Copolymers ethylene with 1-alkenes

We have studied the alkane and alkene yields from the radiolysis of copolymers of ethylene with small amounts of propylene, butene and hexene. These are examples of linear low density polyethenes (LLDPE) and models for LDPE. Alkanes from Ct to C6 are readily observed after irradiation of all the polymers in vacuum. The distribution of alkanes shows a maximum corresponding to elimination of the short-chain branch. This is illustrated in Figure 8 for the irradiation of poly (ethylene-co-1-butene) containing 0.5 branches per 1,000 carbon atoms at 20 C. 

Random copolymers of ethylene and ct-olefins (1-alkenes) can be obtained with Ziegler-Natta catalysts, the most important being those of ethylene and 1-butene (LLDPE) and ethylene with propylene (EPM or EPR and EPDM). Representative reactivity ratios are presented in Table 9.7. It is seen from these values that ethylene is much more reactive than higher alkenes, and the ratios vary with the nature and physical state of the catalyst. In most Instances, riV2 is close to unity. Heterogeneous Ziegler-... 

Ethylene can be copolymerized with alkene compounds or monomers containing polar functional groups, such as vinyl acetate and acrylic acid. Branched ethylene/ alkene copolymers are essentially the same as LDPE, since in commercial practice a certain amount of propylene or hexene is always added to aid in the control of molecular weight. 

In this section we survey the use of metallocenes as catalyst precursors for polymerizations in which the predominant monomer is not ethylene or any a-olefin. The subject of metallocenes as initiators for the cationic polymerization of vinyl monomers is dealt with elsewhere. Except for styrene, true random copolymers of the monomers in this section with alkenes are not produced because the mechanisms for polymerization of the non-olefins differ greatly from that operating in Sinn-Kaminsky catalysis. 

The tensile properties of homogeneous ethylene-alkene statistical copolymers were further explored by Kennedy et al. [64]. It has been shown that the yield stress of a semicrystalline polymer is dependent on its crystal thickness [65, 66]. During uniaxial compression, the yield stress of polyethylene was observed to increase with crystal thickness up to 40 nm, whereas for thicker crystals it leveled off [66]. Since the crystal thickness in homogeneous copolymers decreases with increasing counit concentration, not surprisingly, an inverse correlation was also found between yield stress and counit content [64]. 

The extensive overall crystallization kinetics that have been reported for blends of linear polyethylene with random ethylene-1-alkene copolymers cannot be analyzed in a consistent manner.(40) Hence, they are not reported here. The reason is that the crystallization temperatures were expressed in terms of an arbitrarily defined undercooling, Tq — Tq, where To is the peak crystallization temperature obtained by dynamic cooling. 

Materials that typify thermoresponsive behavior are polyethylene—poly (ethylene glycol) copolymers that are used to functionalize the surfaces of polyethylene films (smart surfaces) (20). When the copolymer is immersed in water, the poly(ethylene glycol) functionaUties at the surfaces have solvation behavior similar to poly(ethylene glycol) itself. The abiUty to design a smart surface in these cases is based on the observed behavior of inverse temperature-dependent solubiUty of poly(alkene oxide)s in water. The behavior is used to produce surface-modified polymers that reversibly change their hydrophilicity and solvation with changes in temperatures. Similar behaviors have been observed as a function of changes in pH (21—24). 

Several reports in which NHC-Pd complexes have been employed to catalyse the copolymerisation of alkenes with CO have appeared over the years. Herrmann and co-workers reported that the chelating dicarbene complex 38 (Fig. 4.14) is active for CO/ethylene [43], The highest TON [(mol ethylene + mol CO) mol Pd ] was 3 075 after a 4 h run. The modest TONs coupled with a very high molecular weight copolymer led the authors to conclude that only a small fraction of the pre-catalyst goes on to form an active species. Low molecular weight (M = 3 790) CO/norbomene copolymer resulted when complex 39 (Fig. 4.14) was tested by Chen and Lin [44]. The catalyst displayed only a very low activity, yielding 330 turnovers after 3 days. 

