Poly short-chain branches

The distribution of volatile products of low molar mass from the irradiation of poly (olefin) s is strongly dependent on the nature of substituents (short-chain branches) on the backbone chain. Hydrogen is the main volatile product with smaller quantities of alkanes and alkenes. 

We have studied the alkane and alkene yields from the radiolysis of copolymers of ethylene with small amounts of propylene, butene and hexene. These are examples of linear low density polyethenes (LLDPE) and models for LDPE. Alkanes from Ct to C6 are readily observed after irradiation of all the polymers in vacuum. The distribution of alkanes shows a maximum corresponding to elimination of the short-chain branch. This is illustrated in Figure 8 for the irradiation of poly (ethylene-co-1-butene) containing 0.5 branches per 1,000 carbon atoms at 20 C. 

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. 

LCT (originally developed for di-block copolymers) was found to be particularly useful to explain miscibility of polyolefin blends where the two resins differ in the type and size of short chain branching. The stractural units of a polymer with two carbons in the main chain can be written as PE = (CH,-CH,), PP = [CH,-CH (CHj)], poly-2-butene (P2B) = [CH (CH3)-CH (CHj)], PIB = [CH -C (CHj) ]jj, poly(4,4-dimethyl 1-butene) (PDMB) = [CH -CH (C Hg)], etc. Three structural parameters (ratio of end to interior groups) have been used to distinguish PO structure r, p, and q. Their values for the model macromolecules discussed above are listed in Table 2.7. 

A transfer to polymer produces a free radical at the point of transfer and more monomer can add on at this point. This causes branching in the polymer. If the transfer reaction occurs intramolecularly as in poly(ethylene), then, short chain branching is the result ... 

The NMR spectroscopy of poly(vinyl chloride), which was reduced with tributyltin hydride, showed that the original polymer contained a number of short four-carbon branches [300]. This, however, may not be typical of all poly(vinyl chloride) polymers formed by free-radical polymerization. It conflicts with other evidence from NMR spectroscopy that chloromethyl groups are the principal short chain branches in poly(vinyl chloride) [301, 302]. The pendant chloromethyl groups were found to occur with a frequency of 2-3/1,000 carbons. The formation of these branches, as seen by Bovey and coworkers, depends upon head to head additions of monomers during the polymer formation. Such additions are followed by 1,2 chlorine shifts with subsequent propagations [301, 302]. Evidence from still other studies also shows that some head to head placement occurs in the growth reaction [303]. It was suggested that this may be not only... 

A transfer to polymer produces a radical site on the polymer, and this site can add on further monomer, thereby producing branched polymer. If transfer to polymer occurs intramolecularly as in poly(ethylene) (see Section 25.2.1), then short-chain branching will result ... 

Short chain branching in polyethylene decreases adsorbed polymer chain on the sur-faee of carbon nanotubes. Simulation study shows that the carbon nanotube seems to inerease more effieiently the polyethylene crystalhnity in the case of the branched chains than in the hnear ones. The presenee of surface groups on carbon nanotubes reduced their ability to nueleate poly(L-laetie aeid). At high supercooling, where homogeneons nucle-ation is prevalent, the addition of earbon nanotubes does not affect the crystallization rate ofPCL. 

The introduction of short-chain branches into polyethylene by use of an alpha-olefin comonomer yields the commercially important polymer called linear low-density polyethylene (LLDPE). These short branches would also be expected to affect the entanglement molecular weight. Based on previously published data on poly (a-olefins) [10, 53], Fetters etal. [60] developed the following empirical equations for estimating the plateau moduli of polymers of this type, given only the average molecular weight per backbone bond, mj,. 

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