Polymer Durability - ACS Publications - American Chemical Society

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Downloaded by 5.62.155.141 on June 22, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch001

Auxiliary Mechanism for Transfer to Monomer during Vinyl Chloride Polymerization Implications for Thermal Stability of Poly(vinyl chloride) W. H. Starnes, Jr. , Haksoo Chung , B. J. Wojciechowski , D. E. Skillicorn , and G. M. Benedikt 1

1

1

2

3

Applied Science Ph.D. Program, Department of Chemistry, College of William and Mary, Williamsburg, VA 23187-8795 The Geon Company Technical Center, Avon Lake, OH 44012 BFGoodrich Company Research and Development Center, Brecksville, OH 44141 1

2 3

Chain transfer to monomer during the free-radical polymerization of vinyl chloride is shown to occur, in part, by a mechanism that begins with the abstraction propagating

of methylene

macroradical

hydrogen from

the polymer

by a

The resultant radical then donates a chlo-

rine atom to the monomer to form an allylic structure that can contribute to the thermal instability

of poly(vinyl

contents found by NMR spectroscopy

chloride).

Double-bond

and their correlation with mo-

lecular weights confirm the operation of this transfer process and verify a new theory for transfer to monomer. The theory involves two transfer constants and, allows their values to be obtained.

W H E N

POLY(VINYL CHLORIDE) (PVC) IS PREPARED AT TEMPERATURES

within the usual commercial range, most if not all of its ethyl-branch segments have the E B structure shown in Scheme 1 (J). This arrangement results from a process that starts with head-to-head addition of monomer and involves the head-to-head radical, 1, and the rearranged radicals, 2 and 3 (I). At a given temperature of polymerization, the sum of the E B and chloromethyl (MB) branch concentrations (2) is independent of the pressure (or molar concen tration) of vinyl chloride (VC) (3). These observations require operation of the 0065-2393/96/0249-0003$12.00/0 © 1996 American Chemical Society Clough et al.; Polymer Durability Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

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POLYMER DURABILITY

entire mechanism in Scheme I (3). They also show that this scheme, as drawn, incorporates all reactions occurring after head-to-head emplacement that have significant structural or kinetic implications (3). Moreover, they lead directly to the following major conclusions (3): 1. Radicals 2 and 3 do not undergo unimolecular β-scission to generate chlorine atoms that become kinetically free. 2. In bulk or suspension polymerizations of V C , rates of propa gation are not controlled by diffusion, even up to conversions of about 90%. 3. The monomer transfer constant, C , that pertains to transfer occurring after head-to-head emplacement (see Scheme I) is a true constant that is independent of the V C concentration (eq 1).


M H H

C ,HH = M

tpMtp +

+

4)

FC

=

W * p

+

FC

')( 3 + * )

P

FC

8

(D

When extrinsic transfer agents are absent, transfer to monomer produces most of the long-chain ends in P V C (4, 5). Yet the instantaneous numberaverage molecular weight ( M J of P V C decreases rapidly with decreasing V C

Scheme I. Mechanistic sequelae of head-to-head addition during the free-radica polymerization of vinyl chloride (VC), where P' is the ordinary head-to-tail macroradical, and the ks are rate constants.

Clough et al.; Polymer Durability Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

1.

STAKNES ET A L .

5

Transfer to Monomer for PVC

concentration (2), despite the constancy of C . These findings necessitate the intervention of an auxiliary monomer-transfer process that increases in importance as the V C content declines. Such a mechanism was described and discussed heretofore (3, 6). This mechanism is depicted in Scheme II, and its first step can be either inter- or intramolecular, as shown. Addition of radical 4 to the monomer would start the growth of a long-branch structure (5), whose presence was supported by N M R observations (7). Alternatively, 4 might undergo a transfer reaction with V C that exhibits second-order kinetics and is, therefore, analogous to the transfers in Scheme I (3). The resultant internal allylic (IA) structure would be a thermally labile site (8, 9). Thus, this auxiliary transfer pathway has potential implications for both the stability of P V C and the overall mechanism for the polymerization of V C . We examine these topics in some detail and present conclusive evidence for the occurrence of chain transfer by the auxiliary route.


