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THE TACTICITY GOVERNED STEREOMICROSTRUCTURE IN POLY(METHYL METHACRYLATE) (PMMA) AS A WAY TO EXPLAIN ITS PHYSICAL PROPERTIES

THE TACTICITY GOVERNED STEREOMICROSTRUCTURE IN POLY(METHYL METHACRYLATE) (PMMA) AS A WAY TO EXPLAIN ITS PHYSICAL PROPERTIES

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N. Guarrotxena



68



stereomicrostructure of PMMA as was done previously for Poly(vinyl chloride) (PVC)

and Polypropylene (PP).



INTRODUCTION

A considerable amount of earlier work in our laboratory has dealt with the relationships

between the physical properties determining processes at molecular level and some tacticity

governed stereomicrostructures of poly(vinyl chloride) and polypropylene polymers [1-10].

These structures are some repeating stereosequences located at the end of isotactic or

syndiotactic sequences, such as mmrm and rrrm tetrads occurring necessarily whenever those

sequences break off respectively. The length of the isotactic or syndiotactic sequence

associated to mmrm or rrrm is very important too. To these may be added moieties of pure

heterotactic and atactic short sequences which both also influence on physical properties. The

frequency of those microstructures along the chain obviously depends on the type of tacticity

statistics in the polymer and then on the polymerization conditions. It is worth noting that an

equal number of r and m placements may give different numbers of each stereomicrostructure

depending on the tacticity distribution statistics of the polymer. This principle led us to

explore the extent to which that statistics and the resulting microstructures are or not a

prominent feature in polymer materials science.

The microstructures so defined are important disruptions of the chain regularity, each

involving changes in either local free volume or rotation mobility facilities or both (2,7-10).

As a consequence, the inter- and intramolecular interactions should be expected to change as

the content of the distinct microstructures changes, and hence .the physical properties of the

polymer should change too without taking place any change in the chemical composition of

the polymer.

By producing polymer samples of different overall microstructure, as accurately stated

especially through NMR spectroscopy, a straight relation between it and most of the physical

properties of PVC and PP materials could be demonstrated in the above quoted work [1-10].

In order to extend this knowledge to other polymers of industrial interest, we endeavoured to

study the Poly(methyl methacrylate) (PMMA) in a similar way to the used before. The major

requirement for such study is to ensure a detailed description of the stereomicrostructure of

some polymer samples prepared under different conditions so that some changes in the

overall tacticity are produced. This is the objective of the present paper where that

stereomicrostructure is assessed for three PMMA industrial samples.



EXPERIMENTAL PART

Materials

Commercial PMMA samples, obtained from Atochem were used in this work. PMMA

samples were purified using tetrahydrofuran (THF, Scharlau) as solvent and water as

precipitating agent, and then washed in methanol and dried under vacuum at 40 ºC for 48 h.

THF was distilled under nitrogen with aluminium lithium hydride (Aldrich) to remove

peroxides immediately before use.



The Tacticity Governed Stereomicrostructure …



69



Characterization of Samples

The tacticity of the three distinct PMMA samples was measured by 1H-NMR

spectroscopy on a Varian UNITY-500 spectrometer operating at 500 MHz with CDCl3 as

solvent at 50ºC. Parameters of 8000 Hz spectral width and 1.9 s pulse repetition rate were

used. The delay time was set at 1.9s. The spectra were obtaining after accumulating 64 scans

with a sample concentration of 10 wt% solutions. The relative peak intensities were measured

from the integrated peak areas, which were calculated with an electronic integrator.

The molecular weight distributions were measured by SEC using a chromatographic

system (515 Waters Division Millipore) equipped with a Waters Model 410 refractive index

detector. THF (Scharlau) was used as the eluent at a flow rate of 1 mL min-1 operated at 25ºC.

Styragel packed columns, HR1, HR3, HR4E and HR5E, were used. PMMA standars (Waters

Associates) in the range between 1.4 x 106 and 3 x 103 g mol-1 were used to calibrate the

columns.



