Branching by transfer

The direction of each chain s growth is indicated by an arrow, which also marks the point at which transfer terminates its growth. Instances in which branching by transfer may assume importance were cited in Chapter VI. 

The degree of branching by transfer with polymer obviously will increase with the conversion since the relative incidence of branching must depend on the ratio of polymer to monomer in the system. To examine the matter from the point of view of reaction rates, let 6 represent the fraction of monomer molecules which have polymerized out of a total of iVo in the system, and let v represent the total number of branches. (At variance with the definition used elsewhere. No is the total number of units polymerized and unpolymerized as well.) The rates of generation of branches and of polymerization can then be written... 

There is little evidence for branching by transfer to monomer in other polymerizations. 

Glycogen Branching Occurs by Transfer of Terminal Chain Segments... 

Branches in PVC can be formed by transfer to polymer during polymerization. Short branches in PVC have usu-... 

The microstrueture of PVC has been the subject of numerous studies (Sections 4.3.1.2 and 6.2.6.3).214 Starnes el n/.l6S determined the long chain branch points by NMR studies on PE formed by Bu,SnlI reduction of PVC. They concluded that the probable mechanism for the formation of these branches involved transfer to polymer that occurred by hydrogen abstraction of a backbone methine by the propagating radical (Scheme 6.32),... 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

Nonlinear structures may arise in vinyl polymerizations through chain transfer with monomer or with previously formed polymer molecules, but such processes usually occur to an extent which is scarcely significant. A more common source of nonlinearity in the polymerization of a 1,3-diene is the incorporation in a growing chain of one of the units of a previously formed polymer molecule. The importance of both branching by chain transfer and cross-linking by addition of a polymer unit increases with the degree of conversion of monomer to polymer. 

The branched trisaccharide 1 was defined as the minimal carbohydrate epitope for effective binding to anti-GAS Abs [79]. The bound conformation of 1 in complex with the IgG mAb Strep 9 was investigated by transferred NOE experiments and found to correspond to a local minimum conformation, which differed from the predominant conformation in the free state by a rotation around the xj/ angle of the o -(l-2) linkage [123]. STD-NMR ex-... 

Poole1305 has reviewed the bacterial cytochrome oxidases, and has drawn attention to features which are not present in the mitochondrial enzyme, and which reflect the metabolic diversity and adaptability of bacteria. These are (1) the synthesis of the oxidases is controlled dramatically by the prevailing environmental conditions (2) some oxidases are multifunctional, and may use electron acceptors other than dioxygen (3) more than one type of oxidase may be present, each terminating a branched electron-transfer pathway. 

As the amount of polymer in the system increases, the probability of interaction of growing free radicals with polymeric (inactive) species either by transfer or, sometimes, by copolymerization through residual double bonds must increase. Because such reactions automatically give rise to branched and crosslinked species, reliable experimentation aimed at studying these processes is difficult to achieve. This follows from the fact that the region of the reaction in which study is essential is precisely that region in which there are rapid changes in the characteristics of the polymer produced, particularly in solubility. This is of... 

The concentration of modifier will tend to diminish during the reaction, and the more reactive the modifier, the more serious is the effect likely to be. An extension of the Flory treatment of crosslinking to take into account the presence of modifier demonstrates the point. When branches arise by transfer to polymer and crosslinks by the subsequent coupling of branched radicals, it is easy enough to show that the rate of formation of crosslinks, X, as a function of conversion is ... 

After debenzylation (Pd-C, cyclohexene) of the hypercores (e.g., 25) by transfer hydrogenation, they were treated with aryl branched, benzyl ether dendrons (Scheme 5.6) that were prepared by similar iterative transformations,1271 i.e., benzylic bromination and phenolic O-alkylation (See Section 5.4.2). Thus, hexaphenol core 27 was reacted with six equivalents of the benzylic bromide building block 28 to give the benzyloxy terminated dendrimer 29. Key features of these dendrimers include cores with flexible alkyl spacers and a three-directional, quaternary carbon branching center. 

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