FREE RADICAL COPOLYMERISATION

Organic peroxides are used in the polymer industry as thermal sources of free radicals. They are used primarily to initiate the polymerisation and copolymerisation of vinyl and diene monomers, eg, ethylene, vinyl chloride, styrene, acryUc acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile, and butadiene (see Initiators). They ate also used to cute or cross-link resins, eg, unsaturated polyester—styrene blends, thermoplastics such as polyethylene, elastomers such as ethylene—propylene copolymers and terpolymers and ethylene—vinyl acetate copolymer, and mbbets such as siUcone mbbet and styrene-butadiene mbbet. 

The mutual polymerisation of two or more monomers is called copolymerisation. This topic has been comprehensively reviewed (4,5). Monomers frequentiy show a different reactivity toward copolymerisation than toward homopolymerisation. In fact, some monomers that can be bomopolymerised only with great difficulty, can be readily copolymerised. One such monomer is maleic anhydride. It is rather inert to free-radical homopolymerisation yet can be copolymerised convenientiy with styrene and many other monomers under free-radical conditions. 

This is a linear polyester containing phthalic anhydride to ensure hydrocarbon solubility and maleic anhydride to enable copolymerisation to take place, esterified with 2-propanediol. The ester is dissolved in styrene which initially acts as the solvent and subsequently as film former when it is copolymerised with the double bond in the ester by free radical induced polymerisation. 

As already shown, it is technically possible to incorporate additive functional groups within the structure of a polymer itself, thus dispensing with easily extractable small-molecular additives. However, the various attempts of incorporation of additive functionalities into the polymer chain, by copolymerisation or free radical initiated grafting, have not yet led to widespread practical use, mainly for economical reasons. Many macromolecular stabiliser-functionalised systems and reactive stabiliser-functionalised monomers have been described (cf. ref. [576]). Examples are bound-in chromophores, e.g. the benzotriazole moiety incorporated into polymers [577,578], but also copolymerisation with special monomers containing an inhibitor structural unit, leading to the incorporation of the antioxidant into the polymer chain. Copolymers of styrene and benzophenone-type UV stabilisers have been described [579]. Chemical combination of an antioxidant with the polymer leads to a high degree of resistance to (oil) extraction. 

Copolymerisation can be brought about by many types of polymerisation reactions. The majority of the commercially important copolymers, however, are made by free-radical, ionic or polycondensation polymerisation. 

In the second stage of the reaction, the free radical produced on the backbone of the base polymer initiates polymerisation which results in the formation of graft copolymerisation as under ... 

Attempts to polymerise isobutene by free radical catalysis have all failed [16,17] and copolymerisation experiments show that the t-butyl radical has no tendency to add to isobutene. The reasons for these facts are not at all obvious. Evidently, they cannot be thermodynamic and therefore they must be kinetic. One factor is probably that the steric resistance to the formation of polymer brings with it a high activation energy [17], and that the abstraction by a radical of a hydrogen atom from isobutene, to give the methallyl radical, has a much smaller activation energy. This reaction will also be accelerated statistically by the presence of six equivalent hydrogen atoms. 

This method involves graft copolymerisation using redox initiators [70] or free radical initiators [71, 72, 73] usually in the solution phase, occasionally under the influence of temperature, predominantly in the latter case. Redox systems have extensively been used to generate active sites especially on the natural polymers [74] (like cellulose). Transition metals viz. Cr+6, V+5, Ce+4,... 

Organoaluminium compounds also produce free radicals by homolytic cleavage of the A1 C bond in the complexed polar monomer [scheme (97)], and thus they initiate the copolymerisation [549,550] ... 

Free radical copolymerisation of divinylbenzene gives crosslinked resins that have been shown to often still bear many unreacted pendant vinyl groups7. These remaining pendant vinyl bonds as well as the crosslinking level can be quantified by FTIR. The value obtained for the produced support is about 3.0 mmol/g of pendant vinyl bonds. 

Copolymers are obtained by free-radical copolymerisation of hexafluoroacetone with, for example, alkenes, tetrafluoroethene and epoxides. 

Tidweli, P. W., and G. A. Mortimer Reactivity in free-radical copolymerisation. Polymer Letters 4, 527 (1966). 

The major limitations of the feed forward control strategy presented here are that (i) it is only as good as the fundamental data which are used in the models and (ii) it can only be used for systems which conform to the conventionally accepted mode of behaviour of free radical chain polymerisation in solution. However, the same approach can be used with the appropriate models for any copolymerisation process. The range of application can be increased by making an arbitary assessment of the parameters necessary for the control models and/or by introducing a feedback loop which incorporates some state measurement device, e.g., an in-line gas chromatograph for measurement of residual monomers concentrations. Such a scheme is shown in Figure 21. 

The role of weak links has been considered in all thermal-initiation possibilities (Scott, 1995), and the tendency of many monomers to copolymerise with trace amounts of oxygen in free-radical polymerization (in the case of styrene, to form a 1 1 copolymer) until it has all been scavenged is well known. 

At least one other possibility exists. Assume, for example, that a very small amount of BPA is used with an excess of ECH under conditions that favored homopolymerisation of ECH (which may be different than conditions used to copolymerise the two). BPA would not be a monomer because it would not occur as a repeat unit, but rather would become a locus of initiation for a homopolymer of ECH. The most accurate description of the substance would be oxirane, (chloromethyl)-, homopolymer, ether with 4,4 -(l-methylethylidene) bis[phenol] (2 1), CASRN 139873-26-0. This principle also applies for cases in which a reactant such as a peroxide is used as a free radical initiator for a vinyl polymer. For example, a copolymer of monomers A, B, and C made using a free-radical initiator D may be called A, copolymer with B and C, D-initiated. Before 1989, the EPA had not informed industry of the need to include free-radical initiators as part of a polymer name, and therefore polymers placed onto the TSCA Inventory before 1989 do not have to include the free-radical initiator in the polymer name, even if it is used at a level of greater than two percent. In the latter case, the polymer would be named as A, polymer with B and C, without reference to the initiator. 

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