Comonomer distribution

The monomers that comprise copolymers can be introduced in one of three general ways, randomly, as regularly alternating series, or as blocks of identical monomers. The polymerization catalyst and the reaction conditions control the type of comonomer distribution produced. 

Alternating copolymers, as illustrated in Fig. 5.8 b), are generally made by condensation polymerization of two different monomers. Such copolymers display regularity and are capable of crystallizing under the appropriate conditions. Examples of such copolymers include nylons 66 and 610, and various types of polyurethane. 

Diblock copolymers, as illustrated in Fig, 5.8 c), comprise homopolymer sequences of the two monomers linked together. The homopolymer blocks may be either compatible or incompatible, depending on their chemical structure. If the sequences are compatible, they will mix to form a material with characteristics similar to those of a blend of the two homopolymers. On the other hand, if the blocks are incompatible, they will tend to segregate from one another to form distinct phases. Each phase will display properties characteristic of the homopolymer, modified by the constraints placed on them by having one end attached 

Triblock copolymers, as shown in Fig. 5.8 d), comprise a central homopolymer block of one type, the ends of which are attached to homopolymer chains of another type. As with other block copolymers, the components of triblocks maybe compatible or incompatible, which will strongly influence their properties. Of particular interest are triblocks with incompatible sequences, the middle block of which is rubbery, and the end blocks of which are glassy and form the minor phase. When such polymers phase-segregate, it is possible for the end blocks of a single molecule to be incorporated into separate domains. Thus, a number of rubbery mid-block chains coimect the glassy phases to one another. These materials display rubber-like properties, with the glassy domains acting as physical crosslinks. Examples of such materials are polystyrene/isoprene/polystyrene and polystyrene/polybutadiene/polystyrene triblock copolymers. 

Multiblock copolymers, as shown in Fig. 5.8 e), with incompatible components form similar structures to those found in diblocks and triblocks. 

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APAOs has limited their utility in a number of applications. The broad MWD produces poor machining and spraying, and the low cohesive strength causes bond failures at temperatures well below the softening point when minimal stress is applied. To address these deficiencies, metallocene-polymerized materials have been developed [17,18]. These materials have much narrower MWDs than Ziegler-Natta polymerized materials and a more uniform comonomer distribution (see Table 3). Materials available commercially to date are better suited to compete with conventional EVA and EnBA polymers, against which their potential benefits have yet to be realized in practice. 

Heterogeneous comonomer distribution Intro molecular-.wittiin molecules... 

Figure 3 Two examples of heterogeneity of comonomer distribution and a possible explanation of the shape of the DSC curve. Source Ref. 32.

When two or more monomers are polymerized into the same molecular chain they produce a copolymer, The distribution of monomers, in terms of their relative concentrations and placements, is responsible for controlling a copolymer s properties. Figure 5.8 illustrates five possible comonomer distributions for a copolymer comprising equal numbers of two types of monomer. The relative concentrations of the different monomers and the lengths of the various blocks can be varied widely. Relatively small changes in comonomer concentration and placement can result in significant changes in physical and chemical properties. Properties that can be modified include such diverse characteristics as extensibility, elastic recovery, modulus, heat resistance, printability, and solvent resistance. 

Monomer reactivity ratios and thus comonomer sequence distributions in copolymers can vary with copolymerization reaction conditions. The comonomer distribution could affect the geometry of the adsorbed polymer - mineral complex and the fines stabilization properties. 

Once assignments are made, C-13 NMR "n-ad" distributions are available. In general, one would like to obtain a distribution over the longest possible sequence length. Relationships, often referred to as the "necessary relationships" exist between n-ad sequences of different lengths. It is possible to reduce any n-ad distribution to m versus r, which correspond to the simplest comonomer distribution but is devoid of any Information concerning sequence length. 

When measuring vinyl polymer tactlclty, one prefers the longest complete n-ad distribution available as well as the translated simplest comonomer distribution, possibly m versus r. An alternative exists to the m versus r distribution In the form of number average or mean sequence lengths. If any vinyl homopolymer Is viewed conceptually as a copolymer of meso and racemic dyads, mean sequence lengths can be determined for continuous runs of both meso and racemic configurations (32), that Is,... 

The mean sequence lengths may offer a better way to present the simple comonomer distribution than meso, racemic distributions because they do reflect the polymer sequential structure. 

When describing polymer tactlcity, one should attempt to obtain the highest complete "n-ad" distribution available as well as a simple "comonomer" distribution. In connection with such a measurement, the mean sequence lengths may offer a viable alternative to the simple m versus r distribution. Useful relationships, which are helpful in establishing particular statistical behaviors, are available. 

The industrial scale fermentative synthesis of PHA uses these pathways to convert the typical nutrients sugar or starch to PHB, but glycerol or palm-oil can also be applied. In addition, copolymers can be produced in this way but special microorganisms, growing condition, and additives are needed. Thus a statistic comonomer distribution starting from 0% (pure PHB) up to 90% co-monomer content can be achieved [35-38]. 

