CHAIN TRANSFER IN —MOLECULAR WEIGHT CONTROL AND MORE

Presented by: Vijay R. Srinivas, Ph.D. Principal Research Scientist Arkema, Inc. Research & Development 900 First Avenue, King of Prussia PA 19406 Phone: 610-878-6656 • Fax: 610-878-6730 • Email: [email protected]

Vijay R. Srinivas is a Principal Research Scientist in the Thiochemical Division of Arkema Inc., based at its R&D Center in King of Prussia, PA. He joined the then Pennwalt Corporation’s Organic Chemicals Division in 1984 after a Post Doctoral stint at the University of Chicago as a Senior Research Chemist to work on Process Development in the thiochemicals area. He was involved with the development of several products and was responsible for implementing their commercial production. He currently is in charge of Technical Service and Technology Marketing for all thiochemical products that go into the energy market – refineries and petrochemicals and the mercaptan products that are used in the and other industries for North America.

He does not understand why he loves to play golf, since he gets his money’s worth whenever he goes out to play. He also likes to play volleyball, but has a slight height disadvantage. 2

Chain Transfer in Polymerizations-Molecular Weight Control and More

Vijay R. Srinivas Arkema Inc.

ABSTRACT:

Control of molecular size and molecular weight distribution is necessary so that the resulting have good processability and have specific properties required for their intended applications. Chain transfer is the process of regulating and controlling molecular weight. Some experts consider this process of chain transfer as the fourth process in polymerizations, along with initiation, propagation and termination. Chain transfer was first observed in radical polymerizations but occurs in ionic polymerizations also.

Alkyl (Mercaptans) have been recognized to be one of the most efficient chain transfer agents in polymerizations that produce polystyrene, styrene-butadiene rubber, ABS terpolymers, polymethacrylates (e.g. PMMA), polyacrylates and other vinyl-type of polymers. Chain transfer can also be used to make low molecular weight polymers (telomers), introduce branching and cross-linking. t-Dodecyl mercaptan (TDM) and n-dodecyl mercaptan (NDM) are examples of the two most common chain transfer agents. In many polymerizations, TDM is the choice due to the fact that its Chain Transfer Constant (CT) is an aggregate of different numbers – since it is a mixture of several tertiary mercaptans of carbon number C10 to C13. NDM, on the other hand is a pure C12 mercaptan with a higher CT, thus giving a narrower molecular weight distribution. Mechanism of the various reactions involved in chain transfer will be discussed. Some strategies involving the use of chain transfer agents – mercaptans in particular, for improving thermal stability of the polymers, increasing branching, providing enhanced cross-linking, will also be discussed.

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Introduction: Kinetics of most polymerization processes involves a fourth step called “Chain Transfer”, in addition to initiation, propagation and termination. The chain transfer reaction can take place with the , solvent, initiator, and or even with a modifier. In all cases the fundamental result is the reduction in the molecular weight of the polymer. The amount of reduction in molecular weight is dependent on the reactivity of the growing macro radical with the small species. In the chain transfer process; the total number of active centers before and after the reaction does not change even though a growing chain is terminated. A term ‘Chain Transfer Constant’ is used to assess the ability of a solvent or other substance added to be a chain transfer agent in polymerizations. This can be defined as:

CT = ktr/kp

Where CT is the chain transfer constant, ktr and kp are the rate constants for the transfer and propagation reactions, respectively.

Chemistry of Chain Transfer:

Polymerization involves mainly three steps, although the fourth one is implicit and very important in controlling the molecular weight of the polymer being formed.

Initiation: I • 2R•

2R• + 2M • 2RM•

kp • • Propagation: RMn + M • RM(n+1)

kt • • Termination: RMn + RMm • RM(m+n)

The chain transfer reaction can take place with any molecular species present during polymerization. In general this takes place via the abstraction of an atom, typically hydrogen or halogen atoms, from a substrate and forming a dead polymer and a new radical. One may consider chain transfer as another mode of termination.

ktr • • Pn + XY • PnX + Y (Chain Transfer)

This is followed by the addition of the formed radical to a monomer to afford a radical species (can be termed as a re-initiation) as below:

ki • • Y + M • YM1 (Reinitiation)

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Substrates containing labile atoms or groups of atoms tend to be active in this reaction.

