Side-chain branching linear polyethylenes

Polyethylene is a major commodity plastic, with more than 33 billion pounds of the resin produced in the United States in 2000 [1]. Polyethylene encompasses a family of semi crystalline polymers with ethylene as the major building block [6]. The resins are loosely grouped into three classes low-density polyethylene (LDPE), high-density polyethylene (HD.PE), and linear-low-density polyethylene (LLDPE). LDPE is a homopoiymer of ethylene with side-chain branching at a frequency... 

Linear polyethylenes are produced in solution, slurry, and increasingly, gas-phase low-pressure processes. The Phillips process developed during the mid 1950s used supported chromium trioxide catalysts in a continuous slurry process (or particle-form process) carried out in loop reactors. Earlier, Standard Oil of Indiana patented a process using a supported molybdenum oxide catalyst. The polyethylenes made by both these processes are HDPE with densities of 0.950-0.965 g/cm and they are linear with very few side-chain branches and have a high degree of crystallinity. 

The more recently developed so-called linear low-density polyethylenes are virtually free of long chain branches but do contain short side chains as a result of copolymerising ethylene with a smaller amount of a higher alkene such as oct-1-ene. Such branching interferes with the ability of the polymer to crystallise as with the older low-density polymers and like them have low densities. The word linear in this case is used to imply the absence of long chain branches. 

Most commercial polymers are substantially linear. They have a single chain of mers that forms the backbone of the molecule. Side-chains can occur and can have a major affect on physical properties. An elemental analysis of any polyolefin, (e.g., polyethylene, polypropylene, poly(l-butene), etc.) gives the same empirical formula, CH2, and it is only the nature of the side-chains that distinguishes between the polyolefins. Polypropylene has methyl side-chains on every other carbon atom along the backbone. Side-chains at random locations are called branches. Branching and other polymer structures can be deduced using analytical techniques such as NMR. 

Random ethylene copolymers with small amounts (4-10 wt-%) of 7-olefins, e.g. 1-butene, 1-hexene, 1-octene and 4-methyl- 1-pentene, are referred to as linear low-density polyethylene, which is a commercially relevant class of polyolefins. Such copolymers are prepared by essentially the same catalysts used for the synthesis of high-density polyethylene [241]. Small amounts of a-olefin units incorporated in an ethylene copolymer have the effect of producing side chains at points where the 7-olefin is inserted into the linear polyethylene backbone. Thus, the copolymerisation produces short alkyl branches, which disrupt the crystallinity of high-density polyethylene and lower the density of the polymer so that it simulates many of the properties of low-density polyethylene manufactured by high-pressure radical polymerisation of ethylene [448] (Figure 2.3). 

The branched polymeric structure shows a conformation for the polymer molecule, where branches or side chains radiate to from a linear polymer. In this case, the monomer, a, is consecutively assembled in the linear chain, and the monomer, p, forms the branches (see Figure 2.42), where both monomers a and p can be identical as in the case of low-density polyethylene. 

Besides crystallinity itself, the kind and amount of side chains is of importance for crazing. A schematic investigation of the influence of chain branches was performed in PE that were introduced via copolymerization [84], A long-chain branched low-density polyethylene (PE-LD) was compared to a linear low-density polyethylene (PE-LLD) with different short chains [85]. Type and concentration of the copolymers were chosen in order to attain the same density of 0.920 g/cm3 and melt flow rate of 25 g/10 min for all polymers. The ESC resistance was measured in a long-term tensile test of notched specimens at 50 °C in 10% Igepal solution. 

Some polymerisations do not continue in a linear fashion, and branching occurs, as shown in Fig. 1.7(a). When branching is prevalent, it can have a serious effect on properties. Thus the polymerisation of ethylene under high-pressure conditions gives a product that has so many side chains on each main chain that crystallisation is appreciably suppressed. This material is softer than the highly crystalline linear form from a catalytic low-pressure process. The two forms of polyethylene may be distinguished by a difference in density between them, the more crystalline material being denser. 

The polyethylene formed in this way is not the perfect linear chain implied by this simple equation. Free radicals frequently abstract hydrogen from the middles of chains in this synthesis, so the polyethylene is heavily branched with hydrocarbon side chains of varying length. It is called low-density polyethylene (LDPE) because the difficulty of packing the irregular side chains gives it a lower density (<0.94 g... 

This radical can now add monomer at the carbon atom next to the R group and thus produce a polymer with a short side chain. (This branched structure is typical of the ordinary polyethylene used in squeeze bottles.) Linear polyethylene produced at low temperatures has a much more rigid structure. Generally speaking, the molecules with a more regular structure produce a more rigid bulk material. 

In polymers with side groups that are a part of the regular structure of one of the monomers, as in the case of polystyrene, polypropylene, polybutyl methacrylate, etc., we generally do not consider these as branches or side chains. Instead, they are called side groups. An exception to this rule is for polyethylene/a-olefin copolymers, the LLDPE family, where the side group is referred to as a side chain. The reason for this exception is that the polymers look exactly like those that could be made by a side chain growth reaction. In fact, it is common for books to list LLDPE as a branched polymer, even though its name, linear low density polyethylene, clearly and correctly describes it as a linear polymer. 

When the layer spacing values are plotted against side chain length, i.e. the number of side branch atoms, a good linearity is obtained, which can be expressed as y=4.90+1.13x. The intercept value 4.90(A) means the layer spacing of Cp-PAM without any side branch appended. This value is known to be 4.7SA. The two values concide well with each other within experimental eiTor. The slope value 1.13(A/atom) indicates an increment of layer spacing per side branch atom. The increment per CHj group in polyethylene is known to be I.25A/CH2. Between these two increment values there is a big difference. 

Polyethylene, the prototype or model chain for linear addition polymers, is composed of ethylene monomer units linked by covalent bonds to form long chains (Figure 1.1a.) But, this linear chain was only produced in the mid 50 s by the Ziegler-Natta catalysts. Before this, the Fawcett process that required high pressure and temperature produced a polyethylene chain that contained many small side chains or branches attached by covalent bonds to the main chain (Figure 1.1b). Nuclear... 

Radical polyethylene does not have a purely linear structure as shown in Fig. 2. If one chain end comes into contact with a lone electron on the inner part of another chain, an H atom may be removed and a side chain substituted for it. High pressure polyethylene therefore has a branched molecular structure and a resulting low density of 0.915-0.94 g/cm. ... 

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