In all the low pressure PE processes the polymer is formed through coordination polymerisation. Three basic catalyst types are used chromium oxide, Ziegler-Natta and single-site catalysts. The catalyst type together with the process defines the basic structure and properties of the polyethylene produced. Apart from the MWD and comonomer distribution that a certain catalyst produces in polymerisation in one reactor, two or more cascaded reactors with different polymerisation conditions increase the freedom to tailor... 

Fig. 10 Tailoring of polymer comonomer distribution gives greatest benefits in HDPE...

Linear NIPAM-co-VP copolymers As discussed in the Experimental Section, hydrophilic comonomer, vinyl pyrrolidone (VP), can be purposely copolymerized into PNIPAM at two different temperatures, 30 °C and 60 °C, respectively, below and above the LCST of PNIPAM homopolymer. At each temperature, the copolymers with two different VP/NIPAM ratios (5 and 10 mol%) were prepared. A proper fractionation of resultant copolymers led to narrowly-distributed long NIPAM-co-VP copolymer chains with a similar length and VP/NIPAM ratio, but different comonomer distributions. 

After investigating the effect of comonomer composition on the chain association as well as the effect of comonomer distribution on the chain folding, Siu et al. [141] extended their study to the effect of comonomer distribution on the chain association. They copolymerized NIPAM and vinyl pyrroli-done (VP) at temperatures, respectively, higher and lower than the LCST, which resulted in segmented and random VP distributions on the PNIPAM chain backbone. The synthesis characterization of these PNIPAM-co-VP amphiphilic copolymers with a similar chain length and comonomer composition, but different comonomer distributions, were described in previous sections. 

The comonomer distribution can be alternated by controlling the synthesis conditions, such as the copolymerization at different reaction temperatures at which the thermally sensitive chain backbone has different conformations (extended coil or collapsed globule). In this way, hydrophilic comonomers can be incorporated into the thermally sensitive chain backbone in a more random or more segmented (protein-like) fashion. On the other hand, short segments made of hydrophobic comonomers can be inserted into a hydrophilic chain backbone by micelle polymerization. One of the most convenient ways to control and alternate the degree of amphiphilicity of a copolymer chain, i.e., the solubility difference of different comonomers in a selective solvent, is to use a thermally sensitive polymer as the chain backbone, such as poly(N-isopropylacrylamidc) (PNIPAM) and Poly(N,N-diethylacrylamide) (PDEA). In this way, the incorporation of a hydrophilic or hydrophobic comonomer into a thermally sensitive chain backbone allows us to adjust the degree of amphiphilicity by a temperature variation. 

For copolymers with some protein-like comonomer distributions, individual copolymer chains can memorize or inherit its parent globular state namely, their folding back into the core-shell nanostructure is much easier, resulting in a smaller and denser single-chain particle in comparison with their counterparts, randomly distributed comonomers on the... 

It must be emphasised that copolymers produced by metallocene-based catalysts display comonomer distributions essentially independent of the chain lengths the copolymers obtained with these catalysts contain larger fractions of higher a-olefins than those obtained with heterogeneous Ziegler-Natta catalysts under comparable conditions [30]. 

The copolymerisation of ethylene oxide and phenyl isocyanate has been found [266] to proceed in the presence of the triethylaluminium-water (2 1) catalyst, although phenyl isocyanate alone could not be polymerised by the same catalyst. The copolymer formed was characterised by an alternating comonomer distribution [scheme (39)] and contained acetalic units in its chains (Table 9.4) ... 

The copolymerisation of dimethylketene and acetaldehyde with diethylzinc as a catalyst produced a crystalline copolymer of alternating comonomer distribution (Table 9.3) [289], Other aldehydes such as w-butyraldehyde, i-butyraldehyde or benzaldehyde were also copolymerised with dimethylketene to produce the respective polyesters [289-318]. 

Products A wide range of bimodal and unimodal products, with a full control of comonomer distribution, can be produced, with densities ranging from 918 to 970 kg/m3 and melt flowrate from less than 0.1 to over 100. The molecular weight distribution can be controlled from narrow to broad. Advanced properties are tailor-made applications such as pipe strength, film bubble stability as well as high ESCR and stiffness in blow molding. Other special applications include extrusion coating and wire cable. 

The excellent performance of metallocenes in copolymerizations also offer improvements in impact copolymers. In the wide variety of properties of impact copolymers, the stiffness of the material is determined by the matrix material, while the impact resistance largely depends on the elastomeric phase. While conventional catalysts show some inhomogeneities in the ethene/propene rubber phase due to crystalline ethene rich sequences, the more homogenous comonomer distribution obtained with metallocene catalysts results in a totally amorphous phase [153]. 

Even the most advanced Ziegler-Natta catalysts contain distinctly non-uniform catalytic sites. Although most of the less selective (and hence undesirable) sites present in catalysts made by older recipes appear to be eliminated or blocked by the inner and outer donors now used to condition these catalysts, the polymers they produce still show large variations in molar mass, stereoregularity and comonomer distributions, which indicate that they originate from distinct catalytic sites. 

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