TABLE-1

CASE Rel. Rate Constants Type of Effect Effect on Rp Effect on Mol. Weight

1 kp>>ktr; ki•kp Normal CT None Decrease Large Decrease 2 kp <

3 kp>>ktr; ki < kp Retardation Decrease Decrease Large Decrease Large Decrease 4 kp <

The effect of chain transfer on the polymerization rate depends on whether the reinitiation rate is comparable to that of the original propagating radical. Table-1 shows the various possible situations one may encounter. In Cases 1 & 2 reinitiation is rapid and one observes no change in the polymerization rate. The same number of monomer is consumed per unit time and a larger number of smaller sized polymer molecules are formed. The relative decrease in the molecular weight (Xn) depends on the transfer constant. When the transfer constant (ktr) is much greater than that of propagation (kp), as in Case-2 above, a very small sized polymer often referred to as a ‘Telomer’ is formed. When reinitiation is slow compared to propagation (Cases-

3 & 4) one observes a decrease in the rate of polymerization (Rp) as well as in the Xn. Again the amount of decrease in the rate depends on the kp and ktr. Retardation in the rate is observed because, with reinitiation being slow, termination of the primary radical occurs, decreasing the rate of polymerization as shown below:

ktp · · Pn + Y • “Dead Polymer Chain”

Where ktp is the rate coefficient for primary radical termination.

Chain transfer is important because it can alter the molecular weight of the polymer in an undesirable fashion. Controlled chain transfer, on the other hand, can be employed to control the molecular weight at a specified level. As mentioned earlier, chain transfer reactions can involve the initiator, monomer, solvent, an added compound (modifier) and the polymer being made.

Chain Transfer to Initiators

Initiators that react with macro radicals, many times form a new radical that is similar to the one obtained by their thermal decomposition. Such initiators can function as chain transfer agents, for e.g. cumyl and t-butyl hydroperoxides in methyl methacrylate polymerizations, wherein the process is faster compared to when benzoyl peroxide or 2,2’-azobis isobutyronitrile (AIBN) is used as an initiator. Most initiators, however, do not encourage chain transfer. Additionally, the initiator concentration is very low and so is its interaction with the growing chain. Thus chain transfer to the initiator is not of much significance; although specific initiators are chosen for specific polymerizations, with their chain transfer properties also contributing to the desired property of the polymer being made.

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Chain Transfer to

Chain transfer to the monomer, however, would be significant if the monomer contains a labile H or other atom. Reaction occurs by the abstraction of a hydrogen atom from the monomer or the donation of a hydrogen atom from the polymer radical to the monomer:

• • Pn + CH2=CHX • PnH + CH2=CX for e.g.

• • Pn + CH2=CH-OCOCH3 • PnH + CH2=CH-OCOCH2

• • Pn + CH2=CH-CH2X • PnH + CH2=CH-CH X

Or

• • Pn-1 –CH2-CHX + CH2=CHX • Pn-1 –CH=CHX + CH3CH X

Chain transfer to the monomer intrinsically limits the length of the macromolecular chain. For vinyl chloride monomers chain transfer to the monomer is one of the important modes of molecular mass control. Rearrangement, elimination of Cl• followed by the addition of Cl• to another monomer unit can take place as shown below:

• • • -CH2 -CHCl-CHCl-CH2 • -CH2 -CHCl-CH -CH2Cl • Cl + -CH2 –CH=CH-CH2Cl or other isomers • • Cl + CH2=CHCl • CH2Cl-CH Cl

Transfer to vinyl acetate also occurs easily, with the hydrogen atoms on the acetate being the most labile. Allylic monomers such as the chloride, acetate etc., show an even greater tendency to give up a hydrogen atom, that results in the formation of a relatively low energy radical, stabilized by delocalization on carbon atoms 1 and 3. Due to this reason, allyl radicals have very little tendency to reinitiate the polymer chain. Therefore the consequence here is a low polymerization rate and a low degree of polymerization. This phenomenon is often referred to as degradative chain transfer. It is important to note that chain transfer to the monomer produces an unsaturated radical, which in many instances, can function both as a co-monomer and as an active center for polymerization, giving rise to a branched macromolecule. Thus, when this happens we can say that chain branching is an indirect consequence of chain transfer to the monomer.

