], Heterotelechelic PCL macroinimer was synthesized in a one-pot enzymatic procedure by using 2-hydroxyethyl a-bromoisobutyrate as a bifunctional initiator. A polymerizable endgroup was introduced by subsequent in sim enzymatic acrylation with vinyl acrylate. Synthesis of branched polymers by self-condensing ATRP of the macroinimers was successfully conducted with and without the addition of MMA as a comonomer. 

The synthesis of triblock copolymers B-A-B can be achieved by means of a bifunctional initiator a bifunctional a,u-dicarbanionic poly-A precursor is formed, and is used in a second step as the initiator for the polymerization of monomer B, with the same conditions to be observed as above. A number of efficient bifunctional initiators are commonly used, in polar solvent media. Recent work, carried out in several laboratories -- t aimed at the preparation of efficient bifunc-tional Initiators soluble in non polar solvents. Such systems... 

Divalent counterions Kinetic measurements using mono- and bifunctional initiators and Ba++ as the counterion in THF were reported by Mathis and Francois (37 ), who applied adiabatic calorimetry. At -7o°C no termination is found and conversion follows first order with respect to monomer concentration. The rate constants do not depend on the concentration of living ends, indicating the absence of free anions. The rate constants are smaller by a factor of 2o as compared with those measured with monovalent counterions. However, they are smaller by a factor of 3 only, compared with those calculated for chains which are intramolecular ly associated (Na+, counterion). The activation energy for PMMA Ba in THF is equal to that for monovalent counterions, but the frequency exponent is smaller by about 1.5 units, reflecting the fact that the transition state for the dianionic ion pair has higher steric requirements. 

Table I Tacticities of samples obtained in DME and THF, using mono- and bifunctional initiators and Na+ and Cs+ as the counterions. I,H,S isotactic (ami), heterotactic (mr/rm) and syn-diotactic (rr) triads m,r meso and racemic dyads p 2mr/H persistence ratio ( p 1 for Bernoullian statistics).

The first method, which is used in many instances, proceeds by anionic block copolymerization. In a first step styrene is anionically polymerized using an efficient bifunctional initiator. The polymer obtained has a well-defined molecular weight, it exhibits a rather narrow molecular weight distribution and it has carbanionic living sites at both ends of its linear chains7. ... 

Polymerization. The polymerization of aziridines takes place in the presence of catalytic amounts of acid at elevated temperatures. The molecular weight can be controlled by the monomer—catalyst ratio, the addition of amines as stoppers, or the use of bifunctional initiators. In order to prevent a vigorous reaction, the heat liberated during the highly exothermic polymerization must be removed by various measures, ie, suitable dilution, controlled metering of the aziridine component, or external cooling after the reaction has started. 

Macromonomers with two methacrylate functionalities (MA-PIB-MA) at both ends of the PIB chain have also been synthesized, by a procedure essentially identical to that reported above, but starting with a bifunctional initiator in the polymerization of IB [103]. Free radical copolymerization of the resulting MA-PIB-MA with 2-(dimethylamino)ethyl methacrylate resulted in amphiphilic networks, with a wide range of mechanical and swelling properties. 

It is the process forming the reactive centers from which macromolecules evolve. It may result from two different mechanisms a nucleophilic attack of the monomer by an organometallic initiator (butyllithium, cumylpotassium, benzylsodium, etc.), or the transfer to the monomer of the counterion and the extra electron of an electron-transfer initiator (lithium or sodium naphtalene, biphenyl.) later the ion-radical monomer having an extra electron in its lowest antibonding n orbital becomes a dicarbanion by dimerization of two activated monomer molecules. The use of a monofunctional initiator leads to a biblock copolymer AB, while that of a bifunctional initiator leads to a triblock copolymer ABA. 

Symmetric BSB copolymers containing between 20 and 80% polybutadiene and covering a large range of molecular weights haw been synthetized by Douy in tetrahydrofuran solution at a low temperature (—70°) using a bifunctional initiator (a-methylstyrenepotassium dianion)66,88. ... 

A comprehensive kinetic model for the free-radical polymerisation of vinyl chloride in the presence of monofunctional and bifunctional initiators,... 

Besides the above-mentioned approaches varying the parameters governing the initiator decomposition, it is worth mentioning an interesting biphasic study based on the difference of solubility of monomers and the use an original amphiphilic bifunctional initiator, the palmitoyl (3-carboxy propionyl) sebacoyl diperoxide 3 [24], In such conditions, block copolymers of water soluble (acrylamide) and oil soluble (butylmethacrylate) monomers can be obtained by interfacial radical copolymerization as follows ... 

The synthesis, starting from a bifunctional initiator followed by quenching the double-headed living ends, gives homotelechelic polymers (method B). Carboxylate-capped telechelic poly(isobutyl vinyl ether) has been obtained in this way [82], where the adduct of a bifunctional vinyl ether with trifluoroacetic acid is the initiator, and the quencher is the malonate anion. For method C, a bifunctional trimethylsilyl enol ether, CH2=C[OSi(CH3)3]C6H4OCH2CH20C6H4[(CH3)3SiO]C=CH2, is a useful terminator (chain coupler) for vinyl ethers [142,147] and a-methyl-styrene [159] (see also Section VI.B.4). 

Bis(chlorodimethylsilyl)benzene-AgPF6 system was shown to act as a bifunctional initiator of substituted oxirane polymerization [42], Tri-methylsilyl iodide and triflate were used also as initiators of the cationic polymerization of oxazolines [43]. In this system, however, in contrast to typical initiation mechanism of oxazoline polymerization, O-silylation leads to initiation, because of the unfavorable charge distribution in N-siiyiated species ... 

In a similar way, using bifunctional initiators, polymers terminated at both ends with polymerizable groups were prepared [171]. 

Boffa, L.S. and Novak, B.M. (1997) Link-functionalized polymers an unusual macromolecular architecture through bifunctional initiation. Macromolecules, 30, 3494. 

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