Hexyl branches

We use carbon-13 NMR spectrometry to identify the monomer units present in copolymers, their absolute concentrations, the probability that two or more monomer units occur in proximity, and long chain branching concentrations. For instance, in the case of polyethylene, we can not only distinguish and quantify ethyl, butyl, and hexyl branches, but we can also determine whether branches are present on carbon backbone atoms separated by up to four bonds. We can compare the observed adjacency of branches to a theoretical value calculated for random comonomer incorporation. By this method, we can determine whether comonomers are incorporated at random, as blocks, or in some intermediate fashion. 

We can incorporate short chain branches into polymers by copolymerizing two or more comonomers. When we apply this method to addition copolymers, the branch is derived from a monomer that contains a terminal vinyl group that can be incorporated into the growing chain. The most common family of this type is the linear low density polyethylenes, which incorporate 1-butene, 1-hexene, or 1-octene to yield ethyl, butyl, or hexyl branches, respectively. Other common examples include ethylene-vinyl acetate and ethylene-acrylic acid copolymers. Figure 5.10 shows examples of these branches. 

The previous sections in this chapter have tried to stress upon the significance of distribution of sequence lengths in polyethylene-based copolymers. The sequence length of interest in a system of ethylene-octene copolymers would be the number of methylene units before a hexyl branch point. As was discussed, this parameter has a greater impact on the crystallization behavior of these polymers than any other structural feature like branch content, or the comonomer fraction. The importance of sequence length distributions is not just limited to crystallization behavior, but also determines the conformational,... 

Coordination copolymerization of ethylene with small amounts of an a-olefin such as 1-butene, 1-hexene, or 1-octene results in the equivalent of the branched, low-density polyethylene produced by radical polymerization. The polyethylene, referred to as linear low-density polyethylene (LLDPE), has controlled amounts of ethyl, n-butyl, and n-hexyl branches, respectively. Copolymerization with propene, 4-methyl-1-pentene, and cycloalk-enes is also practiced. There was little effort to commercialize linear low-density polyethylene (LLDPE) until 1978, when gas-phase technology made the economics of the process very competitive with the high-pressure radical polymerization process [James, 1986]. The expansion of this technology was rapid. The utility of the LLDPE process Emits the need to build new high-pressure plants. New capacity for LDPE has usually involved new plants for the low-pressure gas-phase process, which allows the production of HDPE and LLDPE as well as polypropene. The production of LLDPE in the United States in 2001 was about 8 billion pounds, the same as the production of LDPE. Overall, HDPE and LLDPE, produced by coordination polymerization, comprise two-thirds of all polyethylenes. 

Type of branches—1-butene comonomer gives ethyl branches, 1-hexene comonomer gives butyl branches, and 1-octene comonomer gives hexyl branches. The length of branches has a small influence, but the number of branches is more important since the branches do not take part in crystallization. 

A similar branching effect on the miscibility was observed in the HDPE/ LLDPE-O blends, which were prepared from HDPE (49,400 and 3.60 PDI) and LLDPE-O polymers (69,200-104,000 M and 1.8-3.40 PDI) with 2-87 hexyl branches per 1000 backbone carbons (42). It was observed that the critical branch number in the LLDPE-O component capable of causing immiscibility in the HDPE/ LLDPE-O blend was 50 branches per 1000 backbone carbons, as determined using inverse gas chromatography, rather than the SANS technique. 

Separate crystallization was also reported for blends of LDPEs with LLDPEs having butyl and hexyl branches. Using DSC analysis, Drummond et al. (71) observed that separate crystallization of the individual blend components takes place in the blends of LLDPE-H (146,000 Mw, 3.1 PDl, and 19.9 butyl branches per 1000 backbone carbons) and LDPE (78,700 Mw, 31.0 PDl, and 20.1 branches per 1000 backbone carbons). The DSC studies of Lee et al. (43) revealed that for blends of LLDPE-O (93,100 M , 3.6 PDl, and 15-16 branches per 1000 backbone carbons) with LDPEs (73,000-98,000 M and 8.7-9.2 PDl 32-34 branches per 1000 backbone carbons), the blend components also undergo separate crystallization. Hill et al. (69) reported similar separate crystallization behavior in blends of LLDPE-O (40,000 M , 4.2 PDl, and 15 hexyl branches per 1000 backbone carbons) and LDPE (112,000 M , 12.0 PDl, and 15 short and 10 long branches per 1000 backbone carbons). 

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. 

Degree of branching, commercial resins mol% NMR, ethyl, butyl, and hexyl branches 0.5-7.0 (3,15)... 

Figure 4.6 shows the typical portions of the expanded pyrograms around Cn fragments for a LDPE and those of five model copolymers for methyl, ethyl, butyl, amyl, and hexyl branches. Once relative peak intensities characteristic of the SCBs are determined using well-defined model polymers with known amounts of possible SCBs, the relative abundance of the SCBs in the LDPE can easily be estimated by the peak simulation of the observed isoalkane peaks in the pyrogram of the LDPE. ... 

Figure 4.4 High field C-NMR spectra of low-density polyethylene. (A) Ethyl branched copolymer (B) hexyl branched copolymer (C) LDPE.

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