Free radical polymerization polyolefins

There are two types of polyethylene and polypropylene, called low density and high density. High-density polyolefins are made on a catalyst, while low-density polyolefins are made by free-radical polymerization. Characteristics of these polyolefins are summarized in Table 11-2. 

The free radical polymerized functional polymers with controllable molecular weight are therefore chemically bonded to the side chains of polyolefin (II). One example of PP-g-PMMA [12,13] is shown in Table 1. 

Among the processes used for the formation of polyolefins, the longest-known but least selective one is free radical polymerization. A free radical species X produced e.g. by thermolysis of benzoyl peroxide or by photolysis of azabisisobutyronitrile (AIBN) - can react with the double bond of a vinyl derivative H2C=CHR to form a new radical of the type XCH2-CHR which can then add another H2C=CHR unit repetition of this process leads to polyolefin formation (Figure 2, top). This process works best for vinyl derivatives with unsaturated side groups, which provide resonance stabilization for an adjacent radical centre, e.g. with vinyl and acrylic esters, vinyl cyanides and vinyl chloride and with styrene and 1,3-dienes. It is extensively used in the emulsion polymerization of vinylic and acrylic derivatives and in the light-induced formation of photoresists for the nanofabrication of semiconductor chips and integrated electronic circuits. 

The next major step, almost two decades after the discovery by ICI of then-high-pressure free radical polymerization of ethylene, was the discovery, from 1951, of (metal oxide and organo-metallic) catalysts that produced essentially linear high molecular weight polyethylene (and other polyolefins) under much lower pressures. These catalyst discoveries occurred almost simultaneously and independently in several laboratories in the USA and Europe [12]. 

The production of polyolefins by means of coordination polymerization, which is the highest tonnage polymerization process, is discussed first. The following chapters present the production of polymers by free-radical polymerization in homogeneous, heterogeneous and dispersed (suspension and emulsion) media. Afterwards, the reaction engineering of step-growth polymerization is discussed. The last chapter is devoted to the control of polymerization reactors. 

To prepare polyolefin blends, PO (e.g., EVAc, PE, PP, EPR), with vinyl polymers, 10-200 parts of vinyl monomer [e.g., (meth)acrylates, styrenics, vinyl chloride, glycidyl methacrylate, maleic anhydride, acrylonitrile, divinylbenzene] and 0.01-4.0 parts of a free radical initiator were used to impregnate 100 parts of PO particles at T = 20-130 °C. After 50-99 wt% of the monomer was absorbed, the particles were dispersed in water and the free radical polymerization initiated. Good adhesion between the components in the extruded or molded articles was achieved... 

Free-radical polymerization is used to produce graft copolymers. Free-radical sites on the macromolecule provide sites for an unsaturated copolymer to graft, a method commonly used for polyolefins [9, 14]. 

The unexpected good control in the incorporation of borane groups to polyolefin by metallocene catalysis and the subsequent radical chain extension by the incorporated borane groups prompted us to examine this free radical polymerization mechanism in greater details. Several relatively stable borane-based radical initiators were discovered, which exhibited living radical polymerization characteristics, with a linear relationship between polymer molecular weight and monomer conversion [27] and producing block copolymers by sequential monomer addition [28]. This stable radical... 

Polyethylenes are the commercial polyolefins with the highest tonnage. Figure 8.1 compares the molecular structures of some polyethylene resins made by coordination and free-radical polymerization. (Free-radical polyolefins are not the main topic of this chapter and will be described only in contrast with polyolefins made with coordination catalysts.)... 

The first commercial polyolefin arrived with Imperial Chemical Industries s (ICI) production of free-radical-polymerized, low-density polyethylene (LDPE) in the 1930s. The process was carried out in supercritical ethylene at extreme... 

Global polyolefin demand grew over 5% per year between 1990 and 2010 to a volume of 120 000 000 metric tons. The high-pressure free-radical-polymerized LDPE grew just 1.25% per year. On the other hand, LLDPE, HOPE, and PP grew at 7.6%, 5.3%, and 6.8%, respectively (Figure 18). 

The reactive metal-polymeryl bond has been used to form various chain end functional polyolefins, including hydro-xyls, " azides, amines, and iodo-funcdonal materials (Scheme 16). A number of PEs bearing initiators for free-radical polymerizations and macromonomers have been reported in recent years. These building blocks have been used to synthesize a variety of functional polyolefin block and graft copolymers. These materials show pronuse for improved compatibility and adhesion to polar materials. 

Monomer concentrations and pressures are not very high in Ziegler-Natta polymerization (about 2-3 MPa), contrary to the free radical polymerization of ethylene which requires very high pressures. For obvious reasons, industrial producers of polyolefins focus their attention on the productivity of a catalytic system which corresponds to the mass of polymer produced (and not its rate of formation) per mass unit of transition metal, without mention of time. In Table 8.18 the typical components of a currently used Ziegler-Natta system for the industrial production of polypropylene are given. 

In polyolefins, the chain is propagated by an intermediate free-radical species or by an alkyl species adsorbed onto a solid. Both the free radical and the alkyl have the possibility of termination, and this creates the possibility of growth mistakes by chain transfer and chain-termination steps that create dead polymer before all reactants are consumed. The presence of termination steps produces a broader molecular-weight distribution than does ideal addition polymerization. 

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