Synthesizing block copolymers

Among the pairs of monomers used in synthesizing block copolymers by this method are MMA/AN, C2F4/ MMA, and HEMA/DMA (HEMA = hydroxyethyl methacrylate and DMA = decyl methacrylate). 

It is also important to note that this procedure of synthesizing block copolymers possesses an advantage in that homopolymers are avoided, since all the initiating radicals are attached to a polymer chain. This is not so with many free radical syntheses. 

Generation of radicals by redox reactions has also been applied for synthesizing block copolymers. As was mentioned in Section II. D. (see Scheme 23), Ce(IV) is able to form radical sites in hydroxyl-terminated compounds. Thus, Erim et al. [116] produced a hydroxyl-terminated poly(acrylamid) by thermal polymerization using 4,4-azobis(4-cyano pentanol). The polymer formed was in a second step treated with ceric (IV) ammonium nitrate, hence generating oxygen centered radicals capable of starting a second free radical polymeriza-... 

The principle of synthesizing block copolymers by macroinitiators is as follows ... 

For the modification of PAN properties the methods of synthesizing block copolymers of PAN can be of substantial interest. 

Sutoh and Noda154 succeeded in proving, by synthesizing block copolymers of the structure (Gly-Pro-Pro)n(Gly-Ala-Pro)m-(Gly-Pro-Pro)n, that with increasing imino add content, AS° changes to higher positive values. They do, however, not relate this to lower entropy losses of conformation but to hydrophobic interactions of the proline residues in the helical state. 

Every polymerization method is limited to a certain type and number of monomers, thus preventing the possibility to synthesize block copolymers with a wide combination of monomers. However, recent advances in polymer synthesis enabled the switching of the polymerization mechanism from one type to another, thereby permitting the preparation of block copolymers composed of monomers that can be polymerized by different techniques. 

Crivello et al. synthesized block copolymers consisting of poly(DMS) and vinyl polymer sequences to modify the mechanical properties and solvent resistance of poly(DMS). They used tetraphenylethane derivatives incorporated into the poly(DMS) chain through hydrosilylation (Eq. 26) [124-126] ... 

Living radical polymerizations have received considerable attention because they provide a convenient alternative for synthesizing block copolymers, polymers of narrow polydispersity and complex polymer structures (1-5). Because of their ability to initiate living free radical polymerizations, iniferters have been examined extensively after Otsu et al. (6) introduced them in 1982. In particular, dithiocarbamate derivatives have been studied more closely by several researchers. Lambrinos et al (7) have examined the molecular weight evolution during the polymerization of n-butyl acrylate using p-x ylylene bis(A,A-diethyl... 

The phase morphology of block copolymers can also be visualized by transmission electron microscopy. Figure 10.8 shows the lamellar structure of Fluoro-PSB-IX. From diblock copolymers it is well known that the resulting microphase morphology depends on the volume fraction (< >) of the two phases. By simple adjustment of the relative block lengths we are able to synthesize block copolymers with specific structures.1718... 

There is a current drive in microlithography to define submicron features in bilevel resist structures. The introduction of organometallic components, most notably organosilicon substituents, into conventional resists is one promising approach. To this end, organosilicon moieties have been primarily utilized in starting monomers (1-4) or in post-polymerization functionalization reactions on the polymer (5,6). Little work has been done on the reaction of preformed reactive oligomers to synthesize block copolymer systems. 

The stability of polystyryl carbanions is greatly decreased in polar solvents such as ethers. In addition to hydride elimination, termination in ether solvents proceeds by nucleophilic displacement at the C—O bond of the ether. The decomposition rate of polystyryllithium in THF at 20°C is a few percent per minute, but stability is significantly enhanced by using temperatures below 0°C [Quirk, 2002], Keep in mind that the stability of polymeric carbanions in the presence of monomers is usually sufficient to synthesize block copolymers because propagation rates are high. The living polymers of 1,3-butadiene and isoprene decay faster than do polystyryl carbanions. 

The range of monomers that can be incorporated into block copolymers by the living anionic route includes not only the carbon-carbon double-bond monomers susceptible to anionic polymerization but also certain cyclic monomers, such as ethylene oxide, propylene sulfide, lactams, lactones, and cyclic siloxanes (Chap. 7). Thus one can synthesize block copolymers involving each of the two types of monomers. Some of these combinations require an appropriate adjustment of the propagating center prior to the addition of the cyclic monomer. For example, carbanions from monomers such as styrene or methyl methacrylate are not sufficiently nucleophilic to polymerize lactones. The block copolymer with a lactone can be synthesized if one adds a small amount of ethylene oxide to the living polystyryl system to convert propagating centers to alkoxide ions prior to adding the lactone monomer. 

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

Reactions with monofunctional reagents are for example carried out in order to increase the thermal and/or chemical stability of the end groups (Poly-oxymethylenes, Example 5-7). Reactions with bifunctional reagents can be used to enlarge the degree of polymerization or to synthesize block copolymers (see Sect. 4.2.1). 

Inaki (1992) synthesized a wide range of nucleobase-functionalized random and homopolymers. In addition, Inaki et al. (1980) synthesized block copolymers containing thymine and uracil groups in the main chain through ring-opening cationic and anionic polymerization of cychc derivatives of the nucleobases. 

