Styrene, dual initiator

Becer CR, Paulus RM, Hoppener S et al. (2008) Synthesis of poly(2-ethyl-2-oxazoline)-h-poly(styrene) copolymers via a dual initiator route combining cationic ring opening polymerization and atom transfer radical polymerization. Macromolecules 41 5210-5215... 

Apart from ATRP, the concept of dual initiation was also applied to other (controlled) polymerization techniques. Nitroxide-mediated living free radical polymerization (LFRP) is one example reported by van As et al. and has the advantage that no further metal catalyst is required [43], Employing initiator NMP-1, a PCL macroinitiator was obtained and subsequent polymerization of styrene produced a block copolymer (Scheme 4). With this system, it was for the first time possible to successfully conduct a one-pot chemoenzymatic cascade polymerization from a mixture containing NMP-1, CL, and styrene. Since the activation temperature of NMP is around 100 °C, no radical polymerization will occur at the reaction temperature of the enzymatic ROP. The two reactions could thus be thermally separated by first carrying out the enzymatic polymerization at low temperature and then raising the temperature to around 100 °C to initiate the NMP. Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-MeCL for the synthesis of chiral block copolymers. 

Also, Kerep and Ritter reported a radical chain transfer agent as a dual initiator, FRP-1 [45]. The first step builds on the fact that hydroxyl groups are much better nucleophiles in enzymatic ROP than thiols. Due to the chemoselectivity of the enzyme, PCLs with predominantly thiol endgroups were obtained, which were subsequently used as macroinitiator for styrene. The authors report that the reaction yield can be further increased by microwave irradiation. Although thiols provide less control over the radical polymerization than RAFT agents, the subsequent radical polymerization successfully leads to the synthesis of PCL-Z -PS. 

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

More recently, Heise and Howdle introduced a new strategy to overcome the limitations of the above-described dual-initiator system, by shifting from a metal-catalyzed AROP to an enzymatic AROP. These authors reported that both sequential and simultaneous enzymatic AROP and copper-catalyzed ATRP in toluene of CL and styrene [202], of supercritical CO2 of CL and MMA [203, 204], or of a semifluorinated MA [205] monomer, were possible. In the same way, simultaneous enzymatic AROP and RAFT polymerization of CL and styrene, using a hydroxyl-functionalized trithiocarbonate, resulted in the formation of block copolymers with narrow polydispersities [206]. 

In a paper by Cao and Lee [85], the use of a commercial carboxylated poly(vinyl acetate) as a low-profile additive for a cold-curing UPR system with a styrene monomer was described. A dual-initiator system consisting of methylethylketone peroxide and ferf-butylperoxybenzoate with a cobalt promoter and benzoquinone inhibitor was applied. A potential increase in residual styrene content was taken into account in addition to the low-profile characteristics. Morphology of the UPR samples cured in the temperature range of 35-100 °C was studied. 

Knoop RJl, Habraken GJM, Gogibus N, Steig S, Menzel H et al (2008) Synthesis of poly (benzyl glutamate-b-styrene) rod-coil block copolymers by dual initiation in one pot. J Polym Sci A Polym Chem Ed 46 3068-3077... 

Miktoarm stars of the A(BC)2 type, where A is PS, B is poly(f-bulyl acrylate) (PtBA), and C is PMMA [161] have been synthesized, by using the trifunctional initiator 2-phenyl-2-[(2,2,6,6-tetramethyl)-l-piperidinyloxy] ethyl 2,2-bis[methyl(2-bromopropionato)] propionate (NMP, ATRP) (Scheme 86). In the first step, a PS macroinitiator with dual < -bromo functionality was obtained by NMP of styrene in bulk at 125 °C. This precursor was subsequently used as the macroinitiator for the ATRP of ferf-bulyl acry-... 

Polyester resin coatings are synthesized with components that introduce unsaturation into the polymer chain (—C=C—). The paint is manufactured by mixing a dissolved polyester resin in styrene monomer with pigment and reaction inhibitor. Additional styrene and peroxide are packaged in a separate container and are mixed with the paint when applied using a dual-headed spray gun. Peroxide serves as a radical polymerization initiator for the polyester resin with monomeric styrene and cross-linking. Figure 13.5 shows the chemical structure of an isophthahc polyester resin. 

The polymerization for polyaddition of a monomer that possesses an additional functionality allows the production of dual-function particles. The acyl chloride of the azo-initiator 4,4 -azo-4-cyanopentanoic add was reacted with 2,4-diethyl-l,5-pentanediol to yield a diol-functionahzed monomer, in addition to the azo-bond functional groups [118]. The functionahzed diol was first polymerized in a polyaddition reaction with a diisocyanate subsequently, it was possible to cleave the azo-bonds and to polymerize styrene in the nanodroplets. Such an approach combines free-radical polymerization and polyaddition, to produce hybrid block-copolymer particles. 

Montei and coworkers [240] reported that Nickel complexes [(X,0)NiR(PPh3)] (X = N or P), designed for the polymerization of ethylene, are effective for home- and copolymerization of butyl acrylate, methyl methacrylate, and styrene. Their role as radical initiators was demonstrated from the calculation of the copolymerization reactivity ratios. It was shown that the efficiency of the radical initiation is improved by the addition of PPhs to the nickel complexes as well as by increasing the temperature. The dual role of nickel complex as radical initiators and catalysts was exploited to succeed in the copolymerization of ethylene with butyl acrylate and methyl methacrylate. 

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