Preparation of Block Copolymers by Homogeneous Ionic Copolymerization

Formation of block copolymers by this method depends upon the ability to form “living” chain ends. Among the anionic systems, the following polymerizations fit this requirement:

 1. Polymerizations of nonpolar monomers with alkali metal-aromatic electron transfer initiators in ethers [398].

 2.Polymerizations of nonpolar monomers with organolithium compounds in hydrocarbon solvents [399].

 3. Acrylonitrile polymerizations in dimethyl formamide initiated by sodium triethylthioisopropox-yaluminate at 78C [340].

4. Copolymerizations of hexafluoroacetone and cyclic oxides initiated by CsF [341].

5.Polymerization of alkyl isocyanates initiated by organoalkali species in hydrocarbons at 78C [342]. Among the cationic “living” polymerizations that can be used for block copolymer formation are:

1. Polymerizations of isobutylene [361] and/or vinyl ethers [363] with appropriate catalysts. This includes formation of block copolymers from the two types of monomers [365].

 2. Polymerizations of tetrahydrofuran with the aid of chlorobenzene diazonium hexafluorophosphate [343], triphenylmethyl hexachloroantimonate [344], or phosphorus pentafluoride [445].

3. Polymerization of p-methyl styrene, N-vinylcarbazole, and indene with appropriate catalysts. The preparations by anionic mechanism of A—B—A type block copolymers of styrene and butadiene can be carried out with the styrene being polymerized first. Use of alkyl lithium initiators in hydrocarbon solvents is usually a good choice, if one seeks to form the greatest amount of cis-1,4 microstructure [346]. This is discussed in Chap. 4. It is more difficult, however, to form block copolymers from methyl methacrylate and styrene, because “living” methyl methacrylate polymers fail to initiate polymerizations of styrene [347]. The poly (methyl methacrylate) anions may not be sufficiently basic to initiate styrene polymerizations [345]. A“living” cationic polymerization of tetrahydrofuran, using BH3 as the initiator in the presence of epichlorohydrin and 3,3-bis(chloromethyl) oxacyclo-butane [348], results in formation of block copolymers. Two types form. One is an A—B type. It consists of polytetrahydrofuran blocks attached to blocks of poly(3,3-bis(chloromethyl) oxacyclo-butane). The other one is an A—AB—B type [348]. The preparation of well-defined sequential copolymers by anionic mechanism has been explored and utilized commercially for some time now. Initially, the cationic methods received less attention until it was demonstrated by Kennedy [424] that a large variety of block copolymers can be formed. The key to Kennedy’s early work is tight control over the polymerization reaction. The initiation and propagation events must be fundamentally similar, although not identical [424]: Ion generation:

In this scheme, chain transfer to monomer must be absent and the termination is well defined. Termination:

where X is a halogen, Me is a metal. This allows formation of macromolecules with terminal halogens. They can be used to initiate new and different polymerizations. Three methods were developed to overcome transfer to monomer [424]. These are: (1) use of inifers; (2) use of proton traps; and (3) establishing conditions under which the rate of termination is muchfaster than the rate of transfer to monomer. The first one, the inifer method, is particularly useful in formation of block copolymers. It allows preparation of head and end (a and o) functionalized telechelic polymers. Bifunctional initiators and transfer agents (inifers) are used. The following illustrates the concept [424]:

In the above scheme, the inifer, XRX, is usually an organic dihalide. If chain transferring to the inifer is faster than chain transferring to the monomer, the polymer end groups become exclusively terminated with halogens. It is also possible to carry out “living” cationic polymerization of isobutylene, initiated by a difunctional initiator [435]. This results in a formation of bifunctional “living,” segments of polyisobutylene that are soft and rubbery. Upon completions of the polymerization, another monomer, one that yields stiff segments and has a high Tg, like indene, is introduced into the living charge. Polymerization of the second monomer is initiated from both ends of the formed polyisobutylene. When the reaction is complete, the polymerization is quenched. Preparations of a variety of such triblock and star block polymers have been described [435]. A technique was developed, by introducing cationic to anionic transformation [438]. A “living” carbocationic polymerization of isobutylene is carried out first. After it is complete, the ends of the chains are quantitatively transformed to polymerization-active anions. The additional blocks are then built by an anionic polymerization. A triblock polymer of poly (methyl methacrylate) polyisobutylene-poly (methyl methacrylate) can thus be formed. The transformation takes several steps. In the first one, a compound like toluene is Friedel-Craft alkylated by a,o-di-tert-chloropolyi sobutylene. The ditolylpolyisobutylene, which forms, is lithiated in step two to form a,o-di benzyl lithium polyisobutylene. It is then reacted with 1,1-diphenylethylene to give the corresponding dianion. After cooling to 78C and dilution, methyl methacrylate monomer is introduced for the second polymerization [438] in step 3. Formation of block copolymers from polymers with functional end groups has been used in many ways. In anionic polymerization, various technique were developed for terminating chain growth with reactive end groups. These end groups allow subsequent formations of many different block copolymers. One such active terminal group can be toluene diisocyanate [439]. The isocyanate group located ortho to the methyl group is considerably less reactive toward the lithium species due to steric hindrance [438]. The unreacted isocyanate group can be used for attachment of various polymers that are terminated by hydroxy, carboxy, or amine groups. Other functional compounds that can be used in such reactions are alkyl or aryl halides, succinic anhydride, n-bromophthalimide [448], and chlorosilanes [449]. Because block copolymers can often offer properties that are unattainable with simple blends or random copolymers [364], many efforts were made to combine dissimilar materials, like hydrophilic with hydrophobic, or hard with soft segments, as was shown earlier. One paper [432] describes formation of block copolymers containing helical polyisocyanide and an elastomeric polybutadiene. Compound [(Z₃-C₃H₅)-Ni(OC(O)CF₃)]₂ was used to carry out “living” polymerization of butadiene and then followed by polymerization of tert-butyl isocyanide to a helical polymer.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تشهد تشييع جثمان المحقق الشيخ محمد رضا المامقاني

قسم الشؤون الفكرية يختتم برنامج (فتية سيد الماء) لرعاية الأيتام بنسخته الثانية

لتطوير المهارات الإدارية.. العتبة العباسية المقدسة تطلق الدورة التعليمية الأولى لمسؤولي وحداتها

قسم التطوير يقيم دورة تدريبية عن الظهور الإعلامي لملاكاته

المزيد

الآخبار الصحية

دراسة تكشف "السبب الخفي" وراء ترك التمارين الرياضية

كشف التوقيت "المثالي" لتغيير فرشاة الأسنان

بلاستيك في أنسجة بشرية.. دراسة تكشف مفاجأة طبية

دراسة تحذر من الإفراط في ممارسة الرياضة.. كيف يضر دماءك؟

المزيد

الآخبار التكنلوجية

"ديب سيك" تحجب أحدث نماذجها عن شركات الرقائق الأميركية

"سامسونغ" تطلق سلسلة هواتف "غالاكسي إس 26"

حملات سرية ورسائل تشويه.. كيف يتم استغلال "تشات جي بي تي"؟

أثر نفسي غير متوقع للرياضة.. مرتبط بالغضب

المزيد

مواضيع ذات صلة

Special Reactions for Preparation of Block Copolymers

Simultaneous Use of Free Radical and Ionic Chain-Growth Polymerizations

Block Copolymers of Poly(a-Olefin) s

Polyurethane Ionomers

Polyamide-Polyester Block Copolymers

Block Copolyesters

Block Copolymers

Miscellaneous Graft Copolymerizations

Preparation of Graft Copolymers with Ionic Chain-Growth and Step-Growth Polymerization Reactions

Graft Copolymer Formation with the Aid of High-Energy Radiation

Photochemical Syntheses of Graft Copolymers

Initiations of Polymerizations from the Backbone of Polymers