Perhaloacetaldehyde polymerization

Stereor ularity as a Fimcrion of Side Chain Size in Perhaloacetaldehyde Polymerization... 

The study of the perhaloacetaldehyde polymerization provides an ideal example for the investigation of the polymerizability of the aldehydes, the relationship of the polymer stereoregularity as a function of substituent size and shape. 

No unusual initiators, such as transition metal complexes or heterogeneous catalysts, are needed for perhaloacetaldehyde polymerizations, the polarization of the carbonyl group of the aldehyde monomer is well defined and does not cause the formation of head to head linkages in the polymer. The shorter carbon oxygen single bond (1.43 A) which is formed by ring opening of the carbonyl double bond (1.21 A) has a beneficial effect for the formation of a helical structure for the isotactic polymer and should, consequently, favor the formation of isotactic polymer. 

In order to compare the rates of polymerization of perhaloacetaldehyde polymerization with different initiators, the polymerization of DCBA was studied at -10 C. and at initiator concentrations of 2 mole perr cent. The rate of polymerization of DCBA with pyridine is very fast. The rate of DCBA polymerization with sulfuric acid is much slower but is faster than that of chloral polymerization which is shown in curve A. 

Fieure 1. Rate of perhaloacetaldehyde polymerization NMR study. Initiator, pyridine initiator concentration, 2 mol % polymerization bath temperature, —78°C. B, DCBA C, CDBA D, hrotrud. 

Initiation of perhaloacetaldehyde polymerization must be done above the polymerization threshold temperature (at one molar monomer solutions, this is the ceiling temperature of polymerization) in order to provide complete mixing of initiator and monmer prior to polymerization- To form a homogeneous mixture is particularly important, when the initiation equilibrium is very much on the side of the addition of the initiator to one mole of monomer (Effective initiation), because the polymer which forms rapidly, precipitates and occludes unused initiator. Growing polymer ends are also occluded and the polymerization comes to a standstill both effects cause the polymers to be formed in low yield and/or tow molecular weight-... 

Ue have been able to determine the ceiling temperature of polymerization of the perhaloacetaldehyde polymerization by determining the threshold tempera-tu-... 

Anionically polymerized perhaloacetaldehyde polymers precipitate generally as a gel, are prepared at high polymerization rates and give polymers which cannot be end capped and have apparently occluded end groups with alkoxide character.(12)... 

Perhaloacetaldehyde polymers are of different physical appearance, powdery, have often —OH end groups and are obtained at a very slow polymerization rate. 

The type and combination of the halogen atoms in the trihalomethy1 group would also be a determining factor for the rate of polymerization of the perhaloacetaldehyde and for the location of the ceiling temperature of polymerization. 

The question was now to determine the results of the polymerization experiments with perhaloacetaldehydes with increasing substituent size as shown in Eqn. 4. 

The sulfuric acid initiated polymerization shows induction periods not normally encountered in anionic polymerizations of perhaloacetaldehydes. 

Copolymerization between individual perhaloace-taldehydes are sensitive to reaction conditions, reaction temperature and initiator type. Perhaloacetalde-hydes polymerize quite readily with chloral fluoro-subsitituted perhaloacetaldehydes are more preferentially incorporated into the copolymers. Chloro- and bromo- substituted perhaloacetaldehydes were not very reactive in the copolymerization and chloral rich polymers were obtained. Bromal can be incorporated into copolymers with chloral only with great diffuculty and from a feed mixture containing 25 more % bromal only 1.5 mole % bromal was incorporated into the copolymer. 

Copolymerizations of aldehydes with isocyanates are well known and for perhaloacetaldehydes, aromatic isocyanates are the best comonomers. The polymerization of phenylisocynates with chloral has been most extensively studied. (21,22). The copolymerization of... 

In Table 3 the ceiling temperature of polymerization of all the polyaldehydes are listed together with spectral characteristics of the perhaloacetaldehydes. It may be seen that the T of fluorosubstitu-ted acetaldehydes are higher tha those of chloro- and bromosubstituted perhaloacetaldehydes. The "mixed" perhalaloacetaldehydes have T s somewhere in between those of the three halo-substituted perhaloacetaldehydes. 

It appears that there is a linear relationship of the contribution of temperature increment of each of the C—X bonds of the trihalomethy1 groups to the value of the ceiling temperature of polymerization for each of the perhaloacetaldehydes. Some of the measurements of the threshold temperatures of the more volatile perhaloacetaldehydes are still in the process of being refined. Our calculations and final values will be presented at a later time. 

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