MH H

Experimental Section Subsaturation polymerizations were performed according to a published procedure (2, 10), starting with 296 g of water, 0.164 g of ammonium persulfate, and 1.89 g of a PVC latex resin that was required for use as seed. Polymerization-grade V C was introduced continuously from a colder reservoir (2, 10), and the reaction mixtures were agitated with an efficient stirrer that was operated at the highest possible speed. Polymerizations were stopped before the agglomeration of primary

Ρ·

Ρ·

PH

+

—CHCI-CH-CHCI-

VC^

4

CH -CHCI

vVC

—CH=CH-CHCI—

2

—CHC1-CH-CHCI-

'

A

5

Ρ·

Scheme II. Auxiliary mechanism for transfer to monomer dunng the free-radical polymenzation of vinyl chlonde (VC), where F is the ordinary head-to-tail macroradical, and the ks are composite rate constants as defined in the text.


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POLYMER DURABILITY

particles occurred. The polymers were isolated by filtration, washed three times with water, and then dried thoroughly under vacuum at ca. 40 °C. Number-average molecular weights of PVC specimens were obtained from the equation, log M = 4.6549 + 1.2385[log(IV.)], where I V . is inherent viscosity measured in cyclohexanone, (0.2 g)/(100 mL), at 30 °C, according to A S T M Method D1243-79 (II). Proton N M R measurements were made with a Bruker AMX500 instrument at 50 °C by using dilute solutions of the polymers in tetrahydrofuran-IA + cr


(2)

1.

9


STARNES ET A L .

Table I. Double-Bond Concentrations in PVC Determined by 500-MHz *H NMR Spectroscopy Polym Temp (°C)

M X JQn

82 80* m 70 61 57 56 55 H C l + 4 2

(3)

Equations 2 and 3 are the propagation steps of an ancillary chain reaction that would have produced additional IA moieties (23) and thus would have caused [CH=CH] to be greater than 1 per molecule. Hence, our failure to find that result argues strongly against the formation of a significant number of free chlorine atoms from 4. Instead, we can conclude that 4, like radicals 2 and 3 (3), transfers C l directly to monomer in a process whose overall kinetics are second-order. totaI

Total Monomer-Transfer Constants. The total monomer-trans fer constant ( C ) is defined by eq 4, where C refers to the auxiliary transfer of Scheme II and thus is equal to the IA concentration per monomer unit. M total

Maux

^M.total

^Μ,ΗΗ ^M,aux +

From this equation and our previous arguments, the total double-bond con-


POLYMER DURABILITY

10

tent per V C unit should be equivalent to the C value deduced from the M of the polymer. The literature contains several Arrhenius expressions that are based on M data and can be used to calculate C for conventional polymerizations performed at various temperatures. Four equations of this type, including three reported previously, appear in a paper by Carenza et al. (24). We used these equations to calculate four values of C for each of the temperatures in Table I. Table II compares the averages of these values with the C values obtained with eq 4 from the double-bond concentrations in the third and fourth columns of Table I (data for the subsaturation polymers are irrel evant here and thus were omitted). The agreement is excellent in every case, and this result can be regarded as an additional verification of the mechanistic theory we propose. Mtotal

n

n

Mtotal

Mtotal


M t o t a l

Determination of H e a d - T o - H e a d and Auxiliary Transfer Constants from Molecular Weights. In the transfer process of Scheme II, radical 4 must occur in several microenvironments that differ with respect to local tacticity and the presence or absence of defect structures. Nevertheless, single rate constants can be defined for all of the reactions by which 4 is formed or destroyed. For example, k can be described by eq 5, in which fe , fc , ... are the rate constants for reaction of the various struc tures whose mole fractions are denoted by/ ,/ , ...f , where/ + f + ... +/„ 6

6a

6b

a

b

a

n

= 1.

