Homopolymerization of Macromonomers

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

It could be expected that the rate of polymerization of macromonomers is influenced by the low concentration of the active sites in a macromonomer solution and possibly also because of the lower accessibility of the growing radicals and of the terminal unsaturated bonds of the macromonomer. 

This point of view is however questioned by Yamashita 98) and by Asami21) whose results seem to indicate that the reactivity of a double bond does not depend on the length of the chain, to which it is attached. However, they usually obtained low yields of the products resulting from the polymerization of macromonomers and 

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). 

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  

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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. 

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. 

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. 

Lyotropic liquid-crystalline ordering was also observed for cylindrical brushes formed by the homopolymerization of macromonomers, i.e. macromolecules with polymerizable endgroups (Figure 34) [403]. The extended conformation of these cylindrical brushes is controlled by the coiling and steric repulsion of the side chains and is independent of the unperturbed flexibility of the backbone segments [402]. 

Dense grafting of side chains onto linear backbones, and homopolymerization of macromonomers, are both used to synthesize macromolecular brushes. Steric repulsion of the side chains forces the main chain into an extended wormlike conformation, resulting in liquid-crystalline phases, and lower dynamic shear moduli than linear flexible coils in concentrated solutions [93, 94]. Densely grafted polymeric brushes on sliding surfaces have been found to reduce friction, and therefore have potential for providing biolubrication for artificial implants [95]. 

Among the various means to access these structures, the homopolymerization of macromonomers has proved to be one of the most efficient methods [4,5]. 

As mentioned in the introduction, the homopolymerization of macromonomers can yield similar species, where each monomer unit of the backbone carries a graft [4,5,92]. 

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 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 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... 

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. 

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. 

Since the pioneering work of Tsukahara on the homopolymerization of methacryloyl end-functionalized polystyrene macromonomers to polymacromonomers several chemically different polymacromonomers were successfully synthesized [106-118]. Also different routes to cylindrical brush polymers by grafting from [119-121] and grafting on to [122, 123] techniques were reported, each of which exhibits certain advantages and disadvantages, as outlined in Table 3. 

The self-condensing copper-catalyzed polymerization of macromonomer of poly(tBA) with a reactive C—Br bond (H-6) affords hyperbranched or highly branched poly(tBA).447 Copolymerization of H-1 and TV-cyclohexylmaleimide induced alternating and self-condensing vinyl polymerization.448 The residual C—Cl bond was further employed for the copper-catalyzed radical homopolymerization of styrene to give star polymers with hyperbranched structures. Hyperbranched polymers of H-1 further serve as a complex multifunctionalized macroinitiator for the copper-catalyzed polymerization of a functional monomer with polar chromophores to yield possible second-order nonlinear optical materials.325... 

Extension of macromonomer reinitiation led to copolymerizations with nonpolymerizable monomers .399 401 Methacrylate oligomers cannot be ho-mopolymerized due to the steric bulk of the substituents at what would be the radical center. Thus, they are subject only to reinitiation under CCT conditions. There are many other olefinically unsaturated species which do not undergo homopolymerization or copolymerization with normal free-radical monomers.402 These can by represented by the tetrasubstituted olefin in eq 45, though more often the desired olefin is 1,2-disubstituted. 

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