Linear copolymer, property distributions

Figure 2 Property distributions in a linear copolymer composition distribution, molecular weight distribution and sequence length distribution of poly styrene-co-n-butyl methacrylate). (Styrene units are represented by "A and n-butyl methacrylate units by B".)...

There have been many studies directed at using adsorption and re versed-phase HPLC to separate copolymers by composition (1.-3) interacting problems associated with these approaches ares o The presence of one property distribution interferes with separation on the basis of the other. For example, in adsorption chromatography, the degree of adsorption can be affected by both the molecular weight and by the composition of the molecule. For a linear copolymer, adequate fractionation requires that the ccmposltlon differences completely dominate. 

Along with the isomerism of linear copolymers due to various distributions of different monomeric units in their chains, other kinds of isomerisms are known. They can appear even in homopolymer molecules, provided several fashions exist for a monomer to enter in the polymer chain in the course of the synthesis. So, asymmetric monomeric units can be coupled in macromolecules according to "head-to-tail" or "head-to-head"—"tail-to-tail" type of arrangement. Apart from such a constitutional isomerism, stereoisomerism can be also inherent to some of the polymers. Isomers can sometimes substantially vary in performance properties that should be taken into account when choosing the kinetic model. The principal types of such an account are analogous to those considered in the foregoing. The only distinction consists in more extended definition of possible states of a stochastic process of conventional movement along a polymer chain. 

When the statistical moments of the distribution of macromolecules in size and composition (SC distribution) are supposed to be found rather than the distribution itself, the problem is substantially simplified. The fact is that for the processes of synthesis of polymers describable by the ideal kinetic model, the set of the statistical moments is always closed. The same closure property is peculiar to a set of differential equations for the probability of arbitrary sequences t//j in linear copolymers and analogous fragments in branched polymers. Therefore, the kinetic method permits finding any statistical characteristics of loopless polymers, provided the Flory principle works for all chemical reactions of their synthesis. This assertion rests on the fact that linear and branched polymers being formed under the applicability of the ideal kinetic model are Markovian and Gordonian polymers, respectively. 

Anionic polymerization methods have been used to synthesize a wide variety of macromolecules including linear [1] and cyclic [2] homopolymers, linear copolymers [1], and functional polymers such as macromonomers [3]. These macromolecules are well defined with predetermined molar masses, sharp molar mass distributions, and low compositional heterogeneity. They serve as ideal compounds to establish the relation between the structure, the properties, and theory. 

Two types of well defined branched polymers are acessible anionically star-shaped polymers and comb-like polymers87 88). Such macromolecules are used to investigate the effect of branching on the properties, 4n solution as well as in the the bulk. Starshaped macromolecules contain a known number of identical chains which are linked at one end to a central nodule. The size of the latter should be small with respect to the overall molecular dimensions. Comb-like polymers comprise a linear backbone of given length fitted with a known number of randomly distributed branches of well defined size. They are similar to graft copolymers, except that backbone and branches are of identical chemical nature and do not exhibit repulsions. 

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... 

If you have been working your way through this epic in a more or less linear fashion, then you might have started to ask yourself some fundamental questions such as, How do you know if a vinyl polymer is isotactic, or atactic, or whatever How do you know the composition and sequence distribution of monomers in a copolymer How do you know the molecular weight distribution of a sample This last question will have to wait until we discuss solution properties, but now is a good point to discuss the determination of chain microstructure by spectroscopic methods. The techniques we will discuss, infrared and nuclear magnetic resonance spectroscopy, can do a lot more than probe microstructure, but that would be another book and here we will focus on the basics. 

MAJOR APPLICATIONS POE is a new family of ethylene a-olefin copolymers produced using metallocene catalyst. The uncross-linked polymers referred to in this chapter are known to have only moderate elastomeric recovery properties (up to 96%). These copolymers are characterized by a narrow molecular weight distribution (MWD) (M /Mn = 2-2.5) and homogeneous comonomer distribution.The control of chain microstructure by the use of metallocene catalyst makes it possible to produce poly(a-olefin) copolymers with considerably lower density, which has not been possible before using the conventional Ziegler-Natta catalyst. Some of the highly branched ethylene copolymers presented in the entry on Polyethylene, metallocene linear low-density, in this handbook may be closely related. 

New types of LLDPEs based on the metallocene catalyst technology have been introduced recently in the market place. Such LLDPEs are characterized by narrower molecular weight and homogeneous short-chain branching distribution. Some of the metallocene catalyst based octene-1 LLDPE copolymers made by the Dow Chemical Company are known to have LCB. For the properties of metallocene LLDPE see the entry Polyethylene, metallocene linear low density, in this handbook. 

The different heterogeneities, summarized in the term molecular heterogeneity , can be superimposed one on another, i.e. bifunctional molecules can be linear or branched, linear molecules can be mono- or bifunctional, copolymers can be block or graft copolymers etc. - see Fig. 1. In order to characterize heterogeneous polymers, i.e. polymers which are distributed in more than one property, it is necessary to know a set of functions fi(M), e.g. the molar mass distributions within each other type of heterogeneity. Clearly, it is very difficult in a general case to solve this characterization problem. 

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