For acrylic monomers and styrene, the probability for the chain transfer to occur to the monomer in comparison to propagation is very low (~ 10-5).

Chain Transfer to the Polymer

Chain transfer to polymer results in the formation of a radical site on a polymer chain. The polymerization of the monomer at this site leads to the formation of branched polymer. Arkema Inc. Research & Development King of Prussia, PA 19406 ILC-2005 6

H Y . P + CH C P nH + CH 2 C n 2 .

Y

P

Y CH 2 C

P m

Formation of branched structures as opposed to reduction in molecular mass is the dominant feature of chain transfer to polymer. Transfer to the polymer can be ignored when one is attempting to determine precisely the constants CI, CS, CM for chain transfer to initiator, solvent and monomer respectively, since these are determined from data at low conversions. However, transfer to polymer cannot be ignored or neglected for practical situations where polymerizations are carried out to near completion or high conversions. The effect of chain transfer to the polymer plays a very significant role in determining the physical properties and ultimately the applications of the polymer. Branching, as we know, drastically reduces the cystallinity of a polymer. Hydrogen abstraction results in the formation of active centers inside a preexisting or a growing polymer chain. Subsequent reinitiation and propagation steps give rise to long-chain or short-chain branches.

The following cases can be distinguished, as each, results in the formation of a differently branched polymer:

(i) Intermolecular chain transfer and termination by disproportionation (negligible wrt. transfer) produces branched polymers. (ii) Intermolecular chain transfer and termination by coupling gives rise to branched and cross-linked polymers. (iii) Intramolecular chain transfer can afford polymers with short-chain branches (commonly referred to as a “back-biting reaction involving a quasi-cyclic intermediate) and (iv) Intermolecular chain transfer to preformed polymer of a different structure permits the formation of graft-copolymers.

As examples, the presence of (i) and (iii) above is said to be responsible for the particular structure of low-density and the unique behavior of polyvinyl acetate. Here chain transfer occurs to both the main chain (major) and the acetyl groups, giving rise to two types of branching, one of which can be easily removed by saponification.

As an alternate route, transfer to monomer followed by copolymerization yields almost exclusively hydrolysable branching. As a consequence, polyvinyl alcohol has a more linear structure than the polyvinyl acetate from which it is obtained. The relevance of chain transfer to

Arkema Inc. Research & Development King of Prussia, PA 19406 ILC-2005 7 polymer and consequently of branching is dependent on the concentration of the polymer and thus increases with increasing polymerization yield.

Vinyl Acetate Polymerization . OCOCH2 OCOCH3 . PnH + CH CH CH Pn + CH2 CCH2 2 2 H H P

- [OH ] OCOCH2P CH2 CH CH2 CH2 C CH2 H OH

Linear Polymer

OCOCH3 OCOCH3 . PnH + . Pn + CH2 C CH2 CH2 C CH2 H

OH OCOCH [OH-] 3 CH2 CCH2 CH2 CCH2 P P

Branched Polymer Chain Transfer to Chain Transfer Agent and Solvent

In all of the above cases, one has very little control of the chain transfer process. The role of the solvent or intentionally added chain transfer agent can be well defined, primarily from their activity standpoint and from the concentration used. The chemical and kinetic behavior of several solvents and chain transfer agents are quite similar, in that they possess a loosely bound atom or group of atoms that can homolytically cleave or do so in the presence of a propagating Arkema Inc. Research & Development King of Prussia, PA 19406 ILC-2005 8 radical, under polymerization conditions. The chain transfer constants for various solvents used with different propagating radical species are known.

