L,2.L The Grignard reagent

Much of the controversy regarding the cornposltlon of Grígnard reagents has now been resolved and some excellent reviews Ìrave. 1-3 The reactions of Grignard reagents with varlose organic functional groups have been the c.atal and annual re-vÍe-rvs of the latest advances in organomagnesj,um. Subsequently, many papers were published regarding the excellence and standing of schlenk equlllbrfum L.1-, but most of these earlier conclusions were not valid even on the basis of information known in recent or more recent times.

The exact nature of the association of Grígnard reagents in ether was Ln L969 investigated by Ashby and co-workers l'6r17 and. The phenyl Grfgnard reagents are connected through the dfiner (Fig. 1.2) and are the best representation in this font opinion \das linear polymer ruodel form.

Using di-phenyl magnesium in toluene at -273K, Goode discovered that the stereoblock PMI'Í wa= forr.d?2" The. 34;no"J-, concluded that the mechanism was complex, a marked increase in styrene reactivity. ratio above 253K is observed. The microstructure of nitrel-containing polymers has not been extensively investigated, although isotactic polymethacrylo-.

Stereoregular polyvinyl pyridines have been produced using the 2-vinyl pyridine monomer, but not with 4-vinyl pyridine monomer. The role of complex formaldehyde as a factor influencing the microstructure of the polymer is revealed during polymerization shown.

The alkyl halides used to prepare the appropriate Grignard reagents were analytical reagent grade chemicals. The halides were refluxed over calcium hydrate for 12 hours and recently distilled before use, rejecting a generous fraction of the final distillate.

Tobrcnø

LrZ-dj-b'romoethane (B.D.H. London, analytl-ca1 reagentË) was. dried with calcium chloride, distilled and the lukewarm fraction. Since THF was the solvent used to prepare the initiator solutions, its purity was critical. The inductlon period before the start of the Grlgnard reaction is. depending on the amount of water in the solvent. A large amount of solvent for Grlgnard's preparation treated with 1. l-lithium aluminum hydride.

After refluxing over L1AIHO for 12 h, THF was reduced to 3, and a small fraction was collected. 1-r4 Dioxane (Univar, Aust, anal.y{:ícal reagent) was dropped onto CaH2t, degassed and distilled. was taken under vacuum to fresh CaHZ and then.

L GruLgnand nøa4ønf,t

The induction period at the beginning of the reaction is partly due to the thin layer of orride covering all the nragneslum that has been exposed to air. This drawback was neglected because despite the polarity of the solvent. affects the exchange rate of l-n organomagnetic systems. sectlon f,2,3) effectË Ís mlnor and amount of excess. dioxane rernaínJ-ng in the system after precipitation of the halide-. dio:

Spectrophotometric analysis of decreasing monomer concentration has been successfully applied to styrene polymerizations' - but here again, under certain conditions, the presence of side products that absorb in the same region as the ionomer complicates the mat in our system, MMA Ultraviolet absorption is obscured by the strong absorption of toluene and thus spectrophotometry cannot be used. In systems (such as anionic polymerization) where the average molecular weight (ll) of the polymer increases with time,

Figure 2.4 shows the dilatometer filting apparatus used through-

By adjusting the nitrogen flow, the temperature, measured with a 200 A thermlstor, could be maintained for tJK., a more sophisticated cell holder was designed that could maintain the temperature within tO. In most cases, the polymerizing mixture was sealed by opening into the atmosphere and then poured in tenfold. The solution was acidified with hydrochloric acid to remove the inorganic material and the solution was stirred for several hours.

Instances where the above procedure was not followed were in certain spectroscopic experiments where a single drop of methanol was added to close the rectphone. Polymer samples were dissolved in ethanol-free chloroform to give a final concentration of 3e" w/w and spectra were performed at 312K using a Varian T 60 nmr. The instrument was a newer Associates mod.el 501 ALC/GPC instrument suitable for analytical liquid chromatography as well.

The instrument was calibrated using narrow polystyrene sand grains and the curves are shown in Figure 4.10'. The 'absolute' elution volume was thus replaced by what should probably be called the elution count.

CHAPIER 3

The depenilence of poll.rnerization rate on the initial Grignard reagent concentration, [arr]o, vras determined at 223K by keeping the. monomer concentration and concentration of THF while changing the Grignard reagent concentration. This was achieved by keeping the volume of monomer and tofuene constant and evaporating THF from, or by adding the initiator solution to maintain a constant THF. Deipenclence o-f Íinitial rate of polSnneriza'bion orl. j-initial eoncentr.:at-ion of actÍve n-BulÍg irond.s where;.