RESULTS AND DISCUSSION

Microstructure of the Samples

The 1H-NMR spectra of samples X, Y and Z are compared in Figure 1. The indicated

resonance assignments for the mm, mr and rr centered pentads are those taken from literature

[11]. The spectra are typical of predominantly syndiotactic PMMA but showing different

isotactic contents. The quantitative amount of mm, mr and rr triads, as measured on the

spectra, are given in Table 1. Since the physical properties of poly(vinylchloride), PVC and

polypropylene, PP were demonstrated to relate to the tacticity arrangement along the chain, it

appeared of prime importance to examine this point for samples X, Y and Z. To do that we

first determined the type of repeating sequence statistics, whether Bernoullian or Markovian,

as calculated from the experimental values of mm, mr and rr triads [11]. Secondly the likely

fraction of each individual sequence was calculated according to the respective type of

tacticity distribution. And finally the values so obtained were compared, in a semiquantitative way to those stemming from direct measurement on the spectra, of some of the

pentads. Actually the latter are only approximate because the proximity of signals makes their

deconvolution rather complicate.

The extent to which each sample fits into, or departs from, Bernoullian statistics can be

determined from the mm, rr and mrandrm triad content as measured on the spectra. That

quantity is called the persistant ratio and is defined by ρ=P(s)P(i)/P(is), where P(s)=rr+1/2mr;

P(i)=mm+1/2mr and P(is)=1/2mr. The results obtained are 0.9725, 1.1220 and 1.2709. These

values would seem to indicate that sample X is Bernoullian, while samples Y and Z are

apparently non-Bernoullian isotactic.

Another criterion for Bernoullian statistics is based on the conditional probabilities of

first order Markov statistics, indicating the probability of occurrence of one m or one r diad

preceded by one r or one m diad respectively.



N. Guarrotxena



70



Table 1. 1H NMR data for the PMMA samples

rra



Pmb



Prb



Pm/rb Pr/mb



Pm/mb



Pr/rb



Sample



Mn



mma mra



X



45500



8.39 43.16 48.45 29.97 70.03 0.72 0.308 0.28



0.69



Y



43100



15.16 40.86 43.98 35.59 64.41 0.57 0.31



0.43



0.69



Z



44900



20.93 37.92 41.15 39.89 60.11 0.47 0.31



0.53



0.69



Figure 1. 500 MHz 1H NMR spectra of a X, b Y and c Z PMMA samples measured in CDCl3 at 50ºC.



They are denoted by P(r/m) and P(m/r) [11]. If Bernoullian statistics apply, P(m/r) +

P(r/m)=1. This sum is 1.02, 0.88 and 0.78 for samples X, Y and Z respectively, so confirming

that sample X is Bernooullian and samples Y and Z would tend to be somewhat Markovian.

However the values for Y and Z samples depart no sufficiently from unity for them to be

considered completely Markovian.

As a result the probability of forming pentads in sample X may be easily determined [11],

e.g. [rrrr]=P(s)4; [rmmr]=P(s)2 P(i)2; [rrrm]=P(s)3 P(i)2, etc. In the case of non-symmetric

sequences, like the latter, factor 2 is necessary because both directions should be counted.

The probability of forming pentads in samples Y and Z may be also calculated through

the conditional probabilities [11]. For example an mmrm pentad fraction is given as



The Tacticity Governed Stereomicrostructure …



71



[mmrm]=[mm] P(m/r) P(r/m)+[mr] P(r/m) P(m/m). The values obtained for the most useful

pentads for the purpose of this paper, are given in Table 2.

On the other hand, these values have been also determined from the spectra of Figure 1

by deconvoluting the overall triad signals into the individual pentad signals indicated in

Figure 1. The Origin Program was used. It allows both that deconvolution and the distribution

of every experimental triad percentage into the corresponding pentad percentages, through an

internal mathematical treatment.

