Styrene polymerization catalytic chain transfer

Acrylic Macromonomers. Acrylic-methacrylic macromonomers prepared by catalytic chain transfer using cobalt(II) chelates afford polymer chains, each with an olefinic endgroup (18, 24). Such macromonomers can be polymerized or copolymerized to produce graft polymers that are useful in coatings, fibers, films, and composite materials applications (24). Moreover, one is able to synthesize macromonomers containing several alkylmethacrylates, alkylacrylates, and styrene (IS). 

Catalytic chain transfer in acrylate polymerizations is problematic due to the propensity of acrylates to form stable Co—C bonds between the CCT agent and the propagating radical of both the monomer and its oligomers.268,269,373 It has even been possible to observe a growing polymer chain terminated with a Co—C bond directly by MALDI.374 This bond is stronger than that in the case of styrene. These complications have a direct impact on the use of CCT in acrylic polymerizations. 

Suddaby, K.G., Haddleton, D.M., Hastings, (.)., Richards, S.N., and O Donnell, ).P. (1996) Catalytic chain transfer for molecular weight control in the emulsion polymerization of methyl methacrylate and methyl methacrylate-styrene. Macromolecules,... 

All the above may be illustrated by considering data for bulk, low-conversion polymerization of MMA and styrene (Sty) in the presence of the catalytic chain transfer agent known as To... 

Alkyl Co oxime complexes have been used as chain transfer catalysts in free radical polymerizations.866,867 Regioselective hydronitrosation of styrene (with NO in DMF) to PhCMe=NOH is catalyzed by Co(dmg)2(py)Cl in 83% yield.868,869 Catalytic amounts of the trivalent Co(dmg2tn)I2 (192) (X = I) generate alkyl radicals from their corresponding bromides under mild reaction conditions, allowing the selective preparation of either saturated or unsaturated radical cyclization products.870... 

The catalytic system used to make OBCs uses a chain-shuttling agent (CSA) to shuttle or transfer growing chains between two distinct catalysts with different comonomer (alpha-olefm) selectivity." This is shown in Figure 9. Synthesis of olefin block polymer via chain shuttling requires the chain transfer to be reversible. OBCs are produced in a continuous solution polymerization process more economically favorable than the batch processes employed to make styrenic block copolymers. 

The choice of the most effective catalyst system is highly dependent on the type of olefin under consideration. Polymerization of CO and aliphatic a-olefines is most suitably carried out employing a catalytic system modified with a symmetrical, Cs-bridged aryldiphosphine ligand (2,71). However, these systems are not suitable for copolymerization of CO and styrene (41). For this reaction palladium(II)-based catalysts modified with a conjugated diimine (39,41), a bisoxazoline (43,44), a phosphine-phosphite (43), or a phosphine-imine ligand (43) have been employed, in, under chain-transfer conditions, combination with an oxidant promotor, such as 1,4-benzoquinone or 1,4-naphthoquinone (39-47,72), or a polar, acidic type of solvent (73,74). 

Marks reported that phenylsilane CeHsSiHs acts as a chain transfer agent for homogeneous olefin polymerization catalysts [16]. Newman found that CeHsSiHs increased the catalytic activity in the styrene polymerization [17]. The polymerization conversions with time are shown in Table 4.3. The data... 

Transition metal complexes functioning as redox catalysts are perhaps the most important components of an ATRP system. (It is, however, possible that some catalytic systems reported for ATRP may lead not only to formation of free radical polymer chains but also to ionic and/or coordination polymerization.) As mentioned previously, the transition metal center of the catalyst should undergo an electron transfer reaction coupled with halogen abstraction and accompanied by expansion of the coordination sphere. In addition, to induce a controlled polymerization process, the oxidized transition metal should rapidly deactivate the propagating polymer chains to form dormant species (Fig. 11.16). The ideal catalyst for ATRP should be highly selective for atom transfer, should not participate in other reactions, and should deactivate extremely fast with diffusion-controlled rate constants. Finther, it should have easily tunable activation rate constants to meet sped c requirements for ATRP monomers. For example, very active catalysts with equilibrium constants K > 10 for styrenes and acrylates are not suitable for methacrylates. 

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