Copolymerization macromonomers

In some but not so rare cases, however, reactivity of macromonomers was found to be apparently reduced by the nature of their polymer chains. For example, p-vinylbenzyl- or methacrylate-ended PEO macromonomers, 26 (m=l) or 27b, were found to copolymerize with styrene (as A) in tetrahydrofuran with increasing difficulty (l/rA is reduced to one half) with increasing chain length of the PEO [41]. Since we are concerned with polymer-polymer reactions, as shown in Fig. 3, the results suggest that any thermodynamically repulsive interaction, which is usually observed between different, incompatible polymer chains, in this case PEO and PSt chains, may retard their approach and hence the reaction between their end groups, polystyryl radical and p-vinylbenzyl or methacrylate group. Such an incompatibility effect was discussed in terms of the degree of interpenetration and the interaction parameters between unlike polymers to support the observed reduction in the macromonomers copolymerization reactivity [31,40]. Similar observations of reduction of the copolymerization reactivity of macromonomers have recently been reported for the PEO macromonomers, 27a (m=ll) with styrene in benzene [42], 27b with acrylamide in water [43], and for poly(L-lactide), 28, with dimethyl acrylamide or N-vinylpyr-rolidone in dioxane [44]. 

Use of macromonomers as reactive (copolymerizable) surfactants in heterogeneous systems such as emulsion and dispersion constitutes an increasingly important application in the design of polymeric microspheres, as will be discussed later in Sect. 6. Here the macromonomers copolymerize in situ with some of the substrate comonomers to afford the graft copolymers, the grafts (branches) of which serve as effective steric stabilizers by anchoring their backbone onto the surfaces of the particles. In general, however, the copolymerization reactivities of macromonomers in such systems are not well understood yet. 

Hence, in macromonomer copolymerization the ratio of the molar amounts of A and of M incorporated instantaneously into the copolymer is proportional to the ratio of their molar amounts in the feed. This does not prevent compositional heterogeneity in samples obtained in the case of high conversions however, the amplitude of the fluctuations in the composition generally remains acceptable for conversions up to 50-70 %, because of the low molar concentrations of M usually chosen. 

The most representative research projects concerning graft polymer synthesis by means of macromonomer copolymerization will be reviewed in this section, whenever some kind of characterization of the compounds obtained is presented. Indications on potential applications of these materials will be given whenever available. 

The chain transfer agents (11 X=CH2, A=CH2) are misnamed macromonomers since in this context they do not behave as macromonomers. Copolymerization when it occurs is a side reaction. The mechanism is shown in Scheme 6.19 for MAA Irimer (63). The final product (65) is a also a macromonomcr and formation of the adduct (64) and chain transfer is reversible (see also Section 6.2.7.2 and Section 9.5.2). -" " ... 

To stabilize the micelle structure, core and shell crosslinking has been studied [41,61-63]. Recently, Shen et aL reported that amphiphilic brush copolymers composed of PEO and PCL chains, which were synthesized by macromonomer copolymerization, formed polymeric micelles in which the core-shell interface was crossUnked [64]. The diameters of the crosslinked micelles increased with increasing PCL/PEO chain ratio in the range 27.4-198 nm. The crosslinked micelles were 100 times more stable against dilution compared with micelles from corresponding amphiphilic block copolymers. 

Macro monomers (7, X = CH2, R = polymer chain) can react by a RAFT mechanism as shown in Scheme 16 for MAA trimef (63). The product (65) is also a macromonomef, thus chain transfer is reversible and degenerate. " " These chain transfer agents are frequently called macromonomers even when used as transfer agents. This may appear to be a misnomer, since, when used in this context, they should not behave as macromonomers. Copolymerization when it occurs is a side reaction. The most reported transfer agents of this class are the methacrylate macromonomers (e.g., 66-68) and... 

Macromonomers always lead to the formation of graft copolymers. For example, the vinyl-terminated polystyrene can be copolymerized with ethylene to produce a graft copolymer of polyethylene, whereby the vinyl moiety of polystyrene is integrally polymerized into the linear polyethylene backbone ... 

In the case of polymerization of monosubstituted monomers S, RA) with 66-68, copolymerization of the macromonomer to form a graft copolymer is a significant side reaction.76... 

Copolymerization of macromonomers formed by backbiting and fragmentation is a second mechanism for long chain branch formation during acrylate polymerization (Section 4.4.3.3). The extents of long and short chain branching in acrylate polymers in emulsion polymerization as a function of conditions have been quantified.20 ... 

Macromonomer RAFT polymerization is most effective with methacrylate monomers (Table 9.9).With monosubstituted monomers (e.g. S, acrylates) graft copolymerization, is a significant side reaction which can be mitigated but not eliminated by the use of higher reaction temperatures. 

One of the major advantages of radical polymerization over most other forms of polymerization, (anionic, cationic, coordination) is that statistical copolymers can be prepared from a very wide range of monomer types that can contain various unprotected functionalities. Radical copolymerization and the factors that influence copolymer structure have been discussed in Chapter 7. Copolymerization of macromonomers by NMP, ATRP and RAFT is discussed in Section 9.10.1. 

The grafting through approach involves copolymerization of macromonomers. NMP, ATRP and RAFT have each been used in this context. The polymerizations are subject to the same constraints as conventional radical polymerizations that involve macromonomers (Section 7.6.5). However, living radical copolymerization offers greater product uniformity and the possibility of blocks, gradients and other architectures. 

There have been several studies on the use of RAFT to form polymer brushes by polymerization or copolymerization of macromonomers 348-350. 

Another example of the flexibility of ADMET is the demonstration of successful polymerization of o /v-telechelic diene carbosilane macromonomers.45 The synthesis of macromonomer 30 is achieved using catalyst 23 and copolymerized with a rigid small-molecule diene, 4,4/-di-trans-l-propenylbiphenyl (Fig. 8.17). 

The synthesis of PDMS macromonomers with vinyl silane end-groups and their free-radical copolymerization with vinyl acetate, leading to poly(vinyl acetate)-PDMS graft copolymers, was described 346). The copolymers produced were later hydrolyzed to obtain poly(vinyl alcohol)-PDMS graft copolymers. 

Recently it has been shown that anionic functionalization techniques can be applied to the synthesis of macromonomers — macromolecular monomers — i.e. linear polymers fitted at chain end with a polymerizable unsaturation, most commonly styrene or methacrylic ester 69 71). These species in turn provide easy access to graft copolymers upon radical copolymerization with vinylic or acrylic monomers. 

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