TABLE-2: Chain Transfer Constants for Different Solvent/Radical Systems*

5 5 5 0 5 0 5 SOLVENT Cs*10 at Cs*10 at Cs*10 at 100 C Cs*10 at 80 C Cs*10 at 0 0 0 60 C for 80 C for for ~CH2- for ~CH2- 60 C for • • • • PhCH CH2~ PhCH CH2~ C (CH3)COOCH3 C (CH3)COOCH3 ~CH2- • CHCOOCH3 Benzene 0.18 0.61 1.84 0.75 29.6 Cyclohexane 0.24 0.66 1.6 1.0 65.9 Toluene 1.25 2.98 6.45 5.25 208.9 Ethylbenzene 6.7 10.7 16.2 13.5 551.5 Cumene 8.2 13.1 20.0 19.0 899 Triphenylmethane 35.0 -- 80 Butyl chloride 0.4 -- 3.7 Dichloromethane 1.5 -- 118 Carbon 920 1330 1850 23.9 Tetrachloride

* Data from ‘Polymer Handbook’ Eds. Brandrup J. and Immergut E.H.; John Wiley & Sons, New York, 1975

Compounds with a hydrogen atom attached to a carbon or more commonly to a sulfur atom, a halogen attached to a carbon, a sulfur atom attached to a carbon and/or another sulfur, are common examples of active compounds used for chain transfer. Temperature has an influence on the chain transfer activity of a solvent or agent. As an example, styrene polymerized in the presence of carbon tetrachloride (CCl4), almost always yield lower molecular weight polystyrene than when done so in the absence of a solvent. Additionally, in many cases initiation and termination reactions are negligible compared to the transfer reaction and both chain ends could be derived form the solvent as shown below:

M CCl4 . . . . Pn + CCl4 PnCl + CCl3 CCl3Pn CCl3PnCl + CCl3

It is important to note that not all solvents behave the same in all polymerizations. For example,

CCl4 is not effective in butadiene polymerizations. The efficiency of solvents and chain transfer agents varies by as much as six orders of magnitude as we go from say benzene to carbon tetrabromide to n-dodecyl, t-dodecyl or butyl mercaptan or ethyl thioglycolate etc. In general solvents with high chain transfer ability are used to make low molecular weight liquid polymers. Many a times a solvent with a chemical structure similar to that of the monomer is chosen so that the terminal end groups from the solvent do not effect the polymer structure and properties drastically. This is sometimes referred to as ‘Telomerization’ and the solvent in such a case is known as a ‘Telogen’. For example, polymerization of bromotrifluoro is carried out by this technique using bromotrifluoro methane as the telogen.

In many industrial processes, specific chain transfer agents are intentionally added to control the molecular weight of the polymer. Such agents are specifically called ‘chain modifier’. The modifier prevents the uncontrolled growth of a polymer chain. It terminates growing chains and

Arkema Inc. Research & Development King of Prussia, PA 19406 ILC-2005 9 promotes fresh new chains in such a way that the average molecular weight of the polymer remains within a close range. Mercaptans in general and n-dodecyl mercaptan (NDM) and t-dodecyl mercaptan (TDM) are the most commonly used polymer chain modifiers in the industry. Their interaction with a growing chain can be represented as follows:

. . CH CH + RS CH2 CH + n-C12H25SH 2 2

X X Use of mercaptans to modify the molecular weight dates back to before World War II, especially in the SBR industry. The chain transfer constants for several mercaptans used in various recipes are known. A few select values are represented below in Table-3.

TABLE-3: Chain Transfer Constants for Select Mercaptans

Mercaptan Monomer Temperature(0C) CT Constant Value(s) n-Octyl Mercaptan Butadiene 50 16, 18, 19 n-Decyl Mercaptan Butadiene 50 18.2 n-Tetradecyl Mercaptan Butadiene 50 19.4 n-Butyl Mercaptan Ethylene 130 5.8, 15 Ethyl Mercaptan Methyl Acrylate 50 1.57 n-Butyl Mercaptan Methyl Acrylate 60 1.69 ± 0.17 n-Octyl Mercaptan Styrene 50 19 n-Dodecyl Mercaptan Styrene 60 18.7±1 t-Dodecyl Mercaptan Styrene 50 2.9

Experimentally, one can determine the value of the CT, the chain transfer constant of an added agent using the Mayo equation. Hence mathematically one is able to calculate the amount of chain transfer agent needed to obtain a specifically desired molecular weight of the polymer.