Table 2.The iso, hetero and syndiotactic pentads values calculated through the

conditional of first order Markov statistics probabilities1 for PMMA samples



Triads

rr

mr

mm



Pentads

rrrr

mrrr+mrrm

rmrr+mmrr

mrmr+mmrm

rmmr

mmmr+mmmm



X

0.24

0.294

0.294

0.125

0.088

0.0457



Sample

Y

0.21

0.4545

0.3562

0.2043

0.100 0.1407



Z

0.1959

0.3495

0.3184

0.202

0.094

0.2166



The data so obtained are shown in Table 3. When comparing them to the calculated

assuming Markovian statistical tacticity distribution for samples Y and Z, (Table 2), a strong

divergence appears evident, specially in the order of changing from sample X to sample Z.

This proves the calculation way utilised to be wrong.

Since the departure of samples Y and Z from Bernoullian statistics is not so much great,

an attempt was made to compare the experimental values, and those obtained assuming

Bernoullian statistics for all the samples (Table 4). As can be seen the evolution order of each

pentad is satisfactorily coincident so indicating that Bernoullian statistics apply for all the

samples. The small deviations observed only for rrrr and rmmr pentads obey the contribution

of some little signals around 0.90 ppm, increasing from X to Z sample and being counted as

rrrr pentads, and the experimental uncertainties when deconvoluting the signals at mm region,

respectively.

It is worth noting that the difference between absolute calculated and experimental values

lie within the experimental uncertainties when deconvoluting the signals on the spectra. The

fact that the tacticity statistics of samples Y and Z are closer to, but not exactly the

Bernoullian statistics might also influence the calculated values. What is of major importance

is that there is no change in the evolution order in both sets of values.

Consequently the above results are quite valuable to settle the evolution of any repeating

stereosequence from one sample to the other. The sequences that were proved to be the major

driving force for the physical properties of PVC and PP according to earlier work, are: i) the

average isotactic and syndiotactic sequences length; ii) the mmr-based and the rrm-based

local structures which occur necessarily whenever an isotactic or syndiotactic sequences

breaks off respectively.

Nevertheless, these structures are not active by themselves because it is the occurrence of

either one m placement following mmr or one r placement preceding rrm and the length of the

-mm…- and –rrr….- sequences connected with them, that were identified as a property



N. Guarrotxena



72



determining factor; thence the really important factors are: a) the fraction of mmr followed by

one m placement (-mmrm-structures) and the length of the isotactic sequence preceding

mmrm.

In fact the ratio of –mmrm- repeating stereosequences of at least one heptad in length to

the same shorter ones, was proved to be of major importance; and b) the fraction of rrm

preceded by one or more r placement, i.e. the –rrrm- structures at the end of syndiotactic

sequences; iii) the pure heterotactic –mrmr- sequences and iv) the short atactic moieties like

rmrr, mmrr, mrrm and rmmr.

Table 3. The iso, hetero and syndiotactic pentads values obtained by ined from 1H NMR

spectra (Figure 1) of PMMA samples



Triads

rr

mr

mm



Pentads

rrrr

mrrr+mrrm

rmrr+mmrr

mrmr+mmrm

rmmr

mmmr+mmmm



X

0.395

0.089

0.4197

0.0118



Sample

Y

0.39

0.049

0.397

0.018



0.019 0.064



0.0406 0.111



Z

0.39

0.0245

0.3070

0.072

0.0226

0.1866



Table 4. The iso, hetero and syndiotactic pentads values calculated by assuming

Bernoullian statistics for PMMA samples



Triads

rr

mr

mm



Pentads

rrrr

mrrr+mrrm

rmrr+mmrr

mrmr+mmrm

rmmr

mmmr+mmmm



X

0.24

0.4984

0.2939

0.125

0.044



Sample

Y

0.17

0.3474

0.295

0.163

0.0525



Z

0.13

0.230

0.2882

0.191

0.057



0.0457



0.058



0.0763



The changes of these repeating stereosequences in X, Y and Z samples can be specified

in the light of the above quoted both calculated and experimental results (Tables 3 and 4). It

may be thus stated that:

(1) The average length of isotactic –mmmm…- sequences increases from sample X to

sample Z and so does the content of mmrm sequence and the length of the isotactic

sequence preceding it. As a result the ratio of mmrm stereosequences longer than one

heptad to the shorter ones will increase in the order X
(2) The fraction of –rrrm- structures and the length of the syndiotactic sequence

preceding it, will decrease in the order X>Y>Z.



