General factors controlling the mode of addition

It may be expected that the stereochemistry of monomer addition to the centre will depend on the kind of the centre and on its momentary state. Let us call the radical or ionic end of a growing chain a free centre and an ion pair or complex a bound centre. 

The moment of fixation of a newly added monomer molecule depends on the geometry of the centre [86]. 

A tetrahedral carbon atom in the active centre usually preserves its configuration even during monomer addition. With monomer addition to a chain end, the configuration of the new end depends on the mode of double bond opening, cis [(a) in eqn. (34)] or trans [(b) in eqn. (34)]. If the approaching 

A planar form of the growing chain end is, of course, much more frequent. Most hydrocarbon radicals, carbenium ions, and carbanions are in the sp2 state. From a planar chain end, a specific structure is only formed after the addition of a further unit. When the possibility of (momentary) rotation of the chain end is excluded the sp3 configuration generated from the sp2 state depends on the direction of monomer approach, which is thus one of the factors determining the stereochemistry of addition 

The stereochemistry of addition to a free centre is mostly determined by interactions between the monomer and active centre during approach to the transition state. In simple cases, represented by equations (34) and (35) only the two primary components will interact, and Bernoulli statistics with a single probability parameter Pm will predominate. For Pm = 0.5, the propagation rate constants of isotactic and syndiotactic growth, kpj and k, will differ 

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