External - the first order of initial dependence - the rate of pol¡nler:j-zati.on at its beginning, the concentration of actions in the world under the sariie conditions, ions as sta becl. polymerization obtained from the initial rate of pollzmeri- percentage active compounds n-butyl-magnesium as determined by the equation. determined by the Schlenk equilibrium at that certain temperature" From figure 3.1 and 3.2 it can be concluded, that the initial rate of polymerization is directly proportional to the initial alkyl, .. poll, the rate of merization becomes very slow and unrepeatable. The reason for this is uncertain, but is most likely due to the presence of trace impurities in the initiator, monomer. or. This idea is reinforced by the observation that although the reproducibility is better than five percent for each group of initiators and the monon'rerr correlation between different groups is less reliable. As can be seen, these graphs are linear, even up to 40zr, and the slopes give the percent polling rate.

However, figure 3.4 (b) clearly shows the deviation from linearity of the first-order plot when large conversions are considered. The apparent Ítris discrepancy can be explained on the basis that it is very difficult to distinguish between linear and non-linear. The dependence of external monomer was determined at different initial monomer concentrations and constant [GrrJo and xtnn from.

Figure 3.2. External- first order d"epend.ence of initial- initial-rate of pol¡nler:j-zati.on on ther initia,l concentratic;n of actj.ve n-T3r:Juïg 'borld-s unrler the sariie cond,itions as sta becl

Tigure 3.4(a). Integrated rate plots for the

Rate calculations were carried out taking into account the precautions outlined in section 3.2 and it appears that the external order with respect to monomer is unity.

However, people have not found any evidence throughout the duration of this work to suggest that the ranking with respect to ["rr]o and/ot. The error bars in the dots are not the standard errors of the slopes calculated from the computer plot, but are an estimate of the maximum possible experimental error arising from errors in calibration of volumes, distribution of solutions and extrapolation of converter/time curves, etc. However, it is clear that the degree of polymerization decreases as the mole fraction of THF increases .

The properties are the same as those reported previously for the diethyl ether/toluene system.

Figure 3"5. Depend.ence of initial rate of polynreriza'tion on init:Lal- nonomer eoncentration.

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Õ Õ ffi

Ultraviolet spectra of MS,IA pollination initiated by diphenylhexyl-lithium (Dpltr,) in dioxan9l .ta the sodium salt of. 54,55¡ rnicization of Ir4IutA in the organomagnesium compound at Iow temperatures resulted in the immediate appearance of a. Unfortunately, the lifetime of this color was only 2 minutes at 223K and only slightly longer at lower temperatures and so.

Nevertheless, our spectroscopic studies have yielded some interesting results, which are shown in Figures 3.10 and 3.11. The spectra were performed with air as a reference and as shown in Figure 3.10, all absorptions below I = 280 nm are invisible. The peak position shifted slightly as the temperature vras increased until at 298K, the absorption maximum \^¡as at À = 296 nm.

Õ Æúr

ZB, [tUf1 = 2.8,mol dm-3 the pol'merízing solution turned solid

U for n-Brf4gBr. Possible reasons for this discrepancy will be
MeOMgR
MeOH + R-H
E,F = sr hn antL i values for poly(tof'¿) prepared in the presence of HI.{?n (see text)

The maximum in the Arrhenius plot is partly due to an irreversible termination of the growing chain. Goode * *52a discovered a large fraction of the cyclic trimer, IV in the Ph-MgBr/diethyl ether system at 273K.

9' MexÞ( g

The interesting aspect of these studies was the fact that M was independent of the initial monomeli concentration, e.g. Nevertheless, the comparison of peak heights at the same elution volume for identical sample concentrations can give some idea of the change in the peak. The broadening of the peak on the low MW side will increase this ratio, but will decrease this ratio. special distribution.

The rejection of these particular results will not invalidate any of the conclusions drawn from the kinetic results in chapter 3' (section 6.1). Flourish the discussion in Section 6.2 regarding the stereo-specifying aspect of organomagnesium-initiated polymerization. With each initiator used, as the mole fraction of THF increased, syndiotacticity increased at the expense of isotacticity.

Branching at the o-carbon atom of the alkyl group of the Grignard reagent favors the isotactic arrangement, a fact which has been known for a long time, but. It should be noted here that it is pure enough that it defies the whole sample tactic of a polymer. Most of the experiments were done using n-BuMgBr as initiator, because this system was studied more intensively by dilatometry, however, some of the techniques were extended to.

The experiments comprising this chapter include a detailed description of the development of an adiabatic reaction calorimeter to enable the determination of heats of polmerization. This jacket ended just above that part of the calorimeter which was immersed in the low temperature thermostat. The rise in temperature of the solution during polymerization was recorded continuously and the temperature of the thermostat periodically.