The Tacticity Governed Stereomicrostructure …



73



(3) As indicated by the individual calculated values, the fraction of pure heterotactic

stereosequences, -mrmr..-, hardly changes from one sample to the other. A tiny

tendency towards decreasing from X to Z is however observed.

(4) The short atactic moieties, mrrm, rmrr and mmrr decrease in the order X>Y>Z, this

tendency being significant for mrrm only. The rmmr changes in the reverse order.

On the other hand, it has been extensively conveyed [12,13] that mmrm can adopt GTTGTT and GTGTTT conformation, the equilibrium between them being strongly displaced

towards the latter conformation. It is worthy to note that in PMMA such a displacement

should be much enhanced relative to PVC, because of the more hindered rotation facilities.

Nevertheless, the occurrence of GTTG-TT conformation will decrease in a similar way to

mmrm, i.e. Z>Y>X.

By correlating the changes in all the above repeating stereosequences with those in any

physical property of the samples, the understanding of the processes at molecular level,

involved in that property should take a step further. A considerable amount of work on PVC

and PP makes this prospect quite reliable.



REFERENCES

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]



Vella, N.; Toureille, A.; Guarrotxena, N.; Millán, J. Macromol. Chem. Phys. 1996, 197,

1301-1309.

Guarrotxena, N.; Vella, N.; Toureille, A.; Millán, JL. Macromol. Chem. Phys. 1997,

198, 457-469.

Guarrotxena, N.; Millán, JL.; Vella, N.; Toureille, A. Polymer 1997, 38, 4253-4259.

Guarrotxena, N.; Vella, N.; Toureille, A.; Millán, JL. Polymer 1998, 39, 3273-3277.

Guarrotxena, N.; Toureille, A.; Millán, J. Macromol. Chem. Phys. 1998, 199, 81-86.

Guarrotxena, N.; Millán, J.; Sessler, G.; Hess, G. Macromol. Chem. Phys. 2000, 21,

691-696.

Guarrotxena, N.; Martínez, G.; Millán, J. Polymer 1997, 38, 1857-1864.

Guarrotxena, N.; Martínez, G.; Millán, J. Polymer 2000, 41, 3331-3336.

Guarrotxena, N.; del Val, J.J.; Millán, J. Polymer Bulletin 2001, 47, 105-111.

Guarrotxena, N.; del Val, J.J.; Elicegui, A.; Millán, J. J. Polym. Sci. Polym. Phys. 2004,

42, 2337-2347.

Hatada, K.; Kitayama, T. “NMR Spectroscopy of Polymers”, Chap 3, Springer 2004,

ISBN: 3-450-40220-9.

Guarrotxena, N.; Martínez, G.; Millán, J. Eur Polym J. 33, 1473 (1996) and referentes

cited therein.

Guarrotxena, N.; Schue, F.; Collet, A.; Millán, J. Polym Int. 52, 420 (2003).



In: Advances in Chemistry Research. Volume 8

Editor: James C. Taylor



ISBN 978-1-61209-089-4

©2011 Nova Science Publishers, Inc.



Chapter 7



THE MODELING OF TRANSITION METAL

COMPLEX CATALYSTS IN THE SELECTIVE

ALKYLARENS OXIDATIONS WITH DIOXYGEN:

THE ROLE OF HYDROGEN – BONDING

INTERACTIONS

L. I. Matienko∗, L. A. Mosolova and G. E. Zaikov

Emanuel Institute of Biochemical Physics, Russian Academy of Sciences,

4 Kosygin str., Moscow, 119334 Russia



ABSTRACT

The different methods of improvement of catalytic activity of transition metal

complexes in the oxidations of alkylarens with molecular oxygen are stated briefly. The

offered at first by authors and developed in their works the method of control of catalyst

activity of transition metal complexes with additives of electron-donor mono- or

multidentate exo ligands L2 in the oxidations of alkylarens (ethylbenzene, cumene) with

molecular oxygen into corresponding hydroperoxides is presented. The modeling of

catalytic nickel and iron complexes with use of ammonium quaternary salts and macrocycle polyethers as exo ligands-modifiers is described in detail. The role of the

Hydrogen–Bonding interactions in mechanisms of homogeneous catalysis is discussed.

