Macromonomers homopolymerization

Only few free-radical homopolymeriza ions of macromonomers have been reported so far in spite of the importance of such highly branched compact macromolecules exhibiting very high segment densities in the vicinity of the backbone chain. Before giving some typical examples of macromonomer homopolymerization, attention should be drawn to two specific difficulties involved, namely separation and characterization ... 

Stein166 has indicated that the reactivity of the terminal double bond of the macromonomer (112) is 80% that of VAc monomer. The kinetics of incorporation of 112 have also been considered by Wolf and Burchard175 who concluded that 112 played an important role in determining the time of gelation in VAc homopolymerization in bulk. 

Note 1 The homopolymerization or copolymerization of a macromonomer yields a comb or graft polymer. 

A macromonomer is a macromolecule with a reactive end group that can be homopolymerized or copolymerized with a small monomer by cationic, anionic, free-radical, or coordination polymerization (macromonomers for step-growth polymerization will not be considered here). The resulting species may be a star-like polymer (homopolymerization of the macromonomer), a comblike polymer (copolymerization with the same monomer), or a graft polymer (copolymerization with a different monomer) in which the branches are the macromonomer chains. 

The present article is intended to discuss the state-of-the-art of the design and characterization of the branched polymers obtained by the macromonomer technique, with particular stress on the characterization and the properties of the brush polymers obtained by the homopolymerization of macromonomer. The synthetic aspects of the macromonomer technique, including preparation of various kinds of macromonomers, have been recently reviewed by one of the authors [1]. Therefore, we intend here to outline briefly the macromonomer technique and describe only the very recent important developments in syntheses. Preparation and characterization of the polymeric microspheres by use of macromonomers as reactive (copolymerizable) emulsifiers or dispersants will be described in some detail to represent one of their unique applications. 

Fig. 1 a—f. Various branched architectures obtained by the macromonomer technique a,b comb-like c,d star-like e brush f flower-like, a c, and e are poly(macromonomers) obtained by homopolymerization, while b, d, and f are graft copolymers obtained by copolymerization... 

The variety of branched architectures that can be constructed by the macromonomer technique is even larger. Copolymerization involving different kinds of macromonomers may afford a branched copolymer with multiple kinds of branches. Macromonomer main chain itself can be a block or a random copolymer. Furthermore, a macromonomer with an already branched or dendritic structure may polymerize or copolymerize to a hyper-branched structure. A block copolymer with a polymerizable function just on the block junction may homopolymerize to a double comb or double-haired star polymer. 

So far, a great number of well-defined macromonomers as branch candidates have been prepared as will be described in Sect. 3. Then a problem is how to control their polymerization and copolymerization, that is how to design the backbone length, the backbone/branch composition, and their distribution. This will be discussed in Sect. 4. In brief, radical homopolymerization and copolymerization of macromonomers to poly(macromonomers) and statistical graft copolymers, respectively, have been fairly well understood in comparison with those of conventional monomers. However, a more precise control over the backbone length and distribution by, e.g., a living (co)polymerization is still an unsolved challenge. 

Radical homopolymerization kinetics of some typical macromonomers, such as those from PSt, 23,24 [30,31], and PMMA, 25 [32,33], have been studied in detail by means of ESR methods. 

Homopolymerization of macromonomer provides regular star- or comb-shaped polymers with a very high branch density as shown in Fig. 1 a,c,e. Such polymacromonomers, therefore, are considered to be one of the best models for understanding of branched architecture-property relationships. Their properties are expected to be very different from the corresponding linear polymers of the same MW both in solution and the bulk state. Indeed, during the past decade, remarkable progress has been accomplished in the field of static, dynamic, and hydrodynamic properties of the polymacromonomers in dilute and concentrated solutions, as well as by direct observation of the polymers in bulk. 

Radical homopolymerization and copolymerization of macromonomers are fairly well understood and reveal their characteristic behaviors that have to be compared with those of conventional monomers. A detailed mechanism of the polymer-polymer reactions involved, however, appears still to be an issue. Ionic or, desirably, living polymerization and copolymerization are still an important... 

Characterization of the poly(macromonomers) prepared by homopolymerization has proved that they provide a useful probe for discussing the structural characteristics of the star and brush polymers. Graft copolymers have been and will be a most important area of application of the macromonomer technique since a variety of multi-phased and microphase-separated systems can easily be designed just by an appropriate combination of a macromonomer and a conventional monomer. In general, however, characterization of their absolute MW, branch/backbone composition as well as their distributions remain to be studied in more detail. 

In the first part of this review we shall consider the various pathways that have been used (or attempted) to synthesize macromolecular monomers. We shall critically discuss the efficiency of the methods that have been proposed, together with the procedures used for the characterization of the species obtained. In the second part we shall describe the various attempts to homopolymerize macromonomers and to use them in copolymerization reactions to obtain graft copolymers. We shall include some potential applications of macromonomers as intermediates to the synthesis of new polymeric materials that have been proposed. 

In this section homopolymerization of macromonomers and their copolymerization with vinylic or acrylic monomers will be reviewed. 

Another factor has to be considered. When the macromonomer chain segments can give rise to transfer reactions, the probability of such events is enhanced because the number of chain segments per unsaturation is high. Homopolymerization of such macromonomers should be expected to involve additional branching and possibly crosslinking. A typical example is that of PEO macromonomers because oxyethylene units are known to induce transfer reactions U3). 

The homopolymerization of o)-(4-vinylbenzyl)polystyrene macromonomers was also investigated kinetically by Asami21) under quite different conditions, namely very high amounts of initiator and high overall concentrations. Thus, the molecular weights (even if underestimated) are very low. Under these conditions, the rate of polymerization does not depend on the length of the side chains. However, these particular conditions which favour initiation and termination processes cannot be illustrative of regular polymerizations. 

The anionic homopolymerization of polystyrene macromonomers was carried out successfully. The methacrylic ester sites at the chain end do not require very strong nucleophiles to be initiated diphenylmethylpotassium was used, and the process was carried out at — 70 °C in THF solution24). The products are comparable with those obtained by free-radical polymerization. The molecular weight distribution should be narrower but this cannot be easily checked because these polymer species are highly branched and compact as already mentioned. 

Anionic homopolymerization of poly-THF macromonomers bearing terminal styryl, a-methylstyryl or methacryloyl groups has not been reported so far. 

The homopolymerization of macromonomers has not been studied extensively yet, in spite of the interest involved. The macromolecules that result are very highly branched and compact, and the segment density in the neighbourhood of the backbone chain is exceptionally high. 

Because p-substituted styrenes are more reactive than unsubstituted styrene, the resulting macromonomers should be able to homopolymerize to produce comb polymers, and/or copolymerize with styrene to produce graft copolymers. Copolymerization should also increase at higher conversions when monomer is depleted. 

The selectivity for reaction of carbenium ions with unsaturated oligomers increases not only in the absence of monomer, but also with decreasing temperature [182]. However, unsaturated macromonomers of styrene may dimerize rather than homopolymerize [cf., Eq. (96)]. That is, the molecular weight at complete conversion only doubles, rather than increasing further, because no copolymerization is possible in the absence of monomer. Because molecular weight only doubles [182], dimerization apparently dominates over Friedel-Crafts alkylation. The alkylation may... 

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