Macromonomers free-radical polymerization

Free radical polymerization of macromonomers yields only low degrees of polymerization. This is due to the short life time of the radicals involved, and to the low molar concentration of unsaturations in the medium. 

Kalra et al. studied the synthesis of PPDL graft copolymers following route A as shown in Scheme 6. The macromonomers were obtained by the enzymatic ROP of PDL from HEMA and PEGMA [49]. In a comparative study, Novozym 435 was found to be the most active biocatalyst for this reaction step. Subsequently, graft copolymers were obtained by free-radical polymerization of the macromonomers. A similar approach was published by Srivastava for the HEMA-initiated enzymatic ROP of CL and subsequent free-radical polymerization [50]. 

Initially, the polymerization of macromonomers was achieved by free radical polymerization reactions, which allowed only a limited control of the final properties. With the advent of ROMP and new free radical polymerization techniques, such as atom transfer radical polymerization (ATRP) the control of final properties became more facile (16). ATRP and ROMP techniques can be combined for the synthesis of macroinitiators (17). 

Gramain and Frere [82] observed that the free radical polymerization of co-meth-acryloyl terminated PEO macromonomers in the aqueous phase deviates from the solution polymerization. Polymerizations initiated by KPS in water were much faster than those that proceeded in the solution. Low molecular weight polymers were formed in the aqueous systems (ca. up to 20 macromonomer units were incorporated into polymer molecules). 

The homopolymer and block copolymer macromonomers were copolymerized with MMA by free-radical polymerization in benzene at 60 °C using AIBN as an initiator typical concentration were [MMA]=1.2 mol 1 1 and [macromonomer] =0.020 mol l"1. MMA was completely converted in 18 h and the macromonomers conversion reached more than 70% as determined by lH NMR. Incomplete conversion was explained by steric hindrance. Free-radical copolymerization resulted in high MW graft copolymers with PMMA backbone and relatively rigid, nonpolar poly(P-pinene) branches. 

The use of 2-mercaptoethanol, HS—CH2—CH2- OH, as a transfer agent belongs to the early attempts to synthesize macromonomers by means of free-radical polymerization l0). Methyl methacrylate was employed as the monomer. The formed polymers bear a terminal hydroxy group which was subsequently reacted with methacryloyl chloride. 

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. 

Much attention has been focused on free-radical polymerization in the presence of transfer agents such processes yield -functional precursors that can in turn be reacted with unsaturated compounds carrying an antagonist function. This is the basic principle of what was referred to as a two-step macromonomer synthesis. 

Other teams worked on the functionalization of the aminoxyl group situated at the co position. For instance, the method of Ding et al. [342] is original for the synthesis of a novel series of poly(sodium styrenesulfonate) (PSSNa) macromonomers (compound 3 in Scheme 74) based on stable free radical polymerization in the presence of TEMPO. 

P2VP) molecular brushes via the free radical polymerization of methacryloyl-terminated P2VP macromonomers [14]. (Reprinted with permission of Wiley-VCH.)... 

In 1989, Tsukahara et al reported a pioneering smdy of the free radical polymerization of an oligostyryl macromonomer, that is, an oligostyrene chain terminated with a polymerizable vinyl group at one end. Starting from this, a large number of... 

An example of the macromonomer method is the preparation of graft copolymers of PEO by the free radical polymerization of vinyl acetate in the presence of PEO. The growing vinyl acetate radical would abstract a hydrogen atom from the PEO chain, creating a radical at this site. The newly created radical would then polymerize vinyl acetate to form a branch on the chain. The rather randomly occurring chain transfer reaction would form a graft copolymer of PEO and poly(vinyl acetate). 

Kim et al. [46,47] reported the synthesis of fluorosilicone block copolymers of poly(perfluoroalkylethyl acrylate)-fc-poly(3-[m s(trimethylsilyloxy)-silyl] propyl methacrylates) (PFA-i>-PSiMAs) by a three-step synthetic approach. In the first step, a PFA macromonomer (PFAM) was made by free radical polymerization. Thereafter, a condensation reaction was applied to prepare the PFAM initiator (PFAMI). Finally, the PFAMI and SiMA were reacted to prepare the PFA-i>-PSiMAs block copolymers. In early studies, synthesis of fluorosilicone block copolymers was reported by Boutevin et al. [48-50]. However, two-step hydrosilylation was carried out to prepare the photo-cross-linkahle fluorinated PDMS as reported by Boutevin et al. [48]. In another study, Luo et al. [51] prepared poly(dimethylsiloxane)- -poly(2,2,3,3, 4,4,4-heptafluorobutyl methacrylate- -poly(styrene)... 

Free-radical polymerization has been the most common techniqne for the synthesis of macromonomers becanse of its less demanding experimental conditions, the absence of special purification procednres of reagents used, and its... 

The evolution of the living free-radical polymerization techniques (mainly TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxy) and ATRP methods) very soon led to the synthesis of macromonomers. These methods combine the advantages of the free-radical polymerization with those of the living polymerization techniques, despite the fact that control over the functionalization reaction is not always comparable to the anionic polymerization methods (91,92). 

Table 3. Graft Copolymers Prepared by Macromonomers Synthesized by Free-Radical Polymerization...

Ferrari, R., Yu, Y., MarbideUi, M., Hutchinson, R.A., MoscateUi, D., 2011. e-Caprolactone-based macromonomers suitable for biodegradable nanoparticles synthesis through free radical polymerization. Macromolecules 44 (23), 9205—9212. 

To prepare degradable polymers, graft copolymers of PLA macromonomer and tert-huXy acrylate were prepared by free radical polymerization. An increase in lactic acid units resulted in an increase in degradation rate [84]. ATRP of MMA (96.5%) and (meth)acrylate-terminated PLA macromonomer (Mn 2800g/mol, 3.5%) yielded a homogeneously branched PMMA-g-PLA of low polydispersity index (PDI = 1.15) [85]. The reactivity ratio of MMA for conventional radical polymerization is 1.09 while with ATRP is 0.57. This accounts for the lower PDI of ATRP synthesized PMMA-g-PLA. 

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