Comonomers, linear, random copolymers

Linear, Random Copolymers of Ethylene and Polar Comonomers. 168... 

After five decades of catalyst research there is slowly emerging a family of discrete late transition metal catalysts that are capable of generating high molecular weight, linear, random copolymers of ethylene and polar comonomers such as acrylates. Further advances in the efficiency of these catalysts will likely give rise to new families of commercial polyolefins with a wealth of new performance properties imparted by the polar groups attached to the polymer backbone. 

With block copolymers two types of effect have been observed. In some instances a transition corresponding to each block is observable whilst in other cases a single transition is observed, usually close to that predicted by a linear relationship even where random copolymers show large deviations. This is because the blocks reduce both the contacts between dissimilar comonomer residues and also the disorder of the molecules which occurs in random copolymer systems. 

Many combinations of diacids—diamines and amino acids are recognized as isomorphic pairs (184), for example, adipic acid and terephthalic acid or 6-aminohexanoic acid and 4-aminocyclohexylacetic acid. In the type AABB copolymers the effect is dependent on the structure of the other comonomer forming the polyamide that is, adipic and terephthalic acids form an isomorphic pair with any of the linear, aliphatic C-6—C-12 diamines but not with -xylylenediamine (185). It is also possible to form nonrandom combinations of two polymers, eg, physical mixtures or blends (Fig. 10), block copolymers, and strictly alternating (187—188) or sequentially ordered copolymers (189), which show a variation in properties with composition differing from those of the random copolymer. Such combinations require care in their preparation and processing to maintain their nonrandom structure, because transamidation introduces significant randomization in a short time above the melting point. 

In general, most of the random copolymers form crystals composed of the major comonomer units of more crystallizable comonomer units alone, as incorporation of the minor component units into the crystalline phase need a large amount of excess free energy. So the cocrystallization of polymers is a rare phenomenon and a very few examples, such as poly(vinylidene fluoridej/vinylidene fluoride-tetrafluoro-ethylene copolymers system [58] and high-density polyethylene/linear low-density polyethylene [59], have been reported. Hence, the occurrence of cocrystallization found for the P(3HB-CO-3HV) copolymer is one of the rare examples. 

The copolymerization of ethylene with polar monomers is one potential method of improving various properties of PE [146, 147]. ADMET offers a method of synthesizing linear polymers identical to sequence-specific or random copolymers of ethylene and polar monomers with a variety of comonomer compositions. 

The conductivity is linearly dependent on composition, thus suggesting a substantially homogeneous distribution of the two monomer units with formation of random copolymers chains. These data, while showing the possibility of iiKxiulating solubility and conductivity by copolymerisation of suitable comonomers, are in agreement with the formation of ladder structures by intramolecular conjugation of pyrrole side chains during the oxidation of N-vinylpyrrole derived polymers. 

Copolymerization is the one way to synthesize polymeric materials with desired properties and functions. The cyclic carbonate monomers are successfully copolymerized with various cyclic monomers, such as cyclic carbonates, lactones with/without substituents, lactide, and cyclic phosphates. TMC was copolymerized with lactide by PPL to produce poly(lactide-co-TMC)s having carbonate contents from 0 to 100% and having molecular weights of up to 21000. The glass transition temperature (Tg) of the copolymer was dependent on the carbonate content, and the Tg values linearly decreased from 35° (polylactide) to - 8° [poly(TMC)] [47]. TMC was also copolymerized with medium to large ring-sized lactones. As an example, TMC was copolymerized with PDL in toluene by lipase CA at 70 °C to yield random copolymers [135]. All the poly(PDL-TMC)s were highly crystalline, even those with an equimolar comonomer content and close-to-random distribution. Thermal stability improves with randomization of the comonomer distribution [136]. 

The commercial Ziegler Natta catalyzed linear low density polyethylenes, LLDPEs, are excellent examples of random copolymers with relatively low comonomer contents. The level of comonomer is exceedingly important because it disrupts the crystallization process of very long ethylene sequences (13). By controlling the density of the LLDPE through its comonomer content, a wide and controlled set of physical properties is possible. 

At a fixed crystallization temperature will depend on Ab as indicated in Fig. 10.15 for random copolymers of ethylene. At low crystallization temperatures and large undercoolings, ( will be essentially constant over a relatively large range in comonomer content. At intermediate crystallization temperatures C increases slowly with composition. Over a small range in co-unit content the relation is linear. At the highest crystallization temperatures, the increase in with Ab is more pronounced, but is still linear over a small interval in Ab. 

It must be pointed out that deviations from such a simple relationship do occur. For example, since random copolymerisation tends to promote disorder, reduce molecular packing and also reduce the interchain forces of attraction, the Tg of copolymers is often lower than would be predicted by the linear relationship. Examples are also known where the Tg of the copolymer is higher than predicted. This could occur where hydrogen bonding or dipole attraction is possible between dissimilar comonomer residues in the chain but not between similar residues, i.e. special interchain forces exist with the copolymers. 

During the past four years, linear low-density polyethylene (LLDPE) has probably become the most important of the thermoplastic copolymers. In contrast to the customary practice of producing branched ethylene homopolymer in a high-pressure reaction, a system of copolymerizing ethylene with a-C g olefins at low pressure is used to make LLPDE copolymer. This random copolymerization is commercially carried out in gas-phase, slurry, and solution processes in the presence of a transition metal catalyst 1-butene, 1-hexene, 4-methyl-l-pentene, or 1-octene are choices of comonomer. In the face of plant overcapacity and idle equipment existing at this time, LLDPE can also be made in high-pressure autoclaves and tubular reactors. 

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