The modeling of catalyst activity of complexes Fe(II,III)(acac)n with R4NBr (or 18crown-6) (18C6) in the ethylbenzene oxidation in the presence of small amounts

additives of water (~10-3 mol/l) is analyzed. The role of micro steps of the chain initiation

(O2 activation), and propagation in the presence of catalyst (Cat + RO2•→) in the

mechanism of nickel- and iron-catalyzed oxidation of ethylbenzene is evaluated.







E-mail:matienko@sky.chph.ras.ru



76



L. I. Matienko, L. A. Mosolova and G. E. Zaikov



Keywords: homogeneous catalysis, oxidation, alkylarens, hydroperoxides, dioxygen, Ni(II)-,

Fe(II,III) acetylacetonates, HMPA, DMF, MSt (Na, Li, K), ammonium quaternary salts,

macro-cycle polyethers, PhOH, additives of small amounts of H2O.



1. INTRODUCTION

The major developments in hydrocarbon oxidations have often been motivated by the

need for the ever-growing polymer industry. The functionalization of naturally occurring

petroleum components through reaction with air or molecular oxygen was naturally seen as

the simplest way to derive useful chemicals [1]. The research of N.N. Semenov (gas-phase

oxidation reactions) [2] and later N.M. Emanuel (liquid-phase hydrocarbon oxidation with

molecular oxygen) [3] and others [4] clarified the concepts of chain reactions and put the

theory of free-radical autoxidation on a firm basis. Industrial practice developed alongside.

The development of the industrial processes depends mainly on the investigators ability to

control these processes. The one of the methods of control of the rate and mechanism of the

free-radical autoxidation processes is the change of medium, in which the autoxidation occurs

(the pioneer works of Professor G.E. Zaikov) [5], followed by [1,6]. The homogeneous

catalysis of liquid-phase hydrocarbon oxidation has played no fewer roles in the improvement

of oxidation processes. The selective oxidation of hydrocarbons with molecular oxygen as an

oxidant to desired products is now a foreground line of catalysis and suggests the use of

metal-complex catalysts. In the last years the development of investigations in the sphere of

homogeneous catalysis with metal compounds occurs in two ways – the chain free-radical

catalytic oxidation and catalysis with metal-complexes, modeling the action of ferments. But

the most of the reactions performed at the industrial scale are on autoxidation reactions

mainly because of low substrate conversions at catalysis by biological systems models [1,7].

In works of N.M. Emanuel and his school it was established for the first time that

transition metals compounds participated in all elementary stages of chain oxidation process

with molecular oxygen [8-13]. Later these discoveries were confirmed and described in

reviews and monographs [14-20]. However, there is no complete understanding of

mechanism yet. Special attention was attended to investigation of role of metals compounds

at stages of free radicals generation, in chain initiation reactions (O2 activation) and

hydroperoxides dissociation. Reaction of chain propagation under interaction of catalyst with

peroxide radicals (Cat + RO2•→) is studied insufficiently. Catalysis by nickel compounds

(NiSt, Ni(acac)2) was studied in details only in works L.I. Matienko together with Z.K.

Maizus, L.A. Mosolova, E.F. Brin [12, 21-23].

Solution of the problem of the selective oxidation of hydrocarbons into hydroperoxides,

primary products of oxidation is the most difficult one. High catalytic activity of the majority

of used catalysts in ROOH decomposition doesn't allow suggesting of selective catalysts of

oxidation into ROOH to present day. Application of transition metals salts rarely leads to

significant increase in process selectivity, since transformations of all intermediate substances

are accelerated not selectively [20]. For alkylarens, hydrocarbons with activated C−H bonds

(cumene, ethylbenzene) the problem of oxidation into ROOH at conditions of radical-chain

oxidation process with degenerate branching of chain is solvable, since selectivity of

oxidation into ROOH at not deep stages (∼1-2%) is high enough (S∼80-95%). In this case the



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