Branched chain copolymers

Polymers can display a variety of structures, including linear, cyclic, and branched chains, copolymers in which different repeat units are aligned along the chain in different manners, and star polymers with different numbers of arms. The identification of molecular structure is the first step in the analysis of a polymeric maferial, which may acfually be a homopolymer, a copolymer, or a blend of homopolymers and/or copolymers. 

The chemical features that prohibit crystallinity are main chain flexibility (e.g., rotation), branching, random copolymers or low inter-polymer chain attraction. Normally, polymers are not miscible with each other and on cooling from the melt will separate into different phases. When miscibility is exhibited, e.g., poly(phenylene oxide) (PPO) and PS, crystallisation does not take place. 

Similar to the branches in copolymer stars and miktoarms, the grafted chains in brushes can be of different chemical compositions. Brown et al. [223] studied the microphase separation of grafted mixtures of homopolymer chains composed of immiscible A and B units and also [224] of diblock AB copolymers. In the former case, the brushes expand laterally and then experience lateral microphase separation. In the latter case, however, monomers segregate vertically to the surface forming a three layer structure. 

In this chapter, the big four thermoplastics are covered polyethylene, polypropylene, polyvinyl chloride, and polystyrene. Like most other thermoplastics, they are long-chain polymers that become soft when heated and can be molded under pressure. They are linear- or branch-chained and, except for some exotic copolymers, have little or no cross-linking. Technological advances continue. Research in copolymerization, catalysts, processing, blending, and fabricating continues even as you read this. 

Polymer chains are, in general, regularly built-up, but a few variations are possible. We shall, successively, consider arrangement of monomers, situation of side groups, arrangement round a double bond, branching, and copolymer structure. 

Yet another type of classification of polymers is based on the type of repeating unit. A homopolymer has one type of repeat unit. Copolymers are polymers that have more than one type of monomers or repeat units. If the monomers in a copolymer are distributed randomly along the chain, it is called a regular or random copolymer. If, on the other hand, a sequence of one type of monomer is followed by a sequence of another type of monomer, it is called a block copolymer. If the main chain is one type of monomer and the branch chains are of another monomer, it is called a graft copolymer. 

In an investigation of the effect of chain geometry on the two phase morphology of polystyrenepolyiOTprene block copolymers. Price et al. have studied films of AB linear, AB stars with 2,3 or 4 branches graft copolymers , and ABC... 

ABA tribiock, or all three can be different, as in an ABC triblock copolymer. Obviously, the number of possible block sequences increases rapidly with the number of blocks and the number of different types of block in the chain. One can also synthesize block copolymers with branched architecture, such as star-branched block copolymers, in which each of the arms of the star contains either the same or different block sequences (see Fig. 13-1). One or more of the blocks could also be stiff or liquid crystalline (Chiellini et al. 1994 Chen et al. 1996 Radzilowski et al. 1997 Jenekhe and Chen 1998). For a given type of block copolymer, the degree of polymerization N of the whole molecule, or the degree of polymerization Ni of one or more of the blocks, can be varied. Thus, the number of different types of block copolymers is practically endless. 

Polymer molecules may be linear or branched, and separate linear or branched chains may be joined by crosslinks. Extensive crosslinking leads to a three-dimensional and often insoluble polymer network. Polymers in which all the monomeric units are identical are referred to as homopolymers those formed from more than one monomer type are called copolymers. Various arrangements of the monomers A and B in the copolymer molecules (Fig. 8.1) can be produced with consequent effects on the physical properties of the resulting polymer. Synthetic polymers may have their main chains substituted in different ways, depending on the conditions of the reaction, such that atactic (random), isotactic or syndiotactic forms are produced, as diagrammatically represented in Fig. 8.1. 

Graft polymers consist of a main polymer chain, the backbone, with one or more side chains attached to it through covalent bonds, the branches. Graft copolymers are comb-shaped polymers where the chemical nature of the backbone and the branches differs. The chemical nature and composition of the backbone and the branches differ in most cases. Branches are usually distributed randomly along the backbone, although recently advances in synthetic methods allowed the preparation of better defined structures.88,90... 

Low density polyethylene material has branched chains and limited crystallinity, which lead to an open structure and the low density. It is particularly soft and flexible, transparent to translucent, has good impact resistance and relatively low melting points, which give good heat sealability. Most LDPEs are made by a high pressure polymerisation process starting from ethylene gas. The proportion of crystallinity to amorphous is around 3 2 (i.e. 60-65% crystalline). Recently new linear polyethylene copolymers of 0.89-0.91 (ultra or very low densities) have been developed. Special antioxidant free grades are available for pharmaceutical applications. 

Branched chains formed of a main chain of homopolymer of one monomer type and one or several branches of homopolymer of a second monomer type are called graft copolymers, they are illustrated in Figure 6.5. The nomenclature for these copolymers is polyA-.gra/r-polyB [24]. They are sometimes referred to as poly(A-.gra/r-B). Chapter 10 reviews the topic of graft copolymers, and the reader is also referred to the review by Hadjichristidis [25] for example. CRP has also been used to synthesize this kind of polymer via NMP [26], ATRP [27], and RAFT [28]. 

Another embodiment of this technique was used for the synthesis of high molecular weight, low B arborescent polymers by Yuan and Gauthier in a one-pot synthesis of arborescent PSs [114]. In this case, the anionic copolymerization of styrene (Sty) and 1,3-diisopropenylbenzene (DIB) initiated by iec-butyllithium was carried out in a semibatch process. Following complete monomer conversion, the chains were terminated and the isopropenyl moieties of the DIB units were activated with xec-butyllithium to produce a polyfunctional anionic macroinitiator without additional workup. Further styrene-DIB monomer mixture additions yielded a comb-branched (GO) copolymer, and... 

Graft copolymers A nonlinear or branched block copolymer is formed by attaching chains of one monomer to the main chain of another homopolymer. 

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. 

The symbol -g- may be employed, as in poly(butadiene-g-styrene). The first polymer mentioned forms the backbone chain, and the second the branches. Block copolymers of the type... 

Block copolymer surfactants show a qualitatively similar phase behaviour with temperature to conventional materials. However, solubility requires the presence of branched chains and/or ether links to reduce the occurrence of crystalline polymer with a high melting point. It must be emphasized that commercial polymeric surfactants are polydisperse in both alkyl chain and EO blocks. 

PREPARATION OF HIGHLY BRANCHED GRAFT COPOLYMERS BY CHAIN TRANSFER REACTION... 

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