Such findings allow us to foresee the possibility of synthesizing block copolymers formed alternately by sequences of monomeric units having different natures. 

A rather obvious method to synthesize block copolymers is to use a "living" precursor as the anionic Initiator for the polymerization of second monomer.22However, this method requires that the Initiation reaction be fast, quantitative and free of side reactions. This means that the nucleophlllclty of the carbanlonlc sites should be sufficient to attack the second monomer added In other words, the monomers have to be added In the order of Increasing electroaffinity. 

In view of these advantages it is hardly surprising that efforts to synthesize block copolymers via a radical technique continue. 

Sequential addition methods were utilized to synthesize block copolymers of polybenzvalene with polynorbornene to yield block copolymers of PA and pol-ynorbornene after isomerization [43]. (It should be noted that both benzvalene and polybenzvalenes are sensitive to shock and mechanical stress causing ring strain-promoted explosions in the precursor materials.)... 

McCullough and coworkers had synthesized block copolymers of poly(3-hexylthiophene) and polystyrene or PMA by ATRP of the vinyl monomer from a polythiophene macroinitiator, which was prepared in several steps... 

Shen et al. determined the BD/IP copolymerization parameters for the polymerization with the ternary catalyst system NdN/TIBA/EASC at 50 °C ted = 1.4 and np = 0.6 [92]. Over a wide range of BD/IP copolymer compositions the experimentally determined Tg values significantly deviate from the theoretical curve which was calculated by the Fox equation for random copolymers. Only for IP-contents < lOwt. % does the experimentally determined data coincide with the theoretical curve. Shen et al. also succeeded to synthesize block copolymers comprising poly(butadiene) and poly(isoprene) building blocks [92]. 

In polymerization at low temperatures, the time required to form one polymer chain is long enough to consume one monomer fully and allow the subsequent addition of another one. Thus, it becomes possible to synthesize block copolymers, provided that the polymerization (especially when it is catalyzed by hafnocenes) starts with propylene and, after the propylene is nearly consumed, continues with ethylene. 

In 1998, Chiefari et al. [10] attempted to combine the convenience of radical polymerization with the many benefits of living polymerization, e.g. control of the molecular weight and polydispersity and the possibility of synthesizing block copolymers of complex architecture. They used free-radical polymerization reagents of formula (I) to produce a sequence of reversible addition-fragmentation in which the transfer of the S=C (Z) S moiety between active and dormant chains serves to maintain the living character of the polymerization ... 

UV irradiation of the resulting prepolymers caused a-scission, and benzoyl and polymer bound electron donating radicals are formed in the same manner as described for the low-molar mass analogues. Electron donating polymeric radicals thus formed may conveniently be oxidized to polymeric carboeations to promote cationic polymerization of cyclic ethers. It was demonstrated that irradiation of benzoin terminated polymers in conjuction with pyridinium salts as oxidants in the presence of cyclohexene oxide makes it possible to synthesize block copolymers of monomers with different chemical natures [75] (Scheme 19). 

Such a straightforward and promising investigation needs to be continued since it is an original way to synthesize block copolymers particularly well adapted to acrylic monomers. 

The sequential living polymerization of two monomers (method A) is perhaps the most straightforward method to synthesize block copolymers,... 

In general, styrene and its substituted derivatives are less reactive than vinyl ethers in cationic polymerization, although the reactivity depends considerably on the nature of the substituents. This in turn requires some care in synthesizing block copolymers of styrene derivatives by sequential living cationic polymerization. For example, styrene-methyl... 

Another consequence of living polymerization systems is that they can be used to synthesize block copolymers. Under these conditions, once the initial quantity of monomer in a given reaction is consumed, the resultant polymer chains are terminated with metal carbene end groups that are still active for aUcene metathesis. As long as these carbenes do not react rapidly with the acyclic aUcenes in the polymer chain, addition of a second monomer will result in the synthesis of a block copolymer. This reaction is illustrated in equation (13) for the synthesis of a polymer that consists of block of x repeat units of monomer A followed by a block of y repeat units of monomer B. 

Anionic polymerizations are particularly useful for synthesizing block copolymers. These macromolecules contain long sequences of homopolymers Joined together by covalent bonds. The simplest vinyl-type block copolymer is a two segment molecule illustrated by structure (9-2). This species is called an AB block copolymer, because it is composed of a poly-A block joined to a long sequence of B units. Other common block copolymer structures are shown as (9-3)-(9-6). 

ABA tribiock, or all three can be different, as in an ABC triblock copolymer. Obviously, the number of possible block sequences increases rapidly with the number of blocks and the number of different types of block in the chain. One can also synthesize block copolymers with branched architecture, such as star-branched block copolymers, in which each of the arms of the star contains either the same or different block sequences (see Fig. 13-1). One or more of the blocks could also be stiff or liquid crystalline (Chiellini et al. 1994 Chen et al. 1996 Radzilowski et al. 1997 Jenekhe and Chen 1998). For a given type of block copolymer, the degree of polymerization N of the whole molecule, or the degree of polymerization Ni of one or more of the blocks, can be varied. Thus, the number of different types of block copolymers is practically endless. 

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