Graft polymers Impact polystyrene

Two commercially significant graft copolymers are acrylonitrile—butadiene—styrene (ABS) resins and impact polystyrene (IPS) plastics. Both of these families of materials were once simple mechanical polymer blends, but today such compositions are generally graft copolymers or blends of graft copolymers and homopolymers. 

The relative U.S. production of styrene homopolymer and copolymer resins is also noteworthy (103) (Fig. 6). The impact polystyrene (graft and polymer blend) copolymers are produced in nearly the same quantities as styrene homopolymers. The ABS resins are synthesized in lesser, yet significant, quantities. 

Today the common practice is first to dissolve the rubber in the styrene monomer and then to polymerise the styrene in the usual way. By this process the resultant blend will contain not only rubber and polystyrene but also a graft polymer where short styrene side chains have been attached to the rubber molecules. This gives a marked improvement in the impact strengths that can be obtained. 

The formation of graft copolymers leads to the possibility of combining incompatible polymers in such a way that the components may be distributed homogeneously or they are at least firmly fixed to each other at the phase boundaries, for example, with high-impact polystyrene, where the two components, polystyrene, ( 90%) and rubber ( 10%), are combined through... 

The tendency of polybutadiene to undergo chain transfer reactions can be used for the preparation of impact-resistance polymer where polystyrene has been grafted to polybutadiene as under ... 

The concept is similar to the grafting of plants in botany. To form a styrene-butadiene graft polymer, already polymerized butadiene is dissolved in monomeric styrene and an initiator is added. Because polybutadiene readily undergoes chain transfer at the allylic sites, polystyrene chains grow on the polybutadiene backbone. This forms high impact polystyrene, a low cost plastic that is otherwise too brittle without the grafting. 

Although this method yields a mixture of homopolymer and graft copolymer, and probably also ungrafted backbone polymer, some of the systems have commercial utility. These are high-impact polystyrene (HIPS) [styrene polymerized in the presence of poly(l,3-buta-diene)], ABS and MBS [styrene-acrylonitrile and methyl methacrylate-styrene, respectively, copolymerized in the presence of either poly(l,3-butadiene) or SBR] (Sec. 6-8a). 

The third possibility to prepare graft copolymers is termed grafting onto . This means that a growing chain B attacks the polymer backbone A with formation of a long branch. This attack can be a chain-transfer reaction or a copolymerization with unsaturated groups, for example, in polydienes. These reactions play an important role in the preparation of high impact polystyrene (see... 

Unlike simple mixtures of polystyrene and polybutadiene such blends can be thermoplastically processed without phase separation ( splicing ) Furthermore, they can to a certain extent withstand mechanical impact without disintegration. This is because the above-mentioned graft polymers function also as compatibilizer at the borderline of the hard phase and the rubber-elastic dispersed phase (already at concentrations below 3%). 

The term graft copolymer is used to describe copolymers with long sequences of another monomer (comonomer) as branches on the main polymer chain. Most commercial varieties of high-impact polystyrene (HIP) and copolymers of acrylonitrile, butadiene, and styrene (ABS) are graft copolymen in which the main polymer chain is polybutadiene and the branches are styrene, or styrene and acrylonitrile. Figure 1.12 shows various types of copolymers. 

Phillips catalysts for linear polyethylene and polypropylene and the graft copolymerizations for impact polystyrene and ABS are even younger and have not yet spread into the less industrialized countries of world. The production of polyolefins, poly (vinyl chloride), and styrene resins on a worldwide basis as well as of all synthetic polymers is shown in Figure 3. A comparison of the U.S. production in Figure 1 and in Figure 3 demonstrates the effect of age and dissemination of technology. It shows that relatively more poly (vinyl chloride) but less polyolefins and styrene resins are produced worldwide than in this country. 

Another widely used copolymer is high impact polystyrene (PS-HI), which is formed by grafting polystyrene to polybutadiene. Again, if styrene and butadiene are randomly copolymerized, the resulting material is an elastomer called styrene-butadiene-rubber (SBR). Another classic example of copolymerization is the terpolymer acrylonitrile-butadiene-styrene (ABS). Polymer blends belong to another family of polymeric materials which are made by mixing or blending two or more polymers to enhance the physical properties of each individual component. Common polymer blends include PP-PC, PVC-ABS, PE-PTFE and PC-ABS. 

Graft copolymers are important as elastomeric (e.g., styrene-butadiene rubber (SBR)) and high-impact polymers (e.g., high-impact polystyrene and acrylonitrile-butadiene-styrene (ABS)). 

A high-impact polystyrene that has much better optical clarity than that obtained by usual blending or grafting techniques can be prepared by our technique. Polymers containing 90-95% styrene grafted to polybutadiene rubber by use of 12 mmole RLi-TMEDA/100 gram polymer showed quite good optical clarity. 

Anionic-Radical Combinations. Radical grafting of one monomer on the backbone of another polymer is well known and is the basis of an important commercial process for making high impact polystyrene. Styrene is thermally bulk polymerized in the presence of 5 to 10% (by weight) polybutadiene, the polymerization proceeding by a free-radical grafting path (70). 

Because no termination step exists, styrene/butadiene/styrene (SBS) triblock polymers can easily be made by charging styrene, butadiene, and again styrene, in succession, to the catalyst. Because the polystyrene blocks behave like styrene homopolymer, these triblock polymers are not suitable for use in automobile tires, but they lend themselves well in polyblends and as the backbone in the graft polymerization of Impact polystyrene and ABS... 

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