Chain branches/branching

PVAc is known to contain a significant number of long chain branches. Branches to the acetate methyl may arise by copolymerization of the VAe macromonomcr produced as a consequence of transfer to monomer (Section 6.2.6.2). Transfer to polymer may involve either the acetate methyl hydrogens (Scheme 6.34) or the methine (Scheme 6.35) or methylene hydrogens of the polymer backbone. 

The resonance assignments of the branches are given in the figure. A determination can be made of the number of each type of branch length from measurements of the areas of the respective branch peaks and are reported as number of branches per 1000 CH2. The level of short-chain branches (branches of length 5 or shorter) increases from 6 per KXX) CH2 at a polymerization temperature of 165°C to 20 branches for polymer prepared at 270 C. The typical short-chain branching mechanisms for ethylene homopolymerization are shown in Fig. 7.4. 

The assumption of quasi-stationarity can sometimes be justified if there is no significant chain branching, for example in FIBr fomiation at 200-3 00°C ... 

Effect of Chain Branching on Reactivity of Primary Alkyl Bromides Toward Substitution Under Sn2 Conditions ... 

Chain branching Correction factor Chain branching Correction factor... 

During the polymeriza tion process the normal head-to-tad free-radical reaction of vinyl chloride deviates from the normal path and results in sites of lower chemical stabiUty or defect sites along some of the polymer chains. These defect sites are small in number and are formed by autoxidation, chain termination, or chain-branching reactions. Heat stabilizer technology has grown from efforts to either chemically prevent or repair these defect sites. Partial stmctures (3—6) are typical of the defect sites found in PVC homopolymers (2—5). 

One characteristic of chain reactions is that frequentiy some initiating process is required. In hydrocarbon oxidations radicals must be introduced and to be self-sustained, some source of radicals must be produced in a chain-branching step. Moreover, new radicals must be suppHed at a rate sufficient to replace those lost by chain termination. In hydrocarbon oxidation, this usually involves the hydroperoxide cycle (eqs. 1—5). 

New radicals are introduced by thermolysis of the hydroperoxide by chain-branching decomposition (eq. 4). Radicals are removed from the system by chain-termination reaction(s) (eq. 5). Under steady-state conditions, the production of new radicals is in balance with the rate of radical removal by termination reactions and equation 8 appHes for the scheme of equations 1—5 where r. = rate of new radical introduction (eq. 4). 

Reversibility of Equation 2. Notwithstanding the problems and conflicts, there is widespread agreement that the NTC phenomenon may well be related to the reversibiUty of equation 2 (13,60,63—67) R- + O2 ROO-. In the low temperature regime, the equiUbrium Hes to the right and alkylperoxy radicals are the dominant radical species. They form hydroperoxides, the chain-branching agent, by reaction 3. 

Ethylene oxide is a coproduct, probably formed by the reaction of ethylene and HOO (124—126). Chain branching also occurs through further oxidation of ethylene hydroxyl radicals are the main chain centers of propagation (127). 

Structure. The physical properties of LDPE depend on the molecular weight, the molecular weight distribution, as well as the frequency and distribution of long- and short-chain branching (2). 

The short side chain branching frequency is inversely proportional to polymer crystallinity. Short branches occur at frequencies of 2—50 per 1000 carbons in chain length their corresponding crystallinity varies from 35 to 75%. Directiy proportional to the polymer density, crystallinity can be calculated by the following formula,... 

Both propylene and isobutylene ate comonomers that are incorporated along the chain, resulting in additional short-chain branching. One important factor in controlling polymer crystallinity is the choice of chain-transfer agent. Ethane and methane, for example, are inefficient agents whose presence in the monomer feed stream must be considered in reaction control. 

Crystallinity and Density. Crystallinity and density of HDPE resins are derivative parameters both depend primarily on the extent of short-chain branching in polymer chains and, to a lesser degree, on molecular weight. The density range for HDPE resins is between 0.960 and 0.941 g/cm. In spite of the fact that UHMWPE is a completely nonbranched ethylene homopolymer, due to its very high molecular weight, it crystallines poorly and has a density of 0.93 g/cm. ... 

As a rule, LLDPE resins do not contain long-chain branches. However, some copolymers produced with metallocene catalysts in solution processes can contain about 0.002 long-chain branches per 100 ethylene units (1). These branches are formed in auto-copolymerisation reactions of ethylene with polymer molecules containing vinyl double bonds on their ends (2). 

Dow catalysts have a high capabihty to copolymetize linear a-olefias with ethylene. As a result, when these catalysts are used in solution-type polymerisation reactions, they also copolymerise ethylene with polymer molecules containing vinyl double bonds at their ends. This autocopolymerisation reaction is able to produce LLDPE molecules with long-chain branches that exhibit some beneficial processing properties (1,2,38,39). Distinct from other catalyst systems, Dow catalysts can also copolymerise ethylene with styrene and hindered olefins (40). 

Temperature-risiag elution fractionation (tref) is a technique for obtaining fractions based on short-chain branch content versus molecular weight (96). On account of the more than four days of sample preparation required, stepwise isothermal segregation (97) and solvated thermal analysis fractionation (98) techniques usiag variatioas of differeatial scanning calorimetry (dsc) techniques have been developed. 

Comonomers can be used to create a variety of polymer stmctures that can impart desirable properties. For example, even higher molecular weight PPS polymers can be produced by the copolymerization of a tri- or tetrafunctional comonomer (18). The resultant polymer molecules can have long-chain branching, which can be used to tailor the rheological response of the polymer to the appHcation. 

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