Table II. Comparison of C

Mtotal

Values

Polym Temp (°C) Literature* H NMR 2.7 ± 0.4 82 3.1 ± 0.5 2.4 ± 0.4 2.9 ± 0.5 80 2.2 ± 0.3 2.2 ± 0.3 70 1.6 ± 0.2 1.7 ± 0.3 61 1.2 ± 0.2 1.5 ± 0.2 57 1.5 ± 0.2 1.4 ± 0.2 56 1.1 ± 0.2 1.3 ± 0.2 53 1.1 ± 0.2 1.3 ± 0.2 52 1.3 ± 0.2 1.1 ± 0.1 49 0.8 ± 0.1 40 0.8 ± 0.1 0.6 ± 0.1 0.7 ± 0.1 36 0.8 ± 0.1 36 0.7 ± 0.1 0.6 ± 0.1 32 0.6 ± 0.1 l

NOTE: Values are C

MTOTAL

Χ

b

10. 3

Average values obtained from the four Arrhe nius equations in ref. 24; deviations are average deviations from the mean. Sum of the values in columns 3 and 4 of Table I; deviations are the estimated experimental er rors.

a

b


h

1.


STARNES ET A L .

11

h = fc /a + Kbfb + - + Kfn

(5)

6a

Rate constants k and fc can be defined i n a similar way, and the firstorder constant, k \ can be expressed as a sum of specific rate constants for intramolecular hydrogen abstraction via cyclic transition states having rings of various sizes (eq 6). 8

7

6

*β· = Because C by eq 7:

(6)

is equal to the I A content per monomer unit, it is given

Mmx


κ: + *«/ +... + kj

d[IA]/di

=

^ ,aux M

fc [4][VC] fc [F][VC]

=

8

d [ P V C ] / d i

=

p

fcsW ikptF]

1

where the bracketed terms are concentrations and t is reaction time. Here, [PVC] refers to polymerized monomer units rather than polymer molecules, and head-to-tail propagation is the only reaction of P" that was taken into account. The other reactions of P" can be ignored because the sum of their rates is quite small. Under steady-state conditions, the formation rate of 4 will equal its rate of disappearance. Hence, eq 8 will apply. The combination of eq 8 with eq 7, so as to eliminate [4]/[P*], produces eq 9. Because the recip rocal of the number-average degree of polymerization, (DP)~ , is equal to C ,totai> l 10 follows direcdy from eq 4. Equation 11 then results from the substitution, into eq 10, of the expression for C from eq 9. l

M

e

(

Maux

(feetPVC] + fc ')[F] = (k + e

=

8

(8)

fc (fc [PVC] + k ') 8

6

e

k (k + fc )[VC]

^

p

1

M

c , + M

HH

8

r

(DP)- = C ,

(UP)„

fc )[VC][4]

7

H H

k

+ C , M

^

+

(10)

a m

ui;

fcg)[vc]

When intermolecular hydrogen abstraction by P" is much faster than the analogous intramolecular process (i.e., when fc [PVC] » fc ') eq 11 reduces into eq 12. 6

6

?

This equation can be used in a number of ways to test for the credibility of


POLYMER DURABILITY

12

the dual transfer-process proposal. If the proposal were correct, plots of (DP)" vs. [PVC]/[VC] should produce straight lines whose intercepts are equal to the C values determined by N M R spectroscopy. Moreover, cal culations based on the slopes of such lines should yield values of C that also are equivalent to the corresponding N M R values. A n important paper by Hjertberg and Sôrvik (2) contains the best data that are available for use in plots of eq 12. Most of these data appear in Table III. Because the composite activation energy for transfer to monomer is higher than that for normal chain propagation (24), the molecular weight of P V C should increase when the polymerization temperature is decreased under con trolled conditions that otherwise are the same. This trend is observed for all of the M values in Table III except those of the 45 °C polymers that were made at P/P ratios of 0.76, 0.70,jand 0.61. Calculations based on V C solubility data (2 25) show that the low M values of these specimens could not have been caused by major increases i n the equihbrium [PVC]/[VC] values upon going from 55 °C to 45 °C. These lower M values might have resulted, in stead, from excessive V C starvation brought about by diffusion control, attemptsjx) prevent such control (2, 10) notwithstanding. Regardless, these three M values obviously are in error, and therefore we have not used them for plotting. 1

MH H

Maux


n

Q

n

9

n

n

Figure 2 shows plots of eq 12 based on the other data in Table III [the line for 55 °C also includes a point for P/P = 0.53 (2)]. The [PVC]/[VC] ratios used here were obtained from reported values of (g VC)/(100 g PVC) (2) that were derived from information published previously (25). In all cases, the double-regression fits are quite good, and the C values obtained from their intercepts agree superbly with the values found by N M R spectroscopy (Table IV). A n analogous plot (not shown) was constructed from data given in Table I for the polymers made by us at 55 °C. This plot also was linear (R = 0.97), and its slope (0.076) was similar to that of the 55 °C fine in Figure 2 (0.10). Moreover, Table IV shows that the intercepts of these two fines were identical. Activation energy differences might be expected to cause the kjk ratio to increase as the temperature is raised. This change would increase the slopes of the plots of eq 12. However, that trend is not followed by the fines for Q

MH H

2

v

Table III. Molecular Weights of Subsaturation Polymers P/P

Polym Temp (°C) 0.97 0.92 80 65 55 45

26.7 37.1 48.6 59.8

25.3 35.2 45.0 50.4

0

0.85

0.76

0.70

0.61

26.0 34.1 41.0 46.8

20.9 31.6 37.7 32.0

20.2 30.3 33.1 26.5

19.2 25.2 29.3 24.1

e

e

NOTE: Values are M X 10~. Data are from ref. 2. Validity doubtful; see text for discussion. N

3

0


1.


STARNES ET A L .

13

5

4

(DP)n

2


1

0 0

2

4

6

8 10 12 [PVC]/[VC]

14

16

18

20

Figure 2. Double-regression plots of eq 12 based on data obtained by Hjertberg and Sorvik (2) for subsaturation PVCs. The R values for these plots are as follows: 80 °C, 0.91; 65 °C, 0.90; 55 °C, 0.99; and 45 °C, 0.92 2

55 °C and 65 °C. It may be opposed by an increase in the k7/k8 ratio with increasing temperature. The incursion of the process associated with fc ' is another possible complication in this regard (eq 11). When k » fc [PVC], eq 11 reduces to eq 13: 6

l

6

6

m

-' -

c

-

+

i ^ i w

(13)

Plots of the latter equation (not shown) were made from the data on which the Unes in Figure 2 are based by using V C molar concentrations that were determined in a manner described elsewhere (3). These plots also were linear, and their R values were virtually identical to those of the corresponding plots in Figure 2. However, the C values found from their intercepts (0.5, 0.7, 1.2, and 1.5 for 45 °C, 55 °C, 65 °C, and 80 °C, respectively) were somewhat lower than the N M R values of C . Furthermore, the linearity, per se, of plots of (DP)" versus [ V C ] does not necessarily mean that k§ is much larger than fc [PVC]. The reason is that, at a given temperature, the sum of the molar concentrations of V C and polymerized V C units remains approximately con stant [ ± ( 2 - 5 ) % ] in the concentration ranges of interest to us. [The concen trations of polymerized units upon which this conclusion is based were obtained by assuming volume additivity and using a value of 1.4 g/mL for the density of the polymer (5).] In other words, eq 14 applies when the "constant" total concentration is denoted as A. If eq 11 and 14 are combined to eliminate [PVC], the result is eq 15. 2

MH H

MH H

1

- 1

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POLYMER DURABILITY

14

Table IV. Comparison of C Values

}

Polym Temp (°C) 80 65 55

From M Values * 1.8 ± 0.3 1.3 ± 0.2 0.9 ± 0.1 (0.9 ± 0.1) 0.7 ± 0.1

By *H NMR 1.8 ± 0.3 1.4 ± 0.2 0.95 ± 0 . 1 a

1

n

c

45

0.7 ± 0.1

NOTE: Values are L> mi X 10. 3

M

Values based on data in Table I by taking averages where appropriate. Deviations are the estimated experimental errors. Values are intercepts of the plots in Figure 2 unless noted otherwise; see text for discussion. Deviations are the estimated experimental errors. Value based on data from the present work; see text. a


b

c

[PVC] + [VC] = A /DPI ( D P )

"

-1

U^s

C C m

'

h h

,

*

Polymer Durability - ACS Publications - American Chemical Society

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