Azobisisobutyronitrile chain transfer

In the presence of radical initiators such as benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN), persulfates (S208 ), etc., grafting of vinyl monomers onto polymeric backbones involves generation of free radical sites by hydrogen abstraction and chain transfer processes as described below ... 

The hydrogen abstraction from —SH groups is faster than from —OH groups. Hebeish et al. [9] and Misra et al. [10,11] reported the chain-transfer method of initiation of graft copolymerization onto cellulosic substrates with azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO) as initiators. 

To make further use of the azo-initiator, tethered diblock copolymers were prepared using reversible addition fragmentation transfer (RAFT) polymerization. Baum and co-workers [51] were able to make PS diblock copolymer brushes with either PMMA or poly(dimethylacrylamide) (PDMA) from a surface immobihzed azo-initiator in the presence of 2-phenylprop-2-yl dithiobenzoate as a chain transfer agent (Scheme 3). The properties of the diblock copolymer brushes produced can be seen in Table 1. The addition of a free initiator, 2,2 -azobisisobutyronitrile (AIBN), was required in order to obtain a controlled polymerization and resulted in the formation of free polymer chains in solution. 

Fig. 3-5 Dependence of the degree of polymerization of styrene on the polymerization rate. The effect of chain transfer to initiator is shown for t-butyl hydroperoxide (o), cumyl hydroperoxide ( ). benzoyl peroxide ( ), and azobisisobutyronitrile ( ) at 60°C. After Baysal and Tobolsky [1952] (by permission of Wiley-Interscience, New York).

MMA macromers were prepared by using a cobalt chain-transfer catalyst (CoCTC) and azobisisobutyronitrile (AIBN) as the... 

Favier [2] used t-butyl dithiobenzoate, (II), as the chain transfer agent with 2,2 -azobisisobutyronitrile to prepare poly(A -acryIoyI morpholine), (111), having a Mn > 200,000 daltons with a polydispersity of 1.4. 

Polystyrene was prepared by Benicewicz [3] by bulk polymerization with 2,2 -azobisisobutyronitrile and had a polydispersity of 1.07 using either the low odor thioester, (IV), or thiourethane, (V), as chain transfer agents. 

Reversible addition-fragmentation chain transfer (RAFT) polymerization using 2,2 -azobisisobutyronitrile and either A, A-dimethyl-5-thiobenzoylthiopropionamide or A-dimethyl-5-thiobenzoylthioacetamide as chain transfer agents has been used to prepare low polydispersity poly(A, A-dimethylacrylamide). The chain transfer agents were unusually effective in suppressing free radical termination reaction, thereby mimicking a living polymerization reaction. 

Methyl acrylamidoglycolate methylether (MAGME) (American Cyan-amid) was filtered while warm and recrystallized from xylene (mp 70 73 C). All other monomers were freed from inhibitor on an aluminium oxide column. p-Trimethylsilylstyrene was synthesized from p-chlorostyrene using a Grignard reaction and chlorotrimethylsilane. Azobisisobutyronitrile (AIBN) was used as free-radical initiator in all polymerizations and carbon tetrabromide as chain-transfer agent. Polymerizations were carried out at 60 C in a 50 50 mixture of toluene and butanol under a nitrogen atmosphere. The reaction was carried out for 3,5 h and the formed polymer precipitated in cold diethylether. It was then redissolved and repreciptated prior to further use. once more IR spectra were recorded on a Perkin Elmer 1710 FTIR and NMR on a 200 MHz Bruker WP 200. FTIR was used to determine the co-polymer composition. 

Whether chain transferring can take place to an initiator depends upon its chemical structure. It was believed in the past that chain transferring to cf,a -azobisisobutyronitrile does not occur. Later, it was shown that chain transferring to this initiator does occur as well, at least in the polymerizations of methyl methacrylate. ... 

The reactivity of the initiating radicals toward the backbones can vary and this can also change the efficiency of grafting. Benzoyl peroxide initiated polymerizations of methyl methacrylate monomer, for instance, in the presence of polystyrene yield appreciable quantities of graft copolymers. Very little graft copolymers, however, form when di-r-butyl peroxide initiates the same reactions. Azobisisobutyronitrile also fails to yield appreciable quantities of graft copolymers. This is due to very inefficient chain transferring to the polymer backbones by r-butoxy and isobutyronitrile radicals. 

A RAFT polymerization system consists of initiator, monomer, chain transfer agent, solvent, and temperature. RAFT polymerization can be performed by simply adding a chosen quantity of an appropriate RAFT agent (thiocarbonylthio compounds) to a conventional free radical polymerization. Usually the same monomers, initiators, solvents and temperatures can be used. Because of the low concentration of the RAFT agent in the system, the concentration of the initiator is usually lower than in conventional radical polymerization. Radical initiators such as Azobisisobutyronitrile(AIBN) and 4,4 -Azobis(4-cyanovaleric acid)(ACVA) are widely used as the initiator in RAFT. RAFT polymerization is known for its compatibility with a wide range of monomers compared to other controlled radical polymerizations. These monomers include (meth)acrylates, (meth)... 

More recently it has been shown (6, 7) that zinc dialkyl dithiophosphates also act as chain-breaking inhibitors. Colclough and Cunneen (7) reported that zinc isopropyl xanthate, zinc dibutyl dithiocarbamate, and zinc diisopropyl dithiophosphate all substantially lowered the rate of azobisisobutyronitrile-initiated oxidation of squalene at 60°C. Under these conditions, hydroperoxide chain initiation is negligible, and it was therefore concluded that inhibition resulted from removal of chain-propagating peroxy radicals. Also, consideration of the structure of these zinc dithioates led to the conclusion that no suitably activated hydrogen atom was available, and it was suggested that inhibition could be accounted for by an electron-transfer process as follows ... 

Shaver and coworkers [319] investigated the mechanism of bis(imino)pyridine ligand framework for transition metal systems-mediated polymerization of vinyl acetate. Initiation using azobisisobu-tyronitrile at 120°C results in excellent control over poly(vinyl acetate) molecular weights and polymer dispersities. The reaction yields vanadium-terminated polymer chains which can be readily converted to both proton-terminated poly(vinyl acetate) or poly(vinyl alcohol). Irreversible halogen transfer from the parent complex to a radical derived from azobisisobutyronitrile generates the active species. 

The vast majority of azopolymers developed for optical storage are polyacrylates and polymethacrylates, which are generally prepared by free radical chain polymerization in solution using conventional experimental conditions. For example, azobisisobutyronitrile (AIBN) is used as a thermal initiator in dry organic solvents such as A(A-dimethylformamide (DMF), tetrahydrofuran (THF) or dioxane as the most common. Occasionally, the polymerization process of azobenzene (meth)acrylates can be limited by the radical transfer reaction promoted by the azo group, which seems to be associated with the formation of hydrazyl radicals (Nuyken and Weidner, 1986 Hallensleben andWeichart,1989). 

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