The general structure of linear low density polyethylene is shown in Fig. 18.2 c). Linear low density resins are copolymers of ethylene and 1-alkenes principally 1-butene, 1-hexene, and 1-octene. Comonomer levels range from approximately 2 to 8 mole %. This family of polyethylene is widely known as LLDPE. Linear low density polyethylenes are polydisperse with regard to molecular weight and branch distribution. 

Figure 18,2 d) illustrates the general structure of very low density polyethylene, which we also call ultra low density polyethylene. In common with linear low density polyethylene, these resins are copolymers of ethylene and 1-alkenes. The comonomer level ranges from approximately 8 to 14 mole %. We normally refer to these polymers as VLDPE or ULDPE. The molecules of very low density polyethylene contain a distribution of lengths and branch placements. 

We use single site catalysts primarily to make copolymers of ethylene and 1-alkenes, with densities ranging from approximately 0.87 to 0.93 g/cm3. More recently some higher density grades have been introduced, but these are relatively uncommon at the moment. 

Olefins or alkenes are defined as unsaturated aliphatic hydrocarbons. Ethylene and propylene are the main monomers for polyolefin foams, but dienes such as polyisoprene should also be included. The copolymers of ethylene and propylene (PP) will be included, but not polyvinyl chloride (PVC), which is usually treated as a separate polymer class. The majority of these foams have densities <100 kg m, and their microstructure consists of closed, polygonal cells with thin faces (Figure la). The review will not consider structural foam injection mouldings of PP, which have solid skins and cores of density in the range 400 to 700 kg m, and have distinct production methods and properties (456). The microstructure of these foams consists of isolated gas bubbles, often elongated by the flow of thermoplastic. However, elastomeric and microcellular foams of relative density in the range 0.3 to 0.5, which also have isolated spherical bubbles (Figure lb), will be included. The relative density of a foam is defined as the foam density divided by the polymer density. It is the inverse of the expansion ratio . 

The absence of a second cyclopentadienyl ring coupled with the short bridge gives a very open environment at the metal site. This allows easier access for bulky monomers, including 1-alkenes and norbomene, compared to polymerization with metallocenes. CpA initiators yield ethylene copolymers not easily available with metallocenes. Copolymers containing significant amounts of comonomers such as styrene, norbomene, and a-olefins from 1-hexene to 1-octadecene are easily obtained with CpA, but not with metallocene or traditional Ziegler-Natta initiators. 

Polyethers are typically products of base-catalyzed reactions of the oxides of simple alkenes. More often than not, ethylene oxides or propylene oxides and block copolymers of the oxides are used. A polypropylene oxide-based polymer is built and then capped with polyethylene oxides. An interesting aspect of this chemistry is the use of initiators. For instance, if a small amount of a trifunctional alcohol is added to the reactor, the alkylene oxide chains grow from the three alcohol end groups of the initiator. Suitable initiators are trimethylol propane, glycerol or 1,2,6 hexanetriol. The initiator is critical if one is to make a polyether foam for reasons that we will discuss shortly. 

Linear low-density polyethylene (LLDPE)440-442 is a copolymer of ethylene and a terminal alkene with improved physical properties as compared to LDPE. The practically most important copolymer is made with propylene, but 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene are also employed.440 LLDPE is characterized by linear chains without long-chain branches. Short-chain branches result from the terminal alkene comonomer. Copolymer content and distribution as well as branch length introduced permit to control the properties of the copolymer formed. Improvement of certain physical properties (toughness, tensile strength, melt index, elongation characteristics) directly connected to the type of terminal alkene used can be achieved with copolymerization.442... 

The rates and orientation of free radical additions to fluoroalkenes depend upon the nature of the attacking radical and the alkene, but polar effects again are important For instance, methyl radical adds 9 5 times faster to tetrafluoroethylene than to ethylene at 164 °C, but the tnfluoromethyl radical adds 10 times taster to ethylene [7551 The more favorable polar transition states combine the nucleophilic radical with the electron deficient olefin 17 and vice versa (18) These polar effects account for the tendency of perfluoroalkenes and alkenes to produce highly regular, alternating copolymers (see Chapter starting on page 1101)... 

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