Polyolefin free radical reaction mechanism

T he oxidation of polyolefins under the influence of heat or light is usually regarded as involving a free radical chain mechanism having the following main reactions (3) ... 

Free-radical reaction between polyolefins can be produced by peroxide, high energy radiation, and thermal and/or mechanical shear [12]. When radicals of the two polymers recombine with each other, this immediately produces block or graft copolymers. 

T his paper presents a polymerization reaction, as yet unreported in the literature, wherein block polymerization of a free radical type can be caused to take place onto an actively growing chain which had proceeded by an anionic mechanism. Specifically, a Ziegler type of polymerization, such as that of propylene or ethylene, can be interrupted by adding vinylic monomers and an organic peroxide, and a vinyl polymer grown on the end of the polyolefin. For simplicity we will refer to these types as anionic free radical (AFR) polymerizations. 

Previous investigations have shown that polyisobutene and poly(isobutene-co-iso-prene), i. e. butyl rubber, are unable to cross-link in the presence of free radicals, since extensive chain scissions occur and thus low molecular weight products are formed The degradation mechanism proposed by Loan involves, in the case of polyisobutene, the H abstraction from methyl groups followed by chain scission. Apparently, the formation of secondary alkyl radicals, which are believed to be responsible of polyolefin radical curing is prevented for steric reasons by the presence of two adjacent dimethyl substituted carbon atoms and hence j3 scission reactions prevail. 

Co-reaction of blends to improve the performance has for decades been a practice in the rubber industry. In high-shear mixers some of the chains in rubbers are broken and are reformed by the free-radical mechanism. A similar phenomenon occurs during intensive mixing of polyolefins (see Table 4.34). To enhance this process, sometimes a free-radical source, e.g., peroxides, can be added. 

The adherence to the polymer and elimination of diffusion to the environment are both essential however, they should be mobile enough to move to the surface layer. Novel stabilizers, based on steric hindered amines (HALS) are considered to be very efficient, albeit there is lack of agreement regarding their stabilization mechanism. It is assumed that there exists a mutual reaction of antioxidation and hydroperoxide decomposition combined with the destruction of free radicals. These stabilizers are essential for protecting polyolefins and other polymers. 

Polymers may be attacked by molecular oxygen, ozone, or by indigenous free radicals in the polymer. Thermal-oxidative degradation of polyolefins in air is autocatalytic, i.e., the rate is slow at first but gradually accelerates to a constant value. According to the three-step mechanism outlined below, the RO2 peroxy radicals formed (Step 1) are sufficiently reactive to attack some primary CH bonds of the chain R H (Step 2). The peroxy radical RO2 is thus reformed (Step 3) and can attack another CH bond. This chain reaction continues until termination occurs (Step 4) [1-11]. 

Oxidative degradation of polymers typically follows a free-radical mechanism involving crosslinking and/or chain scission initiated by free radicals from peroxides formed during the initial oxidation step [1-11]. Enhanced stability has been achieved by the use of additives which are frequently called antioxidants or heat stabilizers. One approach employed to reduce the oxidation of polyolefins like PE and PP is to terminate the chain reaction by introducing an antioxidant with a greater affinity than a polyolefin for the peroxy radical RO. Such antioxidants (AH) function by reacting with RO2 to form a relatively inactive radical A, i.e.,... 

SABRA n. Abbreviation or surface activation beneath reactive adhesives, a method of bonding plastics, such as polyolefins and polytetrafluoroethylene, that are normally unreceptive to adhesives without pretreatment. The method consists of mechanical abrasion of the surfaces to be joined to roughen their outer layers scission of bonds with creation of free radicals, and further reaction with primers in the liquid, vapor, or gaseous phase. An adhesive such as an epoxy is then applied. 

Based on the same three considerations (i.e., stability, solubility, and versatility) of the reactive comonomer, we also investigated p-methylstyrene (p-MS) [40 14]. The major advantages of p-MS are its commercial availability, easy incorporation into the polyolefin, and versatility in functionalization chemistry under various reaction mechanisms, including free radical, cationic, and anionic processes. The benzylic protons are known to be readily reactive in many chemical reactions (such as halogenation, metallation, and oxidation) to form a desirable functional group at the benzylic position tmder mUd reaction conditions, as illustrated in Scheme 4. 

A DMTA study of polyolefin-clay nanocomposites has shown that alpha, beta and gamma relaxations of the polymer were affected by polymer chain branching and clay exfoliation level [50]. Salmeron Sanchez and co-workers [51] studied the structure of the system obtained after free radical copolymerisation of ethyl acrylate and hydroxyethyl methacrylate comonomers using dynamic-mechanical and calorimetric techniques. Copolymerisation theory states that the free radical copolymerisation reaction of two monomers may give rise to a copolymer with a different chain composition from that of the random mixture corresponding to the original solution. In this system, the dynamic-mechanical spectra suggested there were two main alpha relaxation processes in the copolymers. 

The most likely reason might be as follows. The basic mechanisms consist of an initiation step in which hydrogen is removed from molecule leaving the free radical. This radical react with oxygen to form a peroxy radical and the autoxidation is started. The temperature dependence of these reactions is the same in the simple polyolefin. 

The chemical mechanisms involved in the action of antioxidants have been discussed in a number of reviews [8,11-18] and the reader is directed to these and the references they contain for more detailed information. Two complementary antioxidant mechanisms are frequently used synergistically in polyolefins. The first is the kinetic chain-breaking hydrogen donor process, (CB-D) summarised in reaction (3). The relatively stable radicals (A) produced (e.g. phenoxyl from phenols and aminoxyl from aromatic amines) carmot continue the kinetic chain and disappear from the system by coupling with other or the same free radicals. However, it should be noted that this process is stoichiometric and hydroperoxides... 

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