Agents that have a CT of 1 or greater are very useful since they can be used effectively in very low concentrations. NDM and TDM are the most common chain transfer agents used as mofidiers. NDM is a pure compound (>98.5% isomer purity of straight chain C12H25SH) and

TDM is a statistical mixture of C10 to C13 mercaptans, predominant in highly branched C12H25SH. If one evaluates the regulating index of the mercaptans, one finds that for normal mercaptans the index drops off rather quickly with increasing carbon number, that for the tertiary mercaptans does not change too much until one goes past 12 carbons. Figure-1 shows the Regulating Index of mercaptans as a function of carbon number. Temperature affects this index also slightly for the tertiary mercaptans, higher the index at higher temperature. These values are approximate since variables such as agitation, emulsifier employed, transport of the active mercaptan between phases etc. impact them. Another aspect of chain transfer studies is that it corroborates the concept of functional group reactivity independent of molecular size. Therefore, one can vary the degree of polymerization by either using different chain transfer agents or by using different amounts of the same transfer agent. Under these conditions the propagation rate constant (kp) is independent of Xn and the transfer constant for a particular agent is independent of the size of the propagating radical. This allows for molecular weight control by a mercaptan throughout the polymerization, and obtain a desired molecular weight distribution. Using mercaptans for example NDM, are known to give totally not cross-linked polymers of desired molecular weight. Observations such as improved stability, possibly due to the suppression of Arkema Inc. Research & Development King of Prussia, PA 19406 ILC-2005 10 undesired termination pathways, show that mercaptans can be used for several purposes in polymerizations.

FIGURE-1

x

de

n I

ng

ti a l

gu

e R

Higher the regulating index of a mercaptan faster is its depletion along the conversion profile. If one plans to conduct a polymerization# to of high C conarvberosnionss , employing a mercaptan with a high regulating index will give a polymer with a broad molecular weight distribution. While theoretically one can calculate this, experimental observations are less drastic since other ingredients in the recipe, such as solvent, monomer, polymer etc. get involved in reducing or modifying the molecular weight. Polymers with a broad molecular weight distribution have been shown to breakdown during milling, because preferential breakdown of higher molecular weight fraction of the polymer takes place. It is important to note that a narrow, optimum molecular weight distribution and free chain ends impart good vulcanizate properties to SBR’s. Mercaptan modifiers with a wide range of regulating indexes are available commercially and

Arkema Inc. Research & Development King of Prussia, PA 19406 ILC-2005 11 with careful selection it is possible to produce polymers with different molecular weight distributions within the confines of the emulsion systems used. Further modification can be effected in SBR polymerizations by the incremental addition of the mercaptan modifier. This technique has been verified experimentally and is used commercially. Here there is a possibility to make a polymer with a specific molecular weight distribution at higher conversions than by using a single aliquot of the modifier. In order to see a significant difference in the molecular weight distribution by using the incremental addition procedure, one needs to use a mercaptan modifier of a relatively high regulating index, which magnifies the differences in the polydispersity. The use of two or more additions (or continuous addition) of mercaptan modifiers does offer the advantage of having a uniform viscosity during the course of the polymerization.

Functionalized mercaptans or di-, tri- mercaptans have been shown to possess unique chain transfer activity and could potentially be used as modifiers not only to control molecular weight but also to obtain controlled amounts of branching and cross-linking.

Conclusion:

Chain transfer is intrinsic to all polymerization processes and can take place to several species present or added. Majority of the processes add an agent to modify or control the process to afford desirable molecular weights that impart beneficial and varied properties to the polymer. The method used to add the chain transfer agent can influence the molecular weight distribution of the polymer being produced. Continuous addition or multiple additions do offer advantages and result in the use of the modifier more efficiently to control molecular weight and polymer viscosity. Additionally, a combination of the different modes of chain transfer can be beneficially used to make uniquely functional polymers with controlled molecular weight distribution, viscosity and properties.

Acknowledgements: The author wishes to thank Arkema in general and its Thiochemical Division for providing the opportunity to present this article.

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