Chain branching during polymerization

More recent studies have shown the consistency of the crystal structure of PAn/ CSA to be variable and highly dependent on the conditions of film preparation. Sara-vanan and coworkers197 have reported a crystallinity as high as 56% in CSA-doped PAn. Djurado and coworkers198 have speculated that the influence of chain branching during polymerization may play a role in the nature of the crystal structure formed. 

Figure 5.13 Example of backbiting reaction to form a long chain branch during the high pressure polymerization of polyethylene...

A major example of the second branched polymer type is the polyethylene that is made by free radical polymerization at temperatures of 100-300°C and pressures of 1,000-3,000 atm. The extent of branching varies considerably depending on reaction conditions and may reach as high as 30 branches per 500 monomer units. Branches in polyethylene are mainly short branches (ethyl and butyl) and are believed to result from intramolecular chain transfer during polymerization (described later in Chapter 5). This branched polyethylene, also called low-density polyethylene (LDPE), differs from linear polyethylene (high-density polyethylene, HDPE) of a low-pressure process so much so that the two materials are generally not used for the same application. 

In vinyl acetate (VA) bulk or solution polymerization systems, side reactions (e.g., chain transfer or termination) will inevitably occur to produce highly branched poly(vinyl acetate) (PVA), based on the nonconjugated nature of the propagating radical. However, the polymerization of VA in nanochannels of [Cu2(terephthalate)2ted] effectively suppresses chain branching during the polymerization, and this results in a constrained chain growth in the narrow 1-D nanochannels [26]. 

Both natural and synthetic rubbers normally have a gel component, which is a part that remains undissolved in a solvent (61,62). The gel component is probably produced by chain branching during the polymerization process or by slight cross-linking when handling rubbers. The most common example is seen in unmilled natural rubbers. When such a component is present, SEC analysis affords only the molecular weight data on the soluble fraction, excepting the gel fraction. In this case, to understand the viscoelastic properties of the rubbers connected with the SEC data is not appropriate because the gel contributes to these properties. Studies of the influence of the gel fraction on the mechanical properties of natural... 

End Groups and Branching. Both saturated and unsaturated end groups can be formed during polymerization by chain transfer to monomer or polymer and by disproportionation. Some of the possible chain end groups are... 

Shimizu and Ohtsu [69] have proposed a chemical method to determine head-to-head structures in PVC. Mitani et al. [70] found 2.5-7.0 head-to-head structures per 1,000 monomer units, increasing with the polymerization temperature. It has not been possible to detect internal head-to-head structure by C-NMR spectroscopy with the detection limit of 2 per 1,000 monomer units [71]. Starnes et al. [71] found evidence for the absence of neighboring methylene groups by C-NMR spectroscopy. However, the proposed reaiTangement of head-to-head units at the radical chain ends resulting in chloromethyl branches [Eq. (6)] would partially explain their consumption during polymerization and their absence in the final product. 

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 ... 

We can create crosslinks during chain growth polymerization by copolymerizing dienes with vinyl monomers. When the two vinyl functions of the diene are incorporated into separate chains, a crosslink is formed. This process is shown in Fig. 2.18. When we use a low concentration of dienes, we produce a long chain branched polymer, while high concentrations of dienes create a highly crosslinked polymer network... 

We can introduce short chain branching into polymers by three methods copolymerization, "backbiting , and chemical modification. The first two occur during polymerization, while the last requires a secondary chemical reaction. Short chain branches have well defined chemical structures, the nature of which we can accurately determine via analytical methods or know, from the structure of the reactants. 

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