Butyl Branch

Figure 1.2 Polyethylene molecule with a short chain (butyl) branch...

Figure 5.11 Example of backbiting reaction to form a butyl branch during the high pressure polymerization of polyethylene...

C are given in Table IV. The relatively greater proportion of 1-butene from ethylene-hexene copolymers indicates that there are two mechanisms for the formation of the butenes, one involving the butyl branches and that this pathway yields a much higher proportion (perhaps 100%) of 1-butene. The C4 hydrocarbons are apparently also formed by fragmentation of chain ends in the polymers. These are probably formed mainly by radiation-induced scission. 

Isolated butyl branches in low-density polyethylene are formed by an intrachain radical rearrangement that is followed by repeated addition of ethylene without further rearrangement. Here, stereochemical selectivity during the formation of CH2R—CH2-CHR—CH2-branches in the free radical initiated polymerization of monosubstituted vinyl monomers is Investigated. The configuration partition functions are denoted by Zm and Zn respectively. They can be written as 2 Um Up Iv1 v2 v3]T and Zr = U U2 Up Ur Up [v3 v2 v3lT. Numerical values... 

HDPE is alinear polymer with the chemical composition ofpolymethylene, (CII2V Depending on application, HDPE molecules either have no branches at all. as in certain injection molding and blow molding grades, or contain a small number of branches which are introduced by copolymerizing ethylene with o-olefins, e g., ethyl branches in the case of 1-butene and -butyl branches in the case of 1-hexene. The number of branches in HDPE resins is low, at most 5 to 10 branches per 1000 carbon atoms in the chain. 

Polyethylenes produced commercially via high pressure, free radical processes have densities around 0.92 g/cc and are referred to simply as "low density" polyethylenes. It has been well established from infrared measurements that these low density polyethylenes possess appreciable quantities of ethyl and butyl branches (1-3) but it was not until C-13 NMR became available that an absolute identification, both qualitatively and quantitatively, of the short branches became possible (4-8). Long chain branching is also present in high pressure process low density polyethylenes and carbon-13 NMR was useful here also in establishing the identity and relative amounts of long versus short chain branches (9-11). 

Polymer "B" is an ethylene-l-hexene copolymer by design. A flow activation energy of 9.6 kcal/mol suggests that long chain branching may be present. The C-13 NMR spectrum, however, is complicated by the presence of butyl branches as in the case of... 

It is to be noted that the reflection assigned to the new phase in butyl branched alkanes is relatively weak compared to the reflections observed for the new phase in ethylene-1-octene copolymer (5.2 mol %). As explained in this chapter, we attribute the new phase to the crystallization of transient layer (butyl branches and fold surface). Considering the anticipated tight folds for butyl branched alkanes, the amount of crystallizable entities in the branched alkanes would be much less than in the ethylene-1-octene copolymers where the loose folds are expected. We would like to mention that, considering the d-value and intensity of the pseudo-hexagonal phase in branched alkanes, this reflection may be referred to as open-orthorhombic phase. 

A second process results in the formation of shorter branches that contain only four carbons. These result when the radical end of a growing polymer chain reaches back and abstracts a hydrogen from itself. Because the cyclic transition state for this abstraction is most favorable when it contains six atoms, four-carbon butyl group branches are formed. The mechanism for the formation of these butyl branches is outlined in Figure 24.2. 

These two branching processes decrease the regularity of the polyethylene macromolecules. Individual polymer chains may have long branches or butyl branches that occur at random positions. As we will see shortly, this irregularity in the structure dramatically affects the physical properties of the polymer. 

Mechanism of the formation of a butyl branch during the polymerization of ethylene. Test yourself on the concepts in this figure at OrganicChemistryNow. 

Polymers Polymers 24.2 Mechanism of the Formation of a Butyl Branch during the... 

Noncyclopentadienyl chromium complexes have also figured in polyolefin catalysis. The /3-diketiminato chromium complexes like (16) polymerize ethylene and copolymerize ethylene and o -olefins in the presence of MAO. The 1,3,5-triazacyclohexane complexes of CrCb (18) polymerize ethylene in the presence of MAO or [HNMe2Ph][B(C6F5)4]/Al-i-Bu3. Some trimers are formed, resulting in butyl branches in the chain higher a-olefins are trimerized. 

The major example of the second branched polymer type is the polyethylene that is made by free-radical polymerization at temperatures between about 100 and 300°C and pressures of 1000-3000 atm (100-300 MPa). Depending on reaction conditions, these polymers will contain some 20 to 30 ethyl and butyl branches... 

Figure 2.2 Mechanism of "backbiting" in formation of short chain branching initiated by attack of radical on a 5 carbon-hydrogen bond. In the reaction above, homolytic bond scission occurs resulting in a free radical on the 5 carbon atom and an n-butyl branch. R is a polymeric alkyl group.

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