Using Bifunctional Initiators

The success of obtaining polystyrene products by free radical processes is affected to a significant extent by the quality and performance of initiators. Monofunctional initiators such as benzoyl peroxide or azobisisobutyronitrile have been utilized in bulk and solution polymerizations for theoretical studies 

Recently, tetrafunctional initiators have also been introduced for styrenics. In 2001, Atofina Chemicals introduced a novel tetrafunctional initiator, Luperox JWEB50, developed specifically for the styrenics industry to produce high molecular weight, high-heat, crystal polystyrene with improved productivity in a cost-effective manner. JWEB50 is a room temperature stable, liquid peroxide with a half-life similar to those of currently used cyclic perketals, appropriate for use in mass polystyrene processes. A unique aspect of 

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

In early work, Prisyazhnyuk and Ivanchev [24] studied the fundamentals of the polymerization using bifunctional initiators. Recent studies on modelling and simulation can be found in a number of publications [25-30]. Model testing results of styrene homopolymerization using an extensive list of homo- and bifunctional initiators can be found in Dhib et al. [31]. 

The synthesis of oy->-dihalogen oligomers is realized by using bifunctional initiators, such as a.a -p-dihaloxylene [128] or arenesulfonyl chlorides [129], Scheme 16 gives some examples of difunctional initiators. 

These catalysts were used for random ester-carbonate bulk copolymerizations. Furthermore, by combining sequential polymerization techniques using bifunctional initiators, the mild polymerization conditions allow for the preparation of block copolymers. Hydrogen bond activation of monomer and initiator/propagating species was proposed as the underlying mechanism, which can be tuned to mitigate adverse side reactions (Scheme 36). 

To obtain telechelic polymers—that are, those carrying a same reactive molecular group at each of their ends—the most direct method is to use bifunctional initiators. Examples of such initiators can be found for all living chain addition polymerizations the example shown below illustrates the bifunctional initiation of living cationic polymerization of vinyl ethers ... 

Polymers obtained by ATRP using bifunctional initiators possess terminal halogen groups that can easily be converted to another functional group. This transformation has been demonstrated by preparing telechelic... 

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

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... 

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... 

Another way of synthesizing B-A-B triblock copolymers is to use a coupling reaction.2 Monocarbanionic poly-B precursor is used to initiate the polymerization of A. The living two block copolymer is then reacted stoichiometrically with an efficient bifunctional coupling agent, such as dibromo-p-xylene or dimethyldichlorosilane, or even phosgene. This coupling reaction yields the triblock copolymers. 

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).

Copolymers. Copolymers from mixtures of different bisphenols or from mixtures of dichlorosulfone and dichlorobenzophenone have been reported in the patent literature. Bifunctional hydroxyl-terminated polyethersulfone oligomers are prepared readily by the polyetherification reaction simply by providing a suitable excess of the bisphenol. Block copolymers are obtained by reaction of the oligomers with other polymers having end groups capable of reacting with the phenol. Multiblock copolymers of BPA-polysulfone with polysiloxane have been made in this way by reaction with dimethyl amino-terminated polydimethylsiloxane the products are effective impact modifiers for the polyethersulfone (79). Block copolymers with nylon-6 are obtained when chlorine-terminated oligomers, which are prepared by polyetherification with excess dihalosulfone, are used as initiators for polymerization of caprolactam (80). 

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. ... 

These sites can be used to initiate the polymerization of a small amount of di-vinylbenzene (DVB)8-10. The bifunctional monomer will polymerize to small tightly crosslinked nodules each of them is connected with the /chain ends which have participated in its initiation process. The average functionality /of the nodules is not directly accessible. However, it was shown in the case of star-shaped molecules that /increases with the overall concentration of the precursor, and, to a smaller extent, with the amount of DVB added per living end./is almost independent of the precursor chain length11. ... 

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. 

In a similar way as has been described for syntheses of type al, the majority of examples of type b involve polycondensation of a,ea bifunctional, small molecule reaction partners. Some examples are the reaction of AIBN or AIBN derivatives with 1,4-cyclohexane bismethyl diamine78), 1,2-ethylene diamine78), 1,6-hexamethylene diamine 78-80 , bisphenol A 78,81 and mono-, di- and tetraethylene glycol 55-64 . In almost all case using the AIBN derivative 4,4 -azobis(4-cyano valeryl chloride), an interfacial polymerization was employed. These polymeric azo compounds could be used as initiators for radical block copolymerizations. 

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. ... 

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 ... 

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