Structures Formed by Backbiting in Ethylene Copolymerization

There are no proven examples of 1,2-hydrogen atom shifts this can be understood in terms of the stereoelectronic requirements on the process. The same limitations are not imposed on heavier atoms e.g. chlorine). The postulate that ethyl branches in reduced PVC are all derived from cbloroethyl branches formed by sequential 1,5-intramolecular hydrogen atom transfers as described for PE (Section 4.4.3,1) has been questioned.It has been shown that many of these ethyl branches are derived from dichloroethyl groups. The latter are formed by sequential 1,2-chlorine atom shifts which follow a head addition (Section 4.3.1.2). 

The high lemperalure polymeri/.alion of acrylates with the backbiting-fragmentation process has been used to synthesize macromonomers based on Interestingly, fragmentation shows a strong preferenee for 

In this section, we consider the kinetics of propagation and the features of the propagating radical (Pn ) and the monomer (M) structure that render the monomer polymerizable by radical homopolymerization (Section 4.5.1). t he reactivities of monomers towards initiator-derived species (Section 3.3) and in copolymerization 

Polymerization thermodynamics has been reviewed by Allen and Patrick, Ivin, Ivin and Busfield, Sawada and Busfield. In most radical polymerizations, the propagation steps are facile (Ap typically 10- M- s -Section 4.5.2) and highly exothermic. Heats of polymerization (A//p) for addition polymerizations may be measured by analyzing the equilibrium between monomer and polymer or from calorimetric data using standard thcrmochcmical techniques. Data for polymerization of some common monomers are collected in Table 4.10. Entropy of polymerization (AS p) data are more scarce. The scatter in experimental numbers for AWp obtained by different methods appears quite large and direct comparisons are often complicated by effects of the physical state of the monomer and polymers i.e whether for solid, liquid or solution, degree of crystallinity of the polymer). 

The value of and the propagation/depropagation equilibrium constant (Aieq) can be measured directly by studying the equilibrium between monomer and polymer or they can be calculated at various temperatures given values of A//p and AAp using cq, 11 and 12 respectively. 

The high temperature polymerization of acrylates with the backbiting-fragmentation process has been used to synthesize macromonomers based on acrylate esters. 277,312 Interestingly, fragmentation shows a strong preference for giving the polymeric macromonomer 64 and a small radical 65. 276.277 An explanation for this specificity has yet to be proposed. 

---

Table 4.9 Structures Formed by Backbiting in Ethylene Copolymerizations ...

---