Ethyl-branched polymers

Short branches, specifically ethyl branches up to about 2 mol%, are formed in the polymerization of ethylene by meso-ansa zirconocenes containing unsubstituted cyclo-pentadienyl and indenyl ligands [Melillo et al., 2002]. Ethyl branches form by an isomerization process in which the usual P-hydride transfer to monomer is immediately followed by reinsertion of the vinyl-terminated polymer into the formed ethyl-zirconium bond. 

As shown in Figure 12.1, ldpe is a highly branched polymer. Infrared spectroscopic studies indicate that there are about 60 ethyl and butyl groups for a molecule of LDPE with a DP of 1000. 

The relative sensitivity of short-chain alkyl branches of different sizes to elimination on irradiation with formation of the corresponding alkane has been variously reported as being constant or varying (13,14). Figure 9 compares G values for formation of the alkane corresponding to the short-chain branch from samples of these three polymers with branch frequencies from 0.5 to 6 per 1,000 C atoms. There is a notably higher scission efficiency for ethyl branches. 

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. 

As was the case for polymer "E", a small amount of ethyl branching was detected in MBS 1483 as indicated in Figure 10 by the resonances observed for the appropriate methine, a2 6 and 2B carbons. (The methyl resonance is not shown.) The concentration of ethyl branches is 3 per 10,000 carbons as determined using a modified version of Equation 4. NBS 1483 was probably prepared using a Ziegler type catalyst system, as suggested by the rela-... 

Copolymers may also be produced with a catalyst containing both chromium oxide and nickel oxide supported on silica-alumina. It is well-known that nickel oxide—silica—alumina by itself makes predominantly butenes from ethylene. In the mixed catalyst, butenes that are formed on nickel oxide copolymerize with ethylene on the chromium oxide to form ethylene-butene copolymers. The fact that infrared shows only ethyl branching in the polymer indicates that the initial product... 

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... 

Hgure 2.11 (a) Linear and ethyl-branched monomers that, when copolymerized together at various ratios, give polymers with a wide range of different ethyl branch con-... 

Tirpak, G. a. Position of ethyl branches in conventional polyethylene made by the free radical process. J. Polymer Sci. 3B, 371 (1965). 

High-density polyethylene has less spurious branching than the low-density material, about 0.3-5 ethyl branches per 1,000 carbon atoms. The much lower extent of branching in HOPE enables closer packing of the polymer chains in the solid state. Closer packing explains both the higher density and the higher crystallinities observed for this material as compared to those of low-density 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. 

An example of this behavior is shown in Table 11. In the reported experiments, a typical Cr/silica catalyst was tested for ethylene polymerization with small amounts of butene added to the reactor. Three different butene isomers were used in three series of experiments 1-butene, 2-butenes (cis and trans), and isobutylene. In the first series, as 1-butene was added to the reactor, the density of the polymer declined significantly, indicating the presence of ethyl branches on the chains from the incorporation of the comonomer (branching disrupts crystallinity and creates more amorphous polymer, which lowers the average density). The MI values of the polymers in this series went up as 1-butene was added, as would be expected from the greater ease with which a (3-hydride can be abstracted from the tertiary carbon resulting from 1-butene incorporation. This is the behavior typical of all a-olefin comonomers. 

Kyu et al. (2) and Ree et al. (3,5) studied blends of LLDPE-B (114,000 M , 4.50 PDI, and 18 ethyl branches per 1000 backbone carbons) and LDPE (286,000 M, 15.98 PDI, and 26 short and 1.6 long branches per 1000 backbone carbons) using in situ small-angle light scattering (SALS) and DSC and found that these blends are miscible across the whole composition range. Similar miscibihty results were reported for LDPE blends with LLDPE-B and LLDPE-O polymers by other research groups. The DSC and DMTA analysis of Lee et al. (43) confirmed that the blends of LLDPEs (LLDPE-B 89,300 M, 3.8 PDI, and 15-16 branches per 1000 backbone carbons LLDPE-O 93,100 M, 3.6 PDI, and 15-16 branches per 1000 backbone carbons) and LDPEs (73,000-98,000 and 8.7-9.2 PDI 32-34 branches per 1000... 

The CROP of N-tetrahydropyranylazlridine results in the tetrahydropyran-protected linear poly(ethylenimine) and, thus, provides straightforward access to linear poly(ethylenimine) by acidic removal of the tetrahydropy-ran groups (Scheme 8.23) [136]. As discussed previously, linear poly(ethylenimine) cannot be prepared by simply polymerizing ethylenimine because of transfer reactions resulting in branched polymers. Poly(ethylenimine)s with polymerizable methacrylate side groups have been reported based on the CROP of 2-(l-aziridinyl)ethyl methacrylate (Scheme 8.23) [137]. Subsequent radical polymerization of the methacrylate moieties results in densely crosslinked... 

The property gap that exists between HDPE and LDPE has been filled by LLDPE. This polymer can be prepared by solution- or gas-phase polymerization, and is actually a copolymer of ethylene with 8 to 10% of an a-olefin, such as but-1-ene, pent-l-ene, hex-l-ene, or oct-l-ene. This produces a chain with a controlled number of short-chain branches and densities intermediate between HDPE and LDPE, thereby allowing it to be prepared in various grades by controlling the type of the comonomer. Thus, the use of oct-l-ene gives a lower-density product than that obtained when but-l-ene is incorporated in the chain because the longer (hexyl) branch in the former pushes the chains further apart than the ethyl branch of the latter, hence lowering the packaging efficiency of the chains. 

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