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THE TACTICITY GOVERNED STEREOMICROSTRUCTURE IN POLY(METHYL METHACRYLATE) (PMMA) AS A WAY TO EXPLAIN ITS PHYSICAL PROPERTIES
stereomicrostructure of PMMA as was done previously for Poly(vinyl chloride) (PVC)
and Polypropylene (PP).
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
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.
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 …
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
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
. 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 . 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.
Table 1. 1H NMR data for the PMMA samples
8.39 43.16 48.45 29.97 70.03 0.72 0.308 0.28
15.16 40.86 43.98 35.59 64.41 0.57 0.31
20.93 37.92 41.15 39.89 60.11 0.47 0.31
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) . 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 ,
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 . For example an mmrm pentad fraction is given as
The Tacticity Governed Stereomicrostructure …
[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
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,
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
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
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
Table 4. The iso, hetero and syndiotactic pentads values calculated by assuming
Bernoullian statistics for PMMA samples
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 …
(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.
Vella, N.; Toureille, A.; Guarrotxena, N.; Millán, J. Macromol. Chem. Phys. 1996, 197,
Guarrotxena, N.; Vella, N.; Toureille, A.; Millán, JL. Macromol. Chem. Phys. 1997,
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,
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,
Hatada, K.; Kitayama, T. “NMR Spectroscopy of Polymers”, Chap 3, Springer 2004,
Guarrotxena, N.; Martínez, G.; Millán, J. Eur Polym J. 33, 1473 (1996) and referentes
Guarrotxena, N.; Schue, F.; Collet, A.; Millán, J. Polym Int. 52, 420 (2003).
In: Advances in Chemistry Research. Volume 8
Editor: James C. Taylor
©2011 Nova Science Publishers, Inc.
THE MODELING OF TRANSITION METAL
COMPLEX CATALYSTS IN THE SELECTIVE
ALKYLARENS OXIDATIONS WITH DIOXYGEN:
THE ROLE OF HYDROGEN – BONDING
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
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.
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.
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 . The research of N.N. Semenov (gas-phase
oxidation reactions)  and later N.M. Emanuel (liquid-phase hydrocarbon oxidation with
molecular oxygen)  and others  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) , 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 . 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