Alternating ethylene-propylene copolymers

The third class of olefin methathesis in Scheme 21.1 is addition metathesis polymerization (ADMET). This reaction is an alternative method to stitch together olefins into polymers, in this case by a combination of dienes with extrusion of ethylene. Control of molecular weight by the ADMET process is less precise than that by ROMP, but this reaction has been used to make polymers with precise architectures, such as polymers that would be perfectly alternating ethylene-propylene copolymers. ... 

Two metallocene catalysts for the formation of alternating ethylene-propylene copolymers. 

In principle the hydrogenation of a diene rubber is one of the simplest reactions. Complete hydrogenation of polybutadiene would give a polyethylene structure whilst hydrogenation of polyisoprene that of an alternating ethylene-propylene copolymer. 

Hydrogenated random copolymers of 1.4- and 1.2-butadiene represent another class of crystalline-amorphous materials [10-15,17). Following hydrogenation, the resultant copolyolefin contains ethylene (E) and 1-butene (B) units. These materials can be described as PEB-n where n denotes the mun-ber of ethyl units per 100 backbone carbons. Eor samples where n < 12 the samples will show partial crystallinity at room temperature while for n > 13 the samples are amorphous in the bulk state. The latter PEB copolymers lack self-assembly capacity. An equivalent amorphous segment is derived from polyisoprene. In this case hydrogenation yields the essentially alternating ethylene-propylene copolymer (PEP). This material also contains about 7% isopropyl units that randomly appear between the ethylene-propylene units. 

To check the correctness of this interpretation von Schooten and co-workers [7] examined the IR spectrnm between 13 and 14 pm of ethylene-propylene copolymers prepared with varions catalyst systems and compared them with the spectra of some model componnds, namely 2,5-dimethylhexane, 2,7-dimethyloctane, 4-methylpentadecane, 4-n-propyltridecane, polypropylene, polyethylene, polybutene-1 and hydrogenated polyisoprene, the last being considered as an ideally alternating ethylene-propylene copolymer. 

Toughened polypropylene may be prepared by block copolymerization in which ethylene monomer is added during the final stages of the polymerization of propylene (4). Thus, some polypropylene chains would contain an end block of rubbery ethylene-propylene copolymer. Alternatively, a blend of an elastomeric copolymer of ethylene and propylene (EPR or EPDM) with isotactic polypropylene (PP) can produce an impact-resistant polymer (5). 

An alternating ethylene-CO copolymer melts at 257 " C. Incorporation of propylene decreases fhe melting point, enabling processing of the materials (e.g. 6 mol% propene 220°C) [35]. Semicrystalline ethylene/propylene/carbon mon-... 

Polyisoprene on hydrogenation gives a rubber directly—an alternating ethylene-propylene polymer. Polybutadiene can give polyethylene on hydrogenation if it is all 1,4 in structure or a variety of flexible-to-rubbery ethylene-butene copolymers as the 1,2 content of the polybutadiene is increased. These polydiene structures can be incorporated as segments in anionic styrene copolymers. 

Polymer (B) Characterization Solvent (A) ethylene/propylene copolymer M /kg.mor = 0.78,M ,/kg.mor = 0.79, 50 mol% propene, alternating ethylene/propylene units from complete hydrogenation of polyisoprene 1-butene C4H8 1992CH1 106-98-9... 

As compounders developed and refined TPO blend technology, PP manufacturers pursued an alternate approach to PP impact modification by developing PP impact copolymers (ICPs). These products are produced in a series of reactors. The first reactor produces PP homopolymer, followed by one or two gas phase reactors in which ethylene is introduced to produce EPR. The gas phase reactor can be either a vertical fluidized bed or a horizontal stirred bed design (6). Because of different reactivity of propylene and ethylene inside the gas phase reactor, complex mixtures of PP with ethylene-propylene copolymers and linear low-density polyethylene (LLDPE) are produced. 

Copolymers.—Both Monomers Vinyl. Ethylene-propylene copolymers have attracted the greatest attention,befitting their industrial importance. A variety of techniques has been used to assign the rather complex C spectra observed, including the synthesis of model oligomers and model polymers e.g. by hydrogenation of isoprene ). Propylene may add by either primary or secondary insertion, and a terpolymerization model has therefore been used to quantify the sequence distribution. Alternatively, Randall has proposed an analysis in terms of —CHa— or —CH(CHs)— units, rather than monomer residues. 

The hydrogenation of cis-1,4 copolymers of B and I would lead to polyolefins with composition and sequence distribution consisting of ethylene (E) blocks and alternating ethylene/propylene (E/P) blocks. These novel polyolefins are difficult or almost impossible to obtain directly by simple polymerization of E and P monomers using any existing polymerization catalysts. Since structural variations in these polyolefins, such as composition aind monomer sequence distribution, would significantly affect the polyolefin properties, the hydrogenated cis-1,4 B/I copolymers with uniformly random distribution of E and E/P imits may serve as model polymers to study structure-property relationships and be useful as polymers with unique properties. 

Diblock copolymers of E-(E/P), where E and (E/P) are block segments of ethylene and alternating ethylene/propylene, respectively, were obtained from the hydrogenation of cis-1,4 B-I diblock copolymers. 

Polyisoprene (hydrogenated natural rubber) is a completely alternating etbylene propylene copolymer (i.e., does not have ethylene or propylene blocking) and is therefore an interesting substance for Py-GC studies. Tbe surface area of tbe main peaks up to C13 obtained by van Schooten and Evenhuis [13, 14] indicate that the unzipping reaction which would yield equal amounts of ethylene and propylene in the hydrogenated pyrolysate takes place to some extent, but is less important than the hydrogen transfer reactions ... 

This is also found in hydrogenated poly-2,3-dimethylbutadiene, used as a model compound and in alternating copolymers of ethylene and butane-2 [2], In the polypropylenes examined by van Schooten and Mostert [3] and in ethylene-propylene copolymers, they found an absorption band near 9.0 pm, although, unlike van Schooten and Mostert [3] (see above), it was much less sharp than the model compound. 

The sequence distribution is random in ethylene-propylene copolymers prepared in the presence of isospecific or not stereospecific catalysts, whereas it tends to the alternance of different units in copolymers prepared in the presence of syndiospecific catalysts [5]. 

Since the same will happen in Reactor 2, in the end the ratio of polypropylene to ethylene-propylene copolymer per particle exiting Reactor 2 will also vary widely, which may be undesirable in some applications. Some of the reactor configurations shown in Figure 8.35 can reduce this phenomenon, particularly the configuration adopted for the gas-phase horizontal reactor, because the residence time distribution of this reactor is the equivalent to about three to four CSTRs in series. (Remember that the residence time of an infinite series of ideal CSTRs is that of a plug-flow reactor.) A more recent solution for this problem, in fact a completely new alternative to tandem reactor technology, is the multizone reactor that will be described in more detail below (see Section 8.6.4). 

A 220-MHz PMR study of ethylene-propylene copolymers is reported (122) and (132). A comparison between the spectra of polypropylene (133) and that of hydrogenated rubber (the model for the alternating copolymers) (122) enabled the authors to assign some resonances in the copolymer spectra, but the assignment of some peaks remains dubious and precludes direct comparison with statistics. 

The chain dimensions of virtually alternating ethylene-propylene model polymers PEP have been studied experimentally [36,38,145,147] and theoretically [160]. PEP is intermediate in structure between PE and PP with substituents on roughly every fourth backbone carbon. The Ca, value of 6.8 for this polymer is also intermediate between those for PP and PE. It reflects the fact that, relative to PP, a lower level of gauche conformers is adequate to relieve steric effects caused by substituents. Similar to PEP, PEB represents copolymers... 

Natta and co-workers [5] determined the degree of alternation of ethylene and propylene units in ethylene-propylene copolymers from the infrared spectrum, using peaks at 13.35, 13.70 and 13.83 pm, the one at 13.70 pm being attributed to a sequence of three methylene groups between branch points, presumably due to the insertion of one ethylene between two similarly oriented propylene molecules ... 

This avoids problems associated with propylene inversion and comonomer sequence assignment. He gives methylene sequence distributions from one to six and larger consecutive methylene carbons for a range of ethylene-propylene copolymers, and uses this to distinguish copolymers which have either random, blocked or alternating comonomer sequences. 

Thermoplastic polyolefins (TPOs) are composite blends of semicrystalline polypropylene and ethylene propylene copolymer (EPR) or ethylene propylene diene monomer (EPDM), widely used in the automotive industry for the production of plastic car parts such as bumper fascia [1]. Polypropylene, which is the major component in such blends, is an inexpensive, easily proccessible polymer, although its poor mechanical properties necessitate the addition of a rubber-dispersed phase. The added rubber acts as an impact modifier by imparting improved ductility, crack resistance, and impact strength to the resulting TPO [1-4]. The cost-effectiveness, light weight, processability, and resilience of TPOs have made them increasingly viable alternatives to steel for bumpers and other car parts. 

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