Branch formation

Copolymerization of macromonomers formed by backbiting and fragmentation is a second mechanism for long chain branch formation during acrylate polymerization (Section 4.4.3.3). The extents of long and short chain branching in acrylate polymers in emulsion polymerization as a function of conditions have been quantified.20 ... 

The branched polymers produced by the Ni(II) and Pd(II) a-diimine catalysts shown in Fig. 3 set them apart from the common early transition metal systems. The Pd catalysts, for example, are able to afford hyperbranched polymer from a feedstock of pure ethylene, a monomer which, on its own, offers no predisposition toward branch formation. Polymer branches result from metal migration along the chain due to the facile nature of late metals to perform [3-hydride elimination and reinsertion reactions. This process is similar to the early mechanism proposed by Fink briefly mentioned above [18], and is discussed in more detail below. The chain walking mechanism obviously has dramatic effects on the microstructure, or topology, of the polymer. Since P-hydride elimination is less favored in the Ni(II) catalysts compared to the Pd(II) catalysts, the former system affords polymer with a low to moderate density of short-chain branches, mostly methyl groups. 

For molecules at a degree of polymerization n or larger, the mathematical model incorporates branch formation reactions which include a free radical of size j and a polymeric specie of degree of polymerization m n. The consequence is the formation of a free radical of molecular size j + m. Furthermore, due to the relatively high concentration initially of the 1,2-polybutadiene constituent at j = n, the derivation assumes that all polymeric species of size j n are unsaturated and are capable of branch and/or crosslink formation. Polymeric species are denoted by Pj free radical intermediates are described by Aj. Therefore, the first activated intermediate capable of formation by branching reactions is Ajj via Aq + Pj, -> Ajj. Conservation laws yield... 

A set of first order differential equations descriptive of the molar concentrations for polymeric species is given. Species, less than size n, are saturated and, therefore, accmulate only within the batch reactor and do not participate in branch/cross-llnk reactions. Molecules greater in size than n are unsaturated and will experience the reaction described by branch formation. 

The term D3 can be expressed in terms of the relative rates of branch formation plus termination compared to propagation. 

The carbonyl C of RCOOH and RCOOR is trigonal sp -hybridized, but that of the intermediate is tetrahedral sp -hybridized. If R in R OH or R in RCOOH is extensively branched, formation of the unavoidably crowded transition state has to occur with greater difficulty and more slowly. 

Acrylates polymerize two orders of magnitude faster than methacrylates by anion catalyzed GTP however, the polymerization dies at about 10,000 MW. During the anion catalyzed polymerization of acrylates the silyl ketene acetal end groups migrate to internal positions. These ketene acetals are too hindered to act as initiators for branch formation [9]. 

Figure 2 shows the effect of the inoculated root tip part on the growth of P. ginseng hairy roots. The end part, whose apical meristem of root tip had been excised prior to inoculation, was grown to a total biomass 1.6 times greater than that of the center parts, which are the root tip part after being excised about 10 mm from the end part. In all experiments, new lateral branch formation was observed on the inoculated main root tip. 

Amylose is synthesized by granular-bound starch synthase, whereas amylopectin is synthesized by soluble starch synthase (Chapter 4).334,339 Because amylose is synthesized by the granular-bound starch synthase in a progressive manner,340 the amylose molecule is likely confined in the granule and has little opportunity to interact and form double helices with other starch molecules to facilitate branch formation. Branching reactions do occur on some amylose molecules, but at a much lower frequency than with amylopectin, and result in slightly branched amylose molecules. 

Scheme 1.41. Back-biting during high-pressure polymerization of ethylene to LDPE results in n-hutyl branch formation.

In the presence of appreciable amounts of polystyrene branches, the polybutadiene backbone may be shielded by the branches. Under these conditions preferential ring alkylation, i.e., branchy branch formation may occur. Further data to corroborate this assumption are presented below. 

The difference in grafted polystyrene and homopolystyrene molecular weights substantiates the proposition of branchy-branch formation. Due to this circumstance it is not possible to determine branch frequencies in the graft copolymer. 

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