Methacrylic monomers, propagation

The free-radical polymerization of methacrylic monomers follows a classical chain mechanism in which the chain-propagation step entails the head-to-taH growth of the polymeric free radical by attack on the double bond of the monomer. Chain termination can occur by either combination or disproportionation, depending on the conditions of the process (36). 

Monomers not amenable to direct homopolymerization using a particular reagent can sometimes be copolymcrizcd. For example, NMP often fails with methacrylates (e.g. MMA, BMA), yet copolymerizalions of these monomers with S are possible even when the monomer mix is predominantly composed of the methacrylate monomer,15j This is attributed to the facility of cross propagation and the relatively low steady state concentration of propagating radicals with a terminal MMA (Section 7.4.3.1). MMA can also be copolymerized with S or acrylates at low temperature (60 C).111 Under these conditions, only deactivation of propagating radicals with a terminal MMA unit is reversible, deactivation of chains with a terminal S or acrylate unit is irreversible. Molecular weights should then be controlled by the reactivity ratios and the comonomer concentration rather than by the nitroxide/alkoxyamine concentration. 

NMP has not been extended to methacrylate monomers, in contrast to ATRP, which is successful with methacrylates. Many attempts to polymerize methacrylates by NMP have been unsuccessful, resulting in low conversions and/or broad PDI [Hawker et al., 2001]. This is generally ascribed to degradation of propagating radicals via [5-hydrogen abstraction... 

Problem 8.4 Lower molecular weights and polymerization rates observed in anionic polymerizations of polar monomers are attributed to the reactivity of the polar substituents toward nucleophiles, leading to termination and side reactions that are competitive with both initiation and propagation. Explain this behavior considering the case of methyl methacrylate monomer. 

Equality of lengths of kinetic chains and the distance between interlocks of the networks forming in the reaction mixture can be considered as the kinetic condition of the onset of autoacceleration, or VR(qa) = ijc [63]. On the other hand, as the polymer solution concentration increases, the distance between interlocking contracts according to the relation i = (where i is the distance between interlocks in the polymer melt). Termination of a propagating chain at polymerisation of methacrylic monomers proceeds mostly by the mechanism of disproportionation that enables one, finally, to give the approximate equality Vr = P as ... 

Similarly, diblock copolymers involving two (or more) methacrylate monomers can generally by polymerized in the order which is most convenient since the two propagating anions should have similar pK s. For example, diblock copolymers of methyl methacrylate and 4-[4 -(4"-methoxyphenyl-azo)phenoxy] butyl methacrylate (PMMA-PMAzM) were synthesized by polymerizing the mesogenic block first (Scheme 23) [46],... 

Zune C, Jerome R (1999) Anionic polymerization of methacrylic monomers characterization of the propagating species. Prog Polym Sci 24 631-664... 

Side reactions like the one shown above can be minimized by using less nucleophilic initiators and low temperatures. This can yield living polymerizations of aciylic and methacrylic monomers. In addition, it is possible to add conunon ions like LiCl to alkyllithium to tighten the ion pairs of the propagating anion-counterion species. That also increases the tendency to form living poly-mers. This approach, however, offers only limited success. 

The propagation mechanism for the free-radical polsrmerization of methacrylic monomers involves the head-to-tail chain addition of monomer imits to grow the polymer chain. Chain termination can occur by either radical combination or disproportionation (41). 

Well-controlled polymerizations of a large number of monomers have been obtained in copper-catalyzed ATRP with sulfonyl halides as initiators, which yield a much faster rate of initiation than monomer propagation. For example, the apparent rate constants of initiation with sulfonyl chlorides are about four (for styrene and methacrylates) and three (for acrylates) orders of magnitude higher than those for propagation (Matyjaszewski and Xia, 2001). 

With both ri and r2 less than unity (styrene-acrylate, styrene-methacrylate), cross-propagation is favored over homopropagation and the copolymer tends toward an alternating structure. The system has an azeotropic composition at which the copolymer composition is exactly equal to the monomer composition. 

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