Polystyrene long-chain branching

Additionally, metallocene catalysts enable the design of catalysts for tailored polyolefins due to the intrinsic nature of the single site. Actually, new polymers which could never have been produced by conventional Ziegler-Natta catalysts, i.e., syndiotactic polypropylene, syndiotactic polystyrene, long chain branched polyolefins, cyclo-olefin polymer, and styrene copolymers, can be obtained by metallocene catalysts. This upcoming new S curve in the polyolefin development cycle means the evolution of new type of polyolefins. 

For polymer melts where the low shear rate limiting viscosity value is r ), r 3t]0 (14). Examples of extensional viscosity growth, either to a steady t](i ) value or to a strainhardening-like mode, are shown in Fig. 3.6 for the linear nonbranched polystyrene (PS), a high density polyethylene (HDPE) that is only slightly branched with short branches, and a long chain-branched low density polyethylene (LDPE) (15). 

Chain branching is a common occurrence during radical polymeriza ] tions and is not restricted to poiyethyiene. Polypropylene, polystyrene, andt poly(methyl methacrylate) all contain branched chains. Studies have shown that short-chain branching occurs about 50 times as often as long-chain branching. 

The synthesis of homo- (Ti-Ti) and heterobinuclear (Ti-Zr) complexes linked by 1,2-G2H4 linker groups as shown in Scheme 316 has been reported. The molecular structures of the dimethylamido derivatives have been determined by X-ray diffraction methods. In the presence of binuclear borate activators, the methyl complexes produce long-chain branched polyethylene and polystyrene in homopolymerization reactions and ethylene-styrene co-polymers. The polymerization behavior differs from that obtained with the mononuclear compound (3-ethylindenylSiMe2-NBiOTiMea (Scheme 3 1 7).762"764... 

Figure 2 Time-dependent elongational viscosity (Ji(f,eo) measured at constant elongation rate for long-chain branched polyethylene (LDPE lUPAC A, LDPE 111), linear polyethylene (HOPE I) and polystyrene (PS I). The number of long-chain branches is indicated by the number of CHj groups per 1000 CHj groups (CHj/lOOOCHj). (From Laun [2], by permission of Plenum Publishing.)...

Interaction chromatography can also be used to fractionate model polymers according to long-chain branching. An application for star-shaped polystyrenes was presented by Chang and co-workers (79). 

Swell represents a recovery of stored elastic energy. The molecular structure, in particular molecular weight and long chain branching, has a profound effect on the swell characteristics, but to date there is no consistent theory to explain the relationships. Early investigations into the phenomenon of swell showed that HDPE appeared to behave in a similar fashion to polystyrene (PS), where an increase in the breadth of the MWD of PS... 

Polystyrene-6-polydimethyl siloxane has been used as an additive to long chain branched polyethylene and narrow molecular weight polystyrene to control barrier properties. 

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. 

These are the linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling. These polymers possess Intermolecular forces of attraction intermediate between elastomers and fibres. Some common thermoplastics are polythene, polystyrene, polyvinyls, etc. 

---