By Frank A. Bovey (Eds.)
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Extra resources for Macromolecules. An Introduction to Polymer Science
A v β a v s 2. 41 Formation of Macromolecules J At/T FIG. 2 . 2 . Semilog plot of R (ave)/K (stationary) vs. At/τ and Δί:( ), theoretical curve (abscissa A log scale); ( - Ο - ) , experimental curve (abscissa A , log scale), arbitrarily chosen; A = abscissa A displaced until both curves coincide (Vollmert, 1 9 7 3 ) . p p l 9 3 2 1 ? sponding to the stationary-state radical concentration (R (stationary)) against log Δί (Fig. 2). The shape of this curve may be calculated theoreti cally (Briers et al, 1926) as a function of log(Ai/r).
The radical concentration rapidly increases to its stationary-state value, and polymerization takes place at a rate equal to kJl , where k is the apparent overall rate constant. During the dark period, the rate of polymerization will be essentially zero since the time interval over which the radicals decay is very short compared with the length of the dark period. 16) 0 3 p 2 1 / 2 p 0 p 12 Q = t 40 Μ. J. Bowden N o w suppose that t h e sector is greatly speeded u p so that the flashes a r e considerably shorter than τ.
Such groups increase the electron density at the double bond, thereby facilitating attack 2. 27 Formation of Macromolecules by a cationic species, and also stabilize the resulting propagating carbenium ion by resonance d e r e a l i z a t i o n of the positive charge. Vinyl c o m p o u n d s containing such substituents as alkyl, alkoxy, phenyl, and alkenyl groups may therefore be expected to polymerize cationically. F o r example, the following resonance structures may be written for vinyl ethers: Η Η R+ + C H = C > R—CH —C 2 Η + 2 OR < > R—CH —C 2 OR OR + Halogens are electron withdrawing by inductive effects and electron donating by resonance effects, but since both effects are weak neither cationic nor anionic polymerization is appreciably facilitated.
Macromolecules. An Introduction to Polymer Science by Frank